Unit 2- PORIFERANS, CNIDARIANS, PLATYHELMINTHES
The sea is inhabited by a large number of animal species (Kingdom
Animalia). They range from microscopic worms to giant squids. Regardless
of their size, each fills a particular niche in the overall marine ecosystem. In
this chapter and the next, we will examine more closely the biology and
ecological roles of the myriad animals without backbones, the invertebrate
animals that are part of the fabric of life in the sea.
-10th Edition Marine Biology, 2016
-
Learning Outcomes
At the end of this unit, you will be able to:

Able to classify and identify marine invertebrates ( PORIFERANS, CNIDARIANS,
PLATYHELMINTHES)
Pretest
Multiple Choice
Directions: Read the following questions and encircle the letter of the correct answer.
1. What is an invertebrate?
a. an animal with a backbone
c. a mammal
b. an animal without a backbone
d. a reptile
2. Symmetry about a central axis
a. asymmetry
c. radial symmetry
b. bilateral symmetry
d. holoblastic
3. A cnidarian body plan characterized by a bowl shape and adapted for a free-swimming life.
a. polyp
c. nematocyst
b. medusa
d. tentacles
4. A type of reproduction in which cells from two parents unite to form the first cell of a new
organism
a. sexual reproduction
c. meoisis
b. asexual reproduction
d. none of the above
Content
INVERTEBRATES
The earth is populated by a wide variety of animals, and it is difficult to arrive at a definition of an
animal that applies to them all. Even though there is great diversity among animals, they all share
four characteristics:
1. Animals are multicellular. This distinguishes them from bacteria and most protists, which are
unicellular.
2. Animal cells are eukaryotic, and they lack cell walls. These characteristics distinguish animals
from bacteria, whose cells are prokaryotic and have cell walls, and from fungi, multicellular algae,
and plants, all of which have cells with rigid cell walls.
3. Animals cannot produce their own food, so they depend on other organisms for nutrients. In
other words, they are heterotrophs.
4. Animals, with the exception of adult sponges, can actively move.
Animals that lack a vertebral column (backbone) are invertebrates, whereas animals that have a
vertebral column are vertebrates. At least 97% of all species of animals are invertebrates. All
major groups, or phyla, of invertebrates have marine representatives, and many are exclusively
marine. Among them, Porifera (sponges), Cnidaria (cnidarians), and Platyheminthes (flatworms)
are the subjects of this unit.
Figure 2.1. Phylogenetic Relationship among the Major Animal Phyla
Note: A photo of “Phylogenetic Relationship among the Major Animal Phyla”. Adapted from “Marine
Biology 10th Edition (p. 118)” by Castro and Huber, 2016. Copyright 2016 by McGraw-Hill Education.
PORIFERAS (Sponges)
Animals in the phylum Porifera (pore bearers) are known informally as sponges. Among the
simplest of animals, sponges are sedentary and were mistaken for plants by the ancient Greeks.
Most species are marine, and they range in size from a few millimeters to a few meters. All are
sessile, living permanently attached to the bottom or some other hard surface. They show an
amazing variety of shapes, sizes, and colors but share a relatively simple body plan. Sponges are
suspension feeders (animals that eat food particles suspended in the water), specifically filter
feeders: They filter out food particles suspended in the surrounding water as they draw it through
their body.
Sponge’s Body Structures
Figure 2.2 Sponges. Collar cells trap food particles in both (a) simple and (b) complex sponges.
Note: A photo of “Sponges”. Adapted from “Marine Biology 10th Edition (p. 119)” by Castro and Huber,
2016. Copyright 2016 by McGraw-Hill Education.

Mesohyl- gelatinous region that separates the two regions that separate the body
-contains live cells

Epidermis- outer layer

Pores (ostia)- numerous tiny pores on the surface allow water to enter and
circulate through a series of canals where plankton and organic particles are
filtered out and eaten
- network of canals and a relatively flexible skeletal framework give most
sponges a characteristic spongy texture

Spongocoel- central cavity where water is drawn after passing though pores
Figure 2.3. Close-up view of a Sponge. (a) Diagrammatic illustration of the body wall of a sponge.
(b) Detail of a choanocyte.

Note:
A photo of “Close-up
view of a where
Sponge”.
Adapted
fromout
“Biology
of the Invertebrates 7th Edition (p.
 Osculumlarger opening
water
flows
the sponge
80)” by Pechenik, 2015. Copyright 2015 by McGraw-Hill Education.


Choanocytes-flagellated cells lining the spongcoel
-create a current that draws water in through the pores and out through the
osculum, maintaining circulation of seawater
-has a thin collar that traps food particles, which are then ingested by the body of
the cell
-capture incoming sperm for fertilization
Amoebocytes (wandering cells)- cells wander throughout the mesohyl by typical
cytoplasmic streaming, with the formation of pseudopodia
-can transport nutrients to other cells of the sponge body
-can produce materials for skeletal fibers (spicules)
-can become any type of sponge cell as needed

Sclerocytes- cells that secrete spicules
Figure 2.4 Binucleate sclerocyte
Note: A photo of “Sponge Spicules”. Adapted from “Biology of the Invertebrates 7th Edition
(p. 81)” by Pechenik, 2015. Copyright 2015 by McGraw-Hill Education.

Spicule-transparent siliceous or calcareous supporting structures of different
and sizes
-together with spongin, maintain shape and discouraging predation

Spongocytes- cells that produce sponging

Spongin- collagenous protein that makes up a tough elastic skeleton
-can be the only means of support, or it can be found together with spicules
shapes
Figure 2.5 Sponge Spicules. (a-g) Representative sponge spicule morphologies
Note: A photo of “Sponge Spicules”. Adapted from “Biology of the Invertebrates 7th Edition
(p. 81)” by Pechenik, 2015. Copyright 2015 by McGraw-Hill Education.
Sponge Size and Body Form
The size of a sponge is limited by its ability to circulate water through its body, and this is
determined by the body form.

Asconoid form is the simplest. It is tubular and always small. Sponges with this type of
body are usually not found alone but in clusters. As a sponge with an asconoid shape
grows, the volume of the spongocoel increases. This increase in spongocoel volume is
not, however, accompanied by a proportional increase in the surface area available for
collar cells. As a result, there are not enough collar cells to move sufficient amounts of
water through the sponge to meet its physiological needs. This is the reason asconoid
sponges
are
always
small.

Syconoid form is when sponges exhibit the first stages of body-wall folding. In syconoid
sponges the body wall has become folded to form internal pockets that are lined with collar
cells. The folding increases the surface area of the collar cell layer, and the reduction of
the spongocoel that occurs because of the folding decreases the volume of water that
needs to be circulated. The more efficient movement of water allows the sponge to grow
larger.

Leuconoid form exhibits highest degree of folding. There are many chambers lined with
collar cells and the spongocoel is frequently reduced to a series of water canals leading
to an osculum. Because the leuconoid body plan is the most efficient arrangement, most
sponge species are built on this plan. It is no wonder that all of the largest sponges have
a leuconoid body type.
Figure 2. 6 Sponge Body Forms.
Note: A photo of “The zonation on rocky shores”. Adapted from “Introduction to Marine Biology 3rd Edition
(p. 194)” by Karleskint et al, 2010. Copyright 2010 Brooks/Cole, Cengage Learning.
Sponges could perhaps be similar to the first multicellular animals, which were probably simple
colonies in which some cells became specialized for such functions as feeding and protection.
Sponge cells are very plastic and easily change from one type to another. If experimentally
separated, the cells can even regroup and form a new sponge.
Figure 2.7 Ability of Sponge to Form New Individuals after being separated. (a)Cells can be
separated through a fine sieve. In a matter of hours, the cells begin to aggregate and reorganize
in seawater (b) and eventually form new sponges (c).
Nutrition and Digestion
Because sponges feed on material that is suspended in seawater, they are called suspension
feeders. They are also referred to as filter feeders because they filter their food from the water.
Large food particles (1 to 50 micrometers, or 0.00004 to 0.002 inch) are engulfed and digested
by pinacocytes and archaeocytes along the sponge’s system of canals that carry water from the
ostia to spongocoels.
Approximately 80% of a sponge’s food is trapped by the choanocytes and consists of smaller
particles (0.1 to 1.0 micrometers, or 0.000004 to 0.00004 inch). Choanocytes strain food from
the water. The beating of their flagella creates a current that draws water in through the ostia and
expels it through the osculum. As the water circulates through the sponge, bacteria, plankton, and
detritus are engulfed and digested by various cells.
Food particles can be transferred to other cells in the sponge by both collar cells and
amoebocytes. Most food particles are digested within the ameobocytes, and these cells also
function in the storage of food. Undigested material and nitrogenous wastes leave the sponge
with the exiting water currents.
Reproduction in Sponges
Many sponges reproduce asexually when branches or buds break off and grow into separate
sponges identical to the original one. Sponges also reproduce sexually by producing gametes.
Unlike most animals, sponge gametes are not produced by gonads. Instead, specialized collar
cells or amebocytes develop into gametes. The gametes are like those of other animals: large,
nutrient-rich eggs and smaller sperm cells with a flagellum.
Most sponges are hermaphrodites, animals in which individuals have both male and female
gonads. Some species, however, have separate males and females. Sponges typically release
sperm into the water. The release of gametes into the water is called broadcast spawning. The
eggs, however, are usually retained inside the body and fertilization takes place internally after
sperm enter the sponge.
Figure 2.8 Sexual Reproduction in Sponges
Note: A photo of “Sexual Reproduction in Sponges”. Adapted from “Marine Biology 10th Edition (p. 120)”
by Castro and Huber, 2016. Copyright 2016 by McGraw-Hill Education.
The early stages of development take place inside the sponge. A tiny, flagellated sphere of cells
is eventually released into the water. This planktonic larva, called the parenchymula larva in most
sponges, is carried by currents until it settles on the bottom and develops into a minute sponge.
Most marine invertebrates have characteristic larvae that eventually change into juveniles that
resemble adults. This drastic change from the larva to a juvenile is called metamorphosis.
Classification of Sponges
Sponges exist in a tremendous variety of colors and shapes, ranging from encrusting forms only
a few millimeters wide to elaborate upright forms over 1 m high. Although most sponges are
immobile, members of a few sponge species can move at speeds of several millimeters per hour,
presumably by the coordinated cytoplasmic movements of individual, amoeboid cells.
Sponges are distributed among the following four classes, based largely upon the chemical
composition and morphology of the support elements:
a. Class Calcarea,
b. Class Demospongiae,
c. Class Hexactinellida
d. Class Homoscleromorpha
Class Calcarea
Members of the class Calcarea bear spicules composed only of calcium carbonate (CaCO3). This
is also the only sponge class to include all three types of body construction; indeed, the only living
asconoid forms are found among the Calcarea.
Class Demospongiae
Members of the largest class (containing at least 80% of all sponge species), the Demospongiae,
are nearly all of leuconoid construction. The supporting spicules and fibers of the Demospongiae
may be composed of sponging and/or silica, but never of CaCO3 .
A small number of species possess neither fibers nor spicules. All freshwater sponges (fewer than
300 species) are found in this class. Interestingly, these freshwater species possess contractile
vacuoles, which are organelles specialized for eliminating water from cytoplasm; they are found
elsewhere only among protozoans.
Class Hexactinellida
Sponges whose bodies are supported entirely by interconnected six-rayed spicules of silica and
chitin are placed in the class Hexactinellida. These sponges, known as the glass sponges, are
marvels of structural complexity and symmetry. The members of some species live in soft
sediment, anchored by tufts of spicules, while members of other species live attached to solid
substratum.
Figure 2.9 Hexactinellid
Hexactinellid canal systems may be either syconoid or leuconoid, but hexactinellids stand apart
from all other sponges in that the entire sponge, including the outer layer is syncytial (having many
nuclei contained within a single plasma membrane) rather than cellular and lacks contractile
elements; thus, there is no pinacoderm layer. The inner, flagellated layer is also syncytial, again
setting the glass sponges apart from all others.
Class Homoscleromorpha
Recent molecular data that incorporate 18S and 28S ribosomal DNA (rDNA) sequences, along
with mitochondrial genome information, support moving these sponges (fewer than 100 species)
from the Demospongiae into this separate class, Homoscleromorpha. Although most
homoscleromorph species lack spicules, when the spicules are present, they are entirely
siliceous, as in hexactinellid sponges, but they have a distinctly different morphology.
Moreoever, the members of this class are unique among sponges in that they have a clear, distinct
basal membrane underlying the epithelium, as well as in the fact that all their epithelial cells bear
cilia.
Ecological Role of Sponges
Although they are simple, sessile animals, sponges interact with other marine organisms in
several ways.




They compete aggressively for space with other sessile organisms.
They are links in some marine food chains.
Sponges form many symbiotic relationships, and they provide habitat for other organisms.
Sponges also play an important role in the recycling of calcium.
Competition
The biggest problem that sponges face is finding enough suitable solid material for attachment.
Consequently, their primary competitors for space are corals and bryozoans. Some sponge
species produce chemicals that either kill corals or inhibit their growth. Other species, such as the
boring sponges (family Clionidae), create their own habitat by boring into corals and dead shells.
Some species of crab known as sponge crabs (family Dromiidae) attach pieces of sponge to
themselves for camouflage and protection and in the process provide a solid surface for the
transplanted sponge species.
Figure 2.10 Sponge crabs camouflage themselves by attaching sponges to their bodies
Predator–Prey Relationships
Very few animals feed on sponges, possibly because ingesting the spicules would be like eating
a mouthful of needles. Many sponges produce chemicals that prevent organisms from settling on
their surface or that deter grazing. A study of sponges found that 9 of 16 Antarctic sponges and
27 of 36 Caribbean species were toxic to fi sh. There are a few species of bony fi shes and
molluscs that will eat sponges, and sea turtles, especially the hawksbill sea turtle (Eretmochelys
imbricata), feed almost exclusively on them. As much as 95% of the turtle’s feces contains glass
spicules, indicating the large role of sponges in the turtle’s diet. The tough lining of the turtle’s
mouth and digestive tract prevents the spicules from injuring the animal.
Figure 2.11. This hawksbill sea turtle is one of the few animals that will feed on sponges.
Symbiotic Relationships
Many sponges are hosts to other organisms. Some of these relationships are mutually beneficial
for both the sponge and its symbionts (mutualism), whereas others seem to favor the symbionts
without harming the sponge (commensalism).





Some species of sponge host large numbers of mutualistic bacteria that provide food to
the sponge while gaining nutrients and protection within the sponge’s system of water
canals.
In fact, in some species, the volume of bacterial cells is twice that of the volume of sponge
cells. Cyanobacteria make up 33% of Verongia, a sponge that lives in shallow, well-lit
habitats.
Many species of sponge that live on coral reefs contain symbiotic cyanobacteria in their
bodies. A sponge provides protection and a sunlit habitat for the cyanobacteria, and the
cyanobacteria provide the sponge with nutrients and oxygen.
Studies of sponges on the Great Barrier Reef of Australia have found that reef sponges
get between 48% and 80% of their food from their symbiotic cyanobacteria. There are also
a few species of sponge that contain symbiotic dinoflagellates, a single-cell,
photosynthetic organism.
The glass sponge known as Venus’s flower basket that is found in the deep waters of the
tropical Pacific Ocean exhibits an interesting symbiotic relationship with certain species of
shrimp (Spongicola). A male and a female shrimp enter the sponge’s spongocoel when
they are young.
CNIDARIA (Cnidarians)
The next level of organizational complexity among animals after the sponges involves quite a big
step: the evolution of multicellular animals with tissues that perform specific functions. This
development makes it possible for organisms to swim, respond to external stimuli, and engulf
prey, among other things.
Cnidarians, sometimes called coelenterates (phylum Cnidaria), include the sea anemones,
jellyfishes (or sea jellies), corals, and their relatives. They are called cnidarians because of the
stinging cell, or cnidocyte that they all possess. This unique cell is used not only in the capture of
prey but also for protection.
Body Organization and Structures
Besides having a tissue level of organization, cnidarians display radial symmetry, where similar
parts of the body are arranged and repeated around a central. If a radially symmetrical animal
were cut like a pizza, all the resulting slices would be similar. Animals with radial symmetry look
the same from all sides and have no head, front, or back. They do, however, have an oral surface,
where the mouth is, and an aboral surface on the opposite side.
Figure 2.12 Body Plan of Cnidarians.
Polyp body plan body that has an opening at one end, the mouth, which is usually surrounded by
a ring of tentacles. The medusa is a free-floating stage that is commonly known as a jellyfish.
Many cnidarians exhibit both body plans during their life cycles, although some, such as corals
and sea anemones, exist only in the polyp stage. Others spend their entire lives as either polyp
or medusa.
The characteristic larva of most cnidarians is the planula, a cylindrical, ciliated stage consisting of
two layers of cells. After a time in the plankton, the planula settles on the bottom and
metamorphoses into a polyp or develops into a medusa.

Mouth- located centrally

Tentacles- slender, finger-like extensions used to capture and handle

Gut- where digestion takes place

Epidermis- outer layer of cells

Gastrodermis- layer of cells that line a large cavity or the gastrovascular cavity

Mesoglea- gelatinous material located between gastrodermis and gastrovascular cavity

Cnidocytes- stinging cells in the tentacles

Cnidae- capsule-like organelle that are capable of exploding outward
-may be specialized for wrapping around small objects, sticking to surfaces,
penetrating surfaces, or secreting proteinaceous toxins, some of which are among
the most deadly toxins known
-function in food collection, defense, and, to some extent, locomotion
Nematocysts- specialized cnidae that contain stinging threads that can penetrate the body
wall of the cnidarian’s prey

food
Each cnida consists of a rounded, proteinaceous capsule, with an opening at one end that is often
occluded by a hinged operculum. Within the capsule is a long, hollow, coiled tube. During
discharge, the hollow tube shoots out explosively from the sac, turning inside out as it goes. The
entire process requires only about 3 ms (milliseconds). Discharge is triggered by a combination
of chemical and tactile stimulation, generally perceived through a cluster of modified cilia (the
cnidocil) that projects from the cnidoblast and by surface chemoreceptors on specialized nearby
cells. Each cnida can be discharged only once.
Figure 2.13. Nematocyst. a) Stages in the discharge of a nematocyst. (b) Penetration of the
nematocyst filament into another animal. (c) Six nematocysts fired by the scyphozoan jellyfish
Cyanea capillata penetrating human skin. (d) Jellyfish (medusa) capturing prey.
Behavior
In contrast to members of the Porifera, cnidarians possess bona fide nerves and muscles.
However, they have no central nervous system. Their nervous system consists instead of a
network of nerve cells (neurons) and their processes (neurites), which generally synapse on one
another repeatedly before terminating at a neuromuscular junction.
Figure 2.14. Cnidarian Nerve Net
Although nerve impulses may cross certain synapses in one direction only, many synapses permit
impulses to pass in both directions. Moreover, a given cell body may give rise to two or more
neurites, radiating in different directions. Thus, a nerve impulse received by one neuron may
proceed in several directions at once.
They are known to attack and even kill anemones from other clones using special nematocysts.
Some scyphozoan medusae have primitive eyes, but the eyes of cubomedusae are known to
form images. Medusae also have statocysts, small, calcareous bodies in fluid-filled chambers
surrounded by sensitive hairs. Statocysts give medusae a sense of balance.
Feeding and Digestion
Cnidarians are primarily carnivorous, although some soft-coral species will also eat
phytoplankton. Cnidarians use their nematocysts primarily to capture prey. In many species,
individuals obtain additional nutrients through the photosynthesizing activities of unicellular algae
living symbiotically in their tissues. In particular, endosymbiotic algae characterize all reef-building
(hermatypic) corals.
Figure 2.15. A Portuguese man-of-war (Physalia physalis) showing its Nematocysts
After ingestion, food passes into the gut, where it is digested. The initial phase of digestion is said
to be extracellular because it takes place outside cells. Intracellular digestion within cells lining
the gut completes the breakdown of food. Cnidarians lack anus. All undigested food material
passes out through the same opening through which the food enters: the mouth. Gonadal
development often takes place within the digestive cavity, and the gametes or embryos must be
released into this cavity before being expelled to the exterior through the mouth.
Respiration
Cnidarians lack gills and other specialized respiratory structures; gases diffuse across all exposed
epidermal and gastrodermal surfaces.
Life Cycle
Cnidarians can reproduce both asexually and sexually, and they exhibit a variety of reproductive
strategies in their life cycles. Asexual reproduction generally occurs in the polyp stage and results
in the formation of more polyps or tiny medusae. Sexual reproduction usually occurs in the
medusa stage.
Taxonomic Classification
Phylum Cnidaria
Subphylum Medusozoa
o
o
o
o
o
o
Class Scyphoza- the true jellyfish
Class Cubozoa- the sea wasps
Class Hydrozoa
 Subclass Hyroidolina
Order Trachylina
 Class Staurozoa
Order Siphonophora
The Myxozoa
Subphylum Anthozoa- the sea anemones, corals, sea whips, sea pens, sea fans,
and sea pansies
o
o
Subclass Hexacorallia (= Zoantharia)
Subclass Octocorallia (= Alcyonaria)
Figure 2. 16. Cnidarian Relationships
Subphylum Medusozoa
This recently established but well-accepted grouping contains all cnidarians except for the sea
anemones and corals. Unlike that of any other metazoan so far studied, the medusozoan
mitochondrial genome is linear rather than circular.
Class Scyphozoa (True Jellyfish)
The Scyphozoa contains fewer than 200 species, all of which are marine and many of which are
quite large (up to about 2 m across). The mesoglea layer of scyphozoans is thick and has the
consistency of firm gelatin; thus, scyphozoans are known collectively as jellyfish.
Morphology
o
Jellyfish morphology is described as medusoid. The body is in the form of an inverted cup,
with nematocyst studded tentacles extending downward from the cup, or bell. The mouth
is borne at the end of a muscular cylinder known as the manubrium.
Figure 2.17. Medusa. (a) In longitudinal section, showing thick layer mesoglea and single opening
to gastrovascular cavity. (b) Lateral view of a medusa.
Feeding and Digestion
o
o
o
o
o
Scyphozoans are characterized by a well-developed system of fluid-filled gastrovascular
canals, ultimately connecting to the mouth through the manubrium.
Food particles captured by nematocysts on the tentacles and/or oral arms are ingested at
the mouth and conveyed to the stomach through the manubrium.
Food is then distributed among four gastric pouches, which contain short, nematocystbearing tentacles (gastric filaments) that secrete an array of digestive enzymes.
The partially digested food particles are then phagocytosed, and digestion is completed
intracellularly, a typical feature of cnidarian biology.
Some scyphozoans also obtain nutrients from certain unicellular algae (zooxanthellae)
that live symbiotically in the jellyfish tissues.
Life Cycle
In adult jellyfish and box jellyfish the sexes are generally separate.
o
o
o
The medusae are the sexual stage in the life cycle, and they release gametes into the
water column, where fertilization and early development take place.
The fertilized eggs develop into planula larvae that spend time in the water column before
settling on a solid surface to form the polyp stage of the life cycle. The polyps reproduce
medusa-like buds by asexual reproduction.
The immature buds are released into the water column to grow into mature medusae.
Figure 2. 18. Moon Jellyfish Life Cycle
Class Cubozoa
The members of the small (about 25 species) but interesting class Cubozoa are called
cubomedusae. Recent molecular data suggest that they are the most derived cnidarians.
Figure 2.19. Box Jellyfish (Chironex sp.)
Morphology
o
All members of the class have a cuboidal swimming bell: The bell is actually square in
transverse section. Each individual bears four tentacles, or four clusters of tentacles,
emerging from the four corners of the bell, near the four rhopalia.
Figure 2.19 The cubomedusan Carybdea sp
o
Cubomedusae are unusually active and strong swimmers for jellyfish and possess an
unusually well-developed nervous system and remarkably complex eyes that can
probably form images
Life Cycle
The polyp resulting from a single planula larva buds off more polyps, each of which develops into
a single medusa. The asexual production of genetically identical individuals (ramets)
characterizing scyphozoans is again achieved in the cubozoan life cycle, but through a different
vehicle.
Cubozoans are restricted to tropical and subtropical areas; they can be quite common in those
waters at certain times of the year.
Class Hydrozoa
Members of the Hydrozoa are characterized by generally greater representation of the polyp
morph in the life cycle than is the case for scyphozoans, although the polyp and medusa morphs
are about equally prominent in a number of hydrozoan species, and the medusa morph dominates
in a few others. In contrast to other cnidarians, the gastrodermal tissue of hydrozoans lacks
nematocysts, and no cells are found within the mesoglea; nematocysts are restricted to the
epidermis. Most hydrozoan species are marine.
Subclass Hydroidolina
Although most members of this subclass are marine, a number of freshwater species also
exist— Hydra, for example.
Morphology
o
o
o
o
The mesoglea layer of the hydrozoan medusa is thick, the mouth is borne at the end
of a manubrium, and ocelli and statocysts are present.
The sense organs may be found at the base of the tentacles, as in scyphozoans, or
between the tentacles.
All medusae are gonochoristic, a given individual being either male or female but
never both. But medusa is smaller and they usually possess a shelf of tissue (the
velum) extending inward from the edge of the swimming bell toward the manubrium
Velum causes water to be ejected from under the swimming bell through a narrower
opening, and thus with greater velocity, when the musculature contracts.
Figure 2.20 Typical Hydrozoan Medusa
Life Cycle
o
The planula (larvae of hydrozoans) develop from fertilized eggs, and the planula
typically metamorphoses into a sessile polypoid individual lacking both statocysts and
ocelli.
Figure 2.21. The planula larva of a hydrozoan, Mitrocomella polydiademata. (b) Ciliated planula
of the Red Sea soft coral Dendronephthya hemprichi
o
o
o
Polyps are structurally and functionally more complex than are the scyphistomae
In Hydra, each polyp is a separate, distinct being, completely responsible for its own
welfare
Unlike that of most other hydrozoans, the Hydra life cycle lacks a medusa stage. Also,
most other hydrozoans are colonial in the polyp stage of the life cycle; that is, a single
planula usually gives rise to a large number of polyps, called zooids (or modules of the
colony), all of which are interconnected and share a continuous gastrovascular cavity
Figure 2.22 (a) A member of the freshwater genus Hydra. (b) A typical colonial hydroid,
Campanularia sp. (c) A single hydrozoan polyp.
o
The zooids are often connected to each other, or to a substrate, by means
of a rootlike stolon. The oral end of a polyp (i.e., the end bearing the mouth and
tentacles) is called the hydranth.
o
In Obelia, alternation between the polyp and medusa forms happen. The polyp stage,
a colony of interconnected polyps in the case of Obelia, is more conspicuous than the
medusa.
Figure 2.23. The life cycle of the hydrozoan Obelia.
Order Siphonophora
o
o
Include the Portuguese man-of-war, are free-floating hydrozoan colonies in which
medusoid and polypoid morphs are present simultaneously in a number of different
incarnations.
Modified medusae serve as modules modified to propel the colony through the water
by jet propulsion (nectophores), or as leaflike defensive modules
o
o
o
Some species exhibit gas-filled (carbon monoxide!) floats called pneumatophores,
which may also derive from basic medusoid architecture.
The mesoglea layer is much reduced or entirely absent in pneumatophores.
Nectophores lack both mouth and tentacles.
Figure 2. 24. Typical pelagic (free-floating) hydrozoans, (a) Nectalia sp. and b) Muggiaea sp.
Class Staurozoa
o
o
o
Staurozoans are mostly small (rarely more than 4 cm long), and mostly found in cold,
shallow waters.
They live attached to seaweeds, rocks, and gravel by a stalk and feed on mostly on small
crustaceans that pass by, catching their prey with the eight sets of tentacles found at their
distal end; the tentacles end in great clusters of nematocysts.
They look like modified medusae, but they live the lives of polyps; they are commonly
referred to as “stalked jellyfish.
Subphylum Anthozoa
Anthozoans (including the sea anemones and the corals) consist of about 6,000 species, nearly
70% of all cnidarian species described. Anthozoans form the sister group to the Medusozoa. All
anthozoans are marine and all exploit the polyp body form and lifestyle exclusively; no trace of
the medusa morph appears in the life cycle
Life Cycle
o
o
Lacking a medusa, gametes are produced directly by the anthozoan polyp. A planula larva
develops from the fertilized egg and metamorphoses to form another polyp. The planula
larvae of some anthozoan species can feed on phytoplankton and other microscopic food
particles; no feeding planulae have been found in other cnidarian groups.
Many anthozoan species also reproduce asexually, often through longitudinal or
transverse fission or through a process of pedal laceration, in which parts of the pedal disc
(foot) detach from the rest of the animal and gradually differentiate to form a new singlemodule ramet.
Feeding and Digestion
o
o
o
o
Anthozoans are primarily carnivorous; they capture food using nematocyst-studded
tentacles and transfer it to a central mouth opening.
The anthozoan mouth opens into a tubular pharynx rather than directly into the
gastrovascular cavity, and one or two discrete, ciliated grooves, called siphonoglyphs,
typically extend down the pharynx from the mouth.
The anthozoan gastrovascular cavity is partitioned by numerous sheets of tissue called
mesenteries or septa. These infoldings of gastroderm and mesoglea greatly increase the
surface area available for secreting digestive enzymes and absorbing nutrients.
Anemones, possess acrorhagi, rings of small, spherical bulges extending around the
circumference of the body column just below the tentacles
Subclass Hexacorallia (= Zoantharia)
o
Hexacorallians possess many tentacles around the mouth opening (usually in some
multiple of 6; hexa = G: six) and six pairs of primary mesenteries. Sea anemone and
scleractinian corals belong to this subclass.
Sea anemone
o
o
Many species, such as anemones, are solitary (i.e., they are independent ramets rather
than colonies of connected modules) and lack any specialized protective covering
Polyps that are larger, heavier, and more complex than hydrozoan polyps
Figure 2.25 Sea anemone. Young anemones temporarily attached around the column of the
parent anemone.
o
o
o
o
o
o
o
o
The gastrovascular cavity is divided into compartments that radiate out from the central
cavity.
Most range from 1.5 to 10 centimeters (3⁄4 to 3 inches) in diameter, but Tealia columbiana
on the North Pacific coast of the United States and Stichodactyla on the Great Barrier
Reef may grow to a diameter of 1 meter (3.3 feet) or more.
Sea anemones inhabit both deep water and shallow coastal water worldwide and are most
diverse in the tropics.
Most species are found attached to a hard surface such as rocks, shells, or submerged
wood, but some burrow in sand or mud.
Sea anemones are capable of expanding, contracting, and reaching with tentacles to
capture their prey. When disturbed, sea anemones will withdraw their tentacles into their
oral openings and contract their bodies.
Although essentially sessile, many species of sea anemone can change location by gliding
on their base, by crawling on the side of their body, or walking on the tentacles. A few
species can detach and swim briefly by rapid contractions of their body stalk or tentacles.
This behavior is used to escape predators such as sea stars.
Some species exhibit aggressive behavior toward other species of sea anemone or clones
of themselves. Cnidocytes on specialized searching tentacles fi re on contact with foreign
anemones.
Both animals then move away from each other. This behaviour may provide for some
spatial separation to avoid crowding.
Coral animals
o
o
o
These species tend to be colonial but not polymorphic. The best known of these colonial
species are the true (or stony) corals, which secrete substantial external calcium
carbonate skeletons.
Also called scleractinian corals (sclero = G: hard) and belong to order Scleractinia.
Majority of these corals form large colonies of small polyps. They may be reef-building
(hermatypic) or not (ahermatypic).
Figure 2.26 Live coral polyps from the Caribbean with their tentacles extended for feeding.
o
o
All members of the colony are connected by a horizontal sheet of tissue. Coral reefs are
the products of reef-building hard corals, coralline red algae, and calcified green algae.
Some hard corals such as mushroom and feather corals (Fungia) have solitary polyps as
large as 25 centimeters (5 inches) in diameter that are similar in structure to sea
anemones.
Figure 2.27 Mushroom coral (Fungia) is an example of a large solitary coral polyp
o
o
o
Coral reefs are especially abundant in tropical areas of the Indo-Pacific, forming chains of
islands and other structures of massive proportions.
The relationship between the anthozoan and its zooxanthellae has an additional effect on
reef growth, quite apart from that mediated by nutritional considerations. Hermatypic orals
calcify faster in the light than in the dark. The implication is that activities of the symbiotic
algae play a role in determining the rate at which anthozoans deposits calcium carbonate,
although the mechanism through which this effect is mediated remains uncertain.
The most likely possibilities are
1. through an effect on availability of bicarbonate ion (HCO3–), an essential constituent of
the calcification process;
2. through a contribution by the algae of a critical component of the organic matrix serving
as the nucleating site for calcium carbonate deposition;
3. through localized removal of soluble phosphate (PO43–) by the algae during
photosynthesis (phosphates are known inhibitors of carbonate calcification); and
4. indirectly, through an influence of elevated dissolved oxygen concentrations on rates of
coral metabolism.
In addition to their importance as reef builders and objects of aesthetic enjoyment, a few
stony coral species are being used in certain surgical procedures.
o small pieces of coral are now being used for human bone grafts, particularly in face
and jaw reconstruction, and in arm and leg surgery
o to repair bird wing fractures
o small coral implants are being used to improve the degree to which artificial eyes
exhibit natural movement
Subclass Octocorallia (= Alcyonaria)
Members of the subclass Octocorallia possess 8 tentacles (and eight primary septa) and generally
have only a single siphonoglyph. The tentacles of octocorallians are pinnate; that is, they bear
numerous lateral outfoldings called pinnules.
o
o
o
All octocorallian species form modular colonies which are often polymorphic. In species
with polymorphic polyps, some individuals cannot feed and function solely in driving water
through the gastrovascular spaces of the colony.
The polyps of octocorals may be embedded in a thick matrix of mesoglea; these are the
soft corals. Soft corals form colonies that look more like plants than animals
In other octocoral species (colonial species with skeletal supports), the polyps are
supported by proteinaceous or calcareous internal skeletons secreted by cells in the
mesoglea. This latter group of octocorals, which belong to order Gorgonaceae, includes
the sea fans and sea whips (known collectively as gorgonians, or horny corals) and the
pipe corals.
Figure 2.29 Octocorals. An orange sea pen, Ptilosarcus (left) and sea pansy, Renilla (right)
o
o
The tissues of octocorallians have long been known to accumulate a variety of unusual
biochemicals derived from fatty-acid metabolism. 25 These compounds are not directly
involved in the metabolic processes of the corals, but seem instead to protect the coral
from predation and overgrowth by other organisms.
Recent research indicates that some of these chemicals kill certain mammalian tumors,
increasing interest in cnidarians as potential sources of effective anticancer agents.
PLATYHELMINTHES (Flatworms)
The platyhelminths are a group of some 34,000 described species, with—at present—no uniquely
defining characters (synapomorphies). The group includes one class of mostly free-living
individuals (the turbellarians) and three classes of exclusively parasitic individuals (the
monogeneans, trematodes, and cestodes) that are believed to have evolved from free-living
turbellarian ancestors; more than 80% of all described platyhelminth species are parasites.
All flatworms are acoelomate, triploblastic, and bilaterally symmetrical. Indeed, the free-living
forms may be the most primitive (i.e., basal) bilateral animals, and the first group to have evolved
a true mesoderm; all coelomate animals may ultimately have evolved from flatworm-like
ancestors.
Perhaps the most conspicuous feature of platyhelminths is that they are flat. They have no
specialized respiratory organs and no specialized circulatory system, although a very few species
possess hemoglobin. Gas exchange is accomplished by simple diffusion across the body surface.
The rate at which such exchange can occur (milliliters of oxygen transported from the surrounding
medium into the tissues per unit of time) depends upon several factors: the oxygen concentration
gradient across the body wall, the body wall’s permeability to gas, the thickness of the body wall,
and the total exposed surface area across which diffusion can occur. By being flat, the flatworms
achieve a high surface area relative to their enclosed volume, and a sufficient amount of gas
exchange can occur to support an active lifestyle despite the lack of gills and an internal circulatory
system.
Metabolic wastes probably move out of flatworms mostly by diffusing across the general body
surface. Again, being flat is helpful in this regard. In addition, most platyhelminths contain a series
of specialized organs called protonephridia (G: first kidney). The typical protonephridium consists
of a group of cilia projecting into a fine-meshed cup. The beating of the cilia within the cup has
been likened to the flickering of a flame; thus, the common name for this type of cell is flame cell.
Other protonephridia take the form of solenocytes, in which a single flagellum is found within the
cup. In both cases, the mesh cup is attached to a long, convoluted tubule that connects to the
outside of the animal through a single excretory pore.
Figure 2.30 Excretory system of free-living flatworm
The vast majority of flatworm species, in all four classes, are simultaneous hermaphrodites; that
is, each individual can, at any one time, function as both a female and a male. In consequence,
sperm exchange and egg fertilization can occur when any individual encounters another individual
of the same species. Individuals generally cannot fertilize themselves; some exceptions are
discussed shortly.
Classification
Phylum Platyhelminthes—the flatworms
Class Turbellaria—the free-living flatworms
Class Cestoda
Subclass Cestodaria
Subclass Eucestoda—the tapeworms
Class Monogenea
Class Trematoda—the flukes
Subclass Digenea
Subclass Aspidogastrea
Class Turbellaria
Only about 15% of all flatworm species are turbellarians (ter-bel-air´-ē-ans). The high surfacearea-to-volume ratio of turbellarians makes them especially prone to dehydration in air, so most
turbellarians live in aquatic environments. Most of the 4,500 turbellarian species are free-living,
but about 150 species are commensal or parasitic with other invertebrates.
Most aquatic turbellarian species are benthic; that is, they live in or on the ocean, lake, pond, or
river bottom. The body’s outer surface is ciliated, often more so on the ventral surface than on the
dorsal surface. Most species move at least partly by secreting mucus from the ventral surface
and beating the ventral cilia within this viscous mucus. As a consequence of being flat, increased
size is accompanied by a substantial increase in the amount of surface area in contact with the
substrate over which the animal is moving. Thus, an increased number of cilia in contact with the
substrate compensates for the increased weight of a larger animal, and the ability to move need
not suffer as the animal grows. In contrast to the monociliated condition of cnidarians and
sponges, each flatworm epidermal cell is multiciliated, bearing several too many cilia.
Nervous System
o
The nervous system consists of a coelenterate-style, diffuse nerve net in the most primitive
turbellarian species. Increasing compactness of the system is associated with
advancement in the class, culminating in the possession of a cerebral ganglion—a
primitive but distinct brain— and from one to three (rarely four) pairs of longitudinal nerve
cords. Such an advanced nervous system also characterizes the parasitic members of the
phylum.
o
Turbellarians typically bear one or more pairs of eyes anteriorly, along with a variety of
cells that sense chemicals (such as potential foods), pressure changes (such as those
produced by water currents), and mechanical stimuli. Individuals in about 10% of the
species also have statocysts, which provide feedback about body orientation.
Reproduction
The most structurally and physiologically sophisticated system found among turbellarians is
the reproductive system. Both male and female reproductive organs are found within a single
individual, and the male system is particularly.
Figure 2.31 Turbellarian reproductive system. (a) Both male and female reproductive organs in a
single individual. (b) Detail of copulatory apparatus and associated structures.
When triclad flatworms mate, each individual typically inserts its penis into the female opening
of the other member of the pair, so sperm transfer is reciprocal. For most other turbellarians,
copulation may occur by hypodermic impregnation, in which the stylets of the penis pierce the
body of the partner. The eggs of each animal are released.
Figure 2.31 Turbellarian flatworms (Pseudoceros bifurcus) joust in a mating ritual known as
penis fencing.
Class Cestoda
Members of the class Cestoda (ses-to9-dah)—most of which are members of the subclass
Eucestoda and commonly known as tapeworms—are all internal parasites; that is, they are
endoparasitic. They primarily parasitize vertebrates, inhabiting various regions of the host
digestive tract. As many as 135 million people worldwide are estimated to have tapeworm
infections.
Exhibits extremely high degree of cestode specialization for an endoparasitic existence. Instead
of the ciliated epidermis characterizing turbellarians, a nonciliated tegument covers the cestodes.
The tegument contains numerous nuclei, but these are not separated by cell membranes; that is,
the tegument is syncytial. The outer surface is outfolded into numerous cytoplasmic projections,
vastly increasing the amount of exposed surface area across which nutrients can be taken up
from the host’s gut. Indeed, the cestode must receive all of its nutrients in this manner, as it has
no mouth or digestive tract of its own at any point in its life cycle.
Although lacking a mouth, cestodes do have an anterior end, which in most species takes the
form of a scolex. The scolex is studded with hooks and/or suckers that are used to maintain
position within the host’s gut. The relatively few cestode species that lack a scolex are mostly
placed within a second subclass, the Cestodaria.
Figure 2.32 Cestode
The essence of tapeworm existence, however, really lies just posterior to the scolex, in a region
known as the neck. A seemingly endless series of sections called proglottids bud from the neck
of most cestodes at a rate of several per day. Only a small number of cestode species, mostly
members of the subclass Cestodaria, do not produce proglottids.
Each proglottid is involved primarily with the process of sexual reproduction. Not only is each
tapeworm a simultaneous hermaphrodite, but so is each proglottid; that, is, each proglottid
contains both male and female reproductive systems.
Figure 2.33 (a) Taenia solium, the pork tapeworm, attached to the intestinal wall of its host. (b)
Scanning electron micrograph of the anterior end of Taenia hydatigena, magnified 170x.
In some species, each proglottid may contain numerous ovaries and as many as 1,000 distinct
testes. The tapeworms might best be thought of as franchisers of eggs and sperm. And like the
best franchising operations, tapeworms daily turn out a large amount of product, qualitatively
similar from proglottid to proglottid.
Each proglottid contains perhaps 50,000 eggs. The eggs are generally fertilized by sperm from a
neighboring cestode, but they can be fertilized by sperm from the same individual—or, in fact,
from the same proglottid.
Generally, the fertilized eggs cannot take up residence in the definitive (final) host immediately;
they must first enter an intermediate host or, in some species, a series of intermediate hosts.
Different cestode species require different intermediate hosts, which include both vertebrates and
invertebrates.
When a fertilized cestode egg is ingested by the appropriate intermediate host, an oncosphere
larva commonly hatches out. Each oncosphere has muscles, flame cells, and most significantly,
3 pairs of hooks with which it attaches to the wall of the host’s digestive tract. The oncosphere
then lyses (dissolves) its way through the intestinal wall, taking up residence as an encysted form
in the coelomic space or in specific organs and tissues of the host.
Among the vertebrates, fishes, cows, pigs, dogs, and sometimes birds may serve as intermediate
hosts. Humans often serve as acceptable final and intermediate hosts. (Think twice before eating
undercooked beef, pork, or fish—and before letting a dog lick your face!) Most invertebrate
intermediate hosts are arthropods.
Class Monogena
Monogenetic flatworms are usually parasitic on the skin or gills of fishes; that is, most are
ectoparasites. This is the flatworm’s sole host, to which it attaches primarily by means of suckers,
hooks, and complex sclerites located at its posterior end. The highly specialized posterior
attachment organ is called the haptor.
An anterior adhesive organ (often called the prohaptor), consisting of suckers and adhesive
glands, aids attachment. There are no intermediate hosts, so the life cycle generally involves the
following stages:
Most monogenean species show a very high level of host specificity, and typically occupy highly
specific sites within a host; members of one monogenean species live only at the base of a fish’s
gill filaments, for example, while members of another species are found only near the tips of the
same filaments.
Only about 8,000 species have been described so far, but about 25,000 species are thought to
exist, which would make this the most speciose group in the Platyhelminthes.
Figure 2.34 Monogean diversity. (a) Choricotyle louisianensis, a monogenetic trematode, taken
from the gills of a fish. (b) Polystomoidella oblongum, taken from the urinary bladder of a turtle.
(c) Oncomiracidium larva of the monogenean Entobdella soleae seen in ventral view. (Larval
length is about 240 μm.)
Class Trematoda
All members of the Trematoda (trem-ah-tōʹ-dah) are parasitic, and most reach adulthood only as
parasites in or on vertebrate hosts.
The outer body layer of adult trematodes, like that of the cestodes and monogeneans, is an
unciliated syncytial tegument. In other respects, the trematode body more closely resembles that
of a turbellarian. Trematodes have a mouth opening and a blind-ended digestive tract that is, with
a few exceptions, bilobed. The body is never segmented. The parasite ingests the host’s tissues
and blood through its mouth.
Trematodes parasitize a wide range of hosts, and most species have complex life cycles with
alternating sexual and asexual stages. Many trematodes require an intermediate host in which
larvae develop before infecting the final host (usually a vertebrate), where the adult worms live.
For example, various trematodes that parasitize humans spend part of their lives in snail hosts.
Schistosomiasis, an often deadly disease prominent in many regions of the world, results from an
infection by trematodes known as “blood flukes” (Schistosoma). Presently, more than 200 million
persons in 77 countries are estimated to suffer from schistosomiasis, making it the second most
prevalent disease in the world.
Schistosoma mansoni, one of the species causing schistosomiasis in humans. Adults are only
about one cm long. Members of this genus are atypical trematodes in that the adults are
gonochoristic, with the adult female nestling within a specialized groove in the body of the larger
male. Fertilized eggs (perhaps 300 per day per female) leave the human host with fecal material,
and miracidia hatch if the feces contact freshwater. The miracidia must locate and bore into the
tissues of the single intermediate host, a snail in the genus Biomphalaria. Each miracidium then
develops into a single sporocyst, which then produces many daughter sporocysts. Many cercaria
larvae emerge from each daughter sporocyst and exit the snail. If the free-swimming cercaria
contacts a human, it bores through the skin and migrates into the circulatory system, within which
it travels through the heart, to the lungs, and thence to the kidneys, where it feeds and grows.
The parasite eventually reaches maturity within the blood vessels of the host intestine, where it
may live for years. Pathology results mainly from inflammation caused by eggs becoming
entrapped in host tissues.
Figure 2.34 Life Cycle of Blood Fluke (Schistosoma mansoni)
The flukes are placed into one major group with over 6,000 species, the digenetic trematodes
(also called “digeneans”), and one much smaller group with fewer than 100 species, the
aspidogastreans (also called aspidobothreans).
A blood fluke, for instance, must evade the immune systems of both snails and humans. By
mimicking the surface proteins of its hosts, the blood fluke creates a partial immunological
camouflage for itself. It also releases molecules that manipulate the hosts’ immune systems into
tolerating the parasite’s existence. These defenses are so effective that individual blood flukes
can survive in humans for more than 40 years.
Look at the figure below. There are two intermediate hosts (a snail, usually in the genus Bithynia
and a fish, of the family Cyprinidae). This fluke can reach sexual maturity only in humans, who
thus serve as the definitive hosts. Encapsulated miracidia pass out of the definitive host’s body
along with feces. The miracidium only hatches following ingestion by the snail. Each miracidium
becomes a single sporocyst, which produces (asexually) many rediae, which, in turn, each
produce (asexually) many cercariae. These leave the snail and swim freely in the surrounding
water. Upon encountering an appropriate fish host, the cercariae bore in and encyst within the
fish muscle. The life cycle reaches completion only when humans eat raw or undercooked fish.
Adulthood is reached within the human bile duct
Figure 2.34 Fluke life cycles. The species are all digenetic trematodes. (a) Chinese human liver
fluke Opisthorchis sinensis.
The sheep liver fluke Fasciola hepatica. This parasite has a snail as the sole intermediate host.
Infected sheep. deposit fluke eggs along with feces. Free-swimming miracidia hatch from the
eggs if the eggs reach freshwater. The miracidia bore into an appropriate snail host; sporocyst,
redia, and cercaria stages follow within the snail. The cercariae leave the snail and encyst to form
a metacercarial resting stage on emergent vegetation. Adulthood is reached if the encysted stage
is ingested by grazing sheep.
Figure 2.35 Fasciola hepatica Life Cycle
Learning Activities
Activity 1: Select one group from the three invertebrates discussed. Provide at least 10
pictures of organisms that belong to that group. Name them properly.
Assessment
Multiple Choice
Directions: Read the following statements, and encircle the letter of the correct answer.
1. These group are the simplest among animals that lives permanently attached to the bottom or
some other hard surface.
a. sponges
c. flat worms
b. cnidarians
d. insects
2. A type of sponge’s body form with the highest degree of folding is
a. asconoid
c. arachnoid
b. syconoid
d. leuconoid
.
3. Flagellated cells that trap food particle and capture incoming sperm for fertilization are
a. amoebocytes
c. choanocytes
b. sclerocytes
d. osculum
4. Cnidarian’s body plan that has an opening at one end that are surrounded by tentacles.
a. medusa
c. asconoid
b. polyp
d. leuconoid
5. Sea anemones, corals, and sea whips are
a. medusozoans
c. hydrozoan
b. cubozoan
d, anthozoan
Matching Type. Match Column A with Column B and write your answer on the space provided.
Column A
Column B
1. Planula
a. finger-like extensions used to capture and handle food
2. Nematocysts
b. endoparasites
3. Tentacles
c. oganisms that possess eight tentacles
4. Acrorhagi
d. highly specialized posterior attachment organ
5. Octocorallians
e. blood flukes
6. Haptor
f. hydrozoan larvae
7. Cestodes
g. transparent siliceous or calcareous supporting structures
8. Schistosoma sp.
h. pores of a sponge
9. Spicule
i. specialized cnidae that contain stinging threads
10. Ostia
j. rings of small, spherical bulges of an anemone