Acoelomates: Phyla Platyhelminthes & Nemertea
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Flatworms in the Phylum Platyhelminthes and ribbon worms in the Phylum Nemertea or
Rhynchocoela are bilateral acoelomates. Thus, like the cnidarians, they lack a coelom.
However, these worms are the simplest animals that are triploblastic (endoderm,
ectoderm & mesoderm) and to have primary bilateral symmetry.
Biological Contributions of Acoelomates
1. basic bilateral body plan
2. mesoderm develops to form an embryonic tissue from which many tissues, organs and
systems can form
3. generally agreed to be (although debatable), the simplest animals to have an organ system
level of organization
4. simplest animals to show cephalization with centralization of a simple “ladder-like”
bilateral nervous system
4. simplest animals to have an excretory system
5. nemerteans are the simplest animals to have a closed circulatory system with blood
6. technically, the rhynchocoel cavity in ribbon worms (nemerteans) is a true coelom,
but since it is only a part of the proboscis it most likely not homologous to the true
coelom seen in other animals
The worms (Vermes, latin) were once classified by zoologists as a somewhat arbitrary and
artificial grouping of many diverse animal forms. This group is now more correctly separated
into different phyla dependent on their individual characteristics.
Phylum Platyhelminthes: Flatworms
There is debate about the validity of this phylum as many believe flatworms evolved from a
planular larva-like ancestor. Others believe this group evolved from a larger coelomate
ancestor whose internal structure changed over evolutionary time.
Average Sizes
¾ millimeters to meters (some internal parasites easily get this long)
¾ most are 1 to 3 cm long
Life Span
¾ days to months (maybe years for some?)
Symmetry & Body Plan
¾ bilateral symmetry with dorsoventrally flattened vermiform (worm-like) shape
¾ no body cavity (acoelomates); spaces between organs filled with parenchyma cells
¾ no metamerism (segmentation)
¾ cephalized
Development & Coelom Formation
¾ true tissues
¾ acoelomate
¾ miracidia and cercaria stages are infective in parasitic forms
¾ triploblastic body plan with endoderm, mesoderm, and ectoderm
Type of Skeleton
¾ none
Appendage Types
¾ none
Acoelomates: Phyla Platyhelminthes & Nemertea
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Basic
¾
¾
¾
¾
Form & Function
no internal body space other than digestive tube (acoelomate)
spaces between organs filled with parenchyma
members of the Class Turbellaria are mostly free-living
Classes Monogenea, Trematoda and Cestoda are entirely parasitic
Specialized Cell Types/Organs
¾ parenchyma (seen in most complex animals) = tissue derived from mesoderm that fills
the space (“packs”) between the endoderm and ectoderm. There are more cells and
fibers found here that in the mesoglea of cnidarians. In some platyhelminthes, noncontractile cell bodies of muscle cells can be found in the parenchyma.
¾ flame cells = comprise protonephridia for excretion (not found in all flatworms). These
cup shaped cells have a tuft of flagella extending from the inner face of the cup. The
cup is elongated into finger-like projections that extend between other projections of a
tubule cell. The flagella beat (like a flickering flame) to push fluid into collecting ducts.
This draws fluid (due to negative pressure) through the projections of the tubule cell.
Sometimes there are microvilli on the wall of the flame cell that may function in the
reabsorption of specific ions and molecules. A small amount of ammonia is excreted by
the protonephridia, but most metabolic wastes are lost by diffusion across the body wall
of the animal. Ammonia is a toxic by-product of the metabolic removal of nitrogen from
proteins and nucleic acids. Most aquatic animals get rid of ammonia by excreting it in
very dilute solutions. Most terrestrial animals convert the ammonia to urea or uric acid,
which conserves water because these less toxic wastes can be transported in the body
in more concentrated form.
¾ rhabdites = rod-like structures in epidermal cells that lie under the epidermis and
produce protective mucous secretions (only seen in members of the Class Turbellaria)
“Organ Systems”
Integumentary System (skin)
¾ syncitial (multinucleate) or cellular epidermis
¾ syncitial tegument (outer covering) seen in Classes Monogenea, Trematoda and
Cestoda
¾ thickened, non-living cuticle that is sometimes molted
¾ cilia present in some (eg. planarian flatworms)
Muscular System
¾ primarily a sheath of muscles cells derived from mesoderm
¾ these flat muscles fibers may run in circular, longitudinal or diagonal directions
Circulatory System
¾ none – nutrient/waste removal by diffusion/osmosis/cell transport
Respiratory System (gas exchange)
¾ none – gas exchange by simple diffusion = cutaneous respiration
Nervous System
¾ a pair of anterior ganglia with ventral longitudinal nerve cords
¾ the longitudinal nerve cords are connected by transverse nerves to form a “ladder-like”
nervous system
Acoelomates: Phyla Platyhelminthes & Nemertea
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Sensory Organs
¾ ocelli or eyespots with light sensitive photoreceptors typical in turbellarians,
monogeneans and larval trematodes; not usually found in adult internal parasites
¾ tactile mechanoreceptors cells abundant on body
¾ chemoreceptor cells abundant on body
¾ planarian flatworms form earlike lobes on the head termed auricles
¾ some species have statocysts for equilibrium (righting/turning over/balance)
Digestive System (food/water supply)
¾ incomplete digestive system with single opening (mouth/anus); wastes can be
regurgitated
¾ cestodes lack a digestive system; other platyhelminthes have a mouth, pharynx
and intestines
¾ there are muscles in the mid-body ventral pharynx of planarians
¾ intestines formed of a series of three branched trunks (one anterior and two
posterior) that form a gastrovascular cavity lined with columnar epithelium
¾ the mouth of trematodes and monogeneans usually opens at or near anterior end of
body
¾ planarians are primarily carnivorous feeding on small crustaceans, nematodes, rotifers
and insects
¾ parasitic forms feed on host animal tissues and fluids and cellular debris
¾ cestodes lack their own digestive system and thus are dependent on their host for
digestion; the digestive enzymes of the host break down nutrients into small molecules
that are then absorbed by the cestode
Immune System
¾ none (?)
Excretory System (waste removal)
¾ system of protonephridia formed of flame cells
¾ the protonephridia of flatworms excrete water and conserve salts = mostly for
osmoregulation and less for metabolic waste removal
¾ freshwater flatworms, such as Planaria, excrete water by means of a network of tubules
that run throughout their bodies.
¾ each tubule ends in a flame cell, which is a tubule with cilia beating inside.
¾ the flame cell and tubule together form a protonephridium
¾ tissue fluid enters tubules, and beating cilia cause this fluid to flow toward the
excretory pore of the worm. The fluid is modified as it flows through the tubules.
¾ the fluid that leaves is less concentrated, so ions are conserved and water and a little
ammonia is excreted.
Reproduction
¾ most forms are monoecious (separate sexes)
¾ relatively complex reproductive systems
¾ well developed gonads, ducts and accessory organs
¾ internal fertilization
¾ direct development in free-swimming flatworms and forms with a single host; direct
development refers to a life cycle that proceeds from zygote to adult without
intervening life stages
¾ complicated life cycle often involving multiple hosts in internal parasites
¾ review life cycles presented in text and in lecture
Acoelomates: Phyla Platyhelminthes & Nemertea
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Mechanism/Mode of Locomotion
¾ in planarians, movement of cilia in epidermis
¾ contraction of longitudinal and circular muscles against parenchyma
¾ free-swimming cercaria larvae, miracidia (ciliated larvae) and attachment with
oral suckers and hooks (scolex) important in parasitic life cycles
Specialized Defenses/Toxin/Poisons
¾ tissue or organ damage by parasitic forms, but not necessarily due to a toxin
Ecology & Adaptive Radiation
¾ marine, freshwater, terrestrial soil or internal parasites
¾ many different animals species from various animal groups (eg. mammals, fish and
snails) can be parasitized by trematodes, cestodes, and monogeneans
Social Organization
¾ none
Communication
¾ none (?)
Thermoregulation
¾ none
Common Trematode Parasites = flukes (see table 14.1 in text)
¾ Blood flukes (Schistosoma spp.)
¾ Liver flukes (Clonorchis sinesis)
¾ Lung flukes (Paragonimus spp.)
¾ Intestinal flukes (Fasciolopsis buski)
Common Cestode Parasites = tapeworms (see table 14.2 in text)
¾ Beef tapeworm (Taenia saginata)
¾ Pork tapeworm (Taenia solium)
¾ Fish tapeworm (Diphyllobothrium latum)
¾ Dog tapeworm (Dipylidium caninum)
¾ Unilocular hydatid (Echinococcus granulosus)
Acoelomates: Phyla Platyhelminthes & Nemertea
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Phylum Nemertea (Rhynchocoela): Ribbon worms
Nemertea = “unerring one” – refers to aim of proboscis (Greek)
Rhynchocoela = “hollow beak” (Greek)
Average Sizes
¾ centimeters to meters
¾ usually less than 20 cm long
Life Span
¾ days to months (maybe years for some?)
Symmetry & Body Plan
¾ eversible proboscis = long muscular tube that is quickly thrust out to grab prey
¾ bilateral symmetry with ribbon-like body shape; contractile body that is
cylindrical at the anterior end and more dorsoventrally flattened at posterior end
¾ no body cavity (acoelomates); spaces between organs filled with parenchyma cells
¾ nemerteans are the simplest animals to have a closed circulatory system with blood
¾ technically, the rhynchocoel cavity in ribbon worms (nemerteans) is a true coelom,
but since it is only a part of the proboscis it most likely not homologous to the true
coelom seen in other animals
¾ no metamerism (segmentation)
¾ cephalized
Development & Coelom Formation
¾ true tissues
¾ acoelomate
¾ triploblastic body plan with endoderm, mesoderm, and ectoderm
Type of Skeleton
¾ none
Appendage Types
¾ none
Basic
¾
¾
¾
Form & Function
no internal body space other than digestive tube (acoelomate)
spaces between organs filled with parenchyma that is partially gelatinous
mostly free living with few parasitic forms
Specialized Cell Types/Organs
¾ eversible proboscis = unique to nemerteans; long muscular tube that is quickly
thrust out to grab prey; in some the proboscis can stretch considerable (meters);
the proboscis lies in the rhynchocoel cavity above the digestive tract
¾ parenchyma (seen in most complex animals) = partially gelatinous tissue derived from
mesoderm that fills the space (“packs”) between the endoderm and ectoderm.
¾ flame cells = comprise protonephridia for excretion. In nemerteans, protonephridia are
found closely associated with the circulatory system and they appear to be truly for
excretion of metabolic wastes versus the primarily smoregulatory function of
protonephridia in Platyhelminthes.
“Organ Systems”
Integumentary System (skin)
¾ epidermis with cilia and gland cells
Acoelomates: Phyla Platyhelminthes & Nemertea
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Muscular System
¾ body wall muscles comprised of outer circular and inner longitudinal layers with
diagonal fibers in between these two layers
Circulatory System
¾ blood-vascular system = closed circulatory system with two or three longitudinal
trunks
¾ blood flow is caused by contractile walls of vessels and movements of the body
Respiratory System (gas exchange)
¾ none – gas exchange by simple diffusion = cutaneous respiration
Nervous System
¾ a pair of anterior ganglia (“four lobed brain”) with one or more pairs of ventral
longitudinal nerve cords; middorsal/midventral cords seen in some species
¾ the longitudinal nerve cords are connected by transverse nerves
Sensory Organs
¾ sensory ciliated pits or head slits
¾ tactile mechanoreceptor sensory cells
¾ ocelli in some
Digestive System (food/water supply)
¾ complete digestive system with tube that runs from mouth to anus
¾ mouth is anterior and ventral with digestive tract extending entire length of body
¾ evolution of an anus is a significant advancement as regurgitation of wastes is no
longer required and thus ingestion and defecation can occur simultaneously
¾ the proboscis lies in the rhynchocoel above the digestive tract, but not attached to it
¾ the proboscis is a long, blind ended muscular tube that open at the anterior end to a
pore superior to the mouth
¾ muscular pressure on the fluid in the rhynchocoel causes rapid eversion of the
slimy proboscis that wraps around prey and may stab it with a stylet if present (not
found in all nemerteans)
¾ toxins may be released by the proboscis as the prey is drawn toward the mouth and
swallowed whole
¾ cilia move food products through intestines; there are no muscles in digestive tube
wall
¾ digestion is primarily extracellular in gut
¾ most are active predators on annelids and other small invertebrates; a few are
scavengers
Immune System
¾ none (?)
Excretory System (waste removal)
¾ system of two or more coiled canals of protonephridia formed of flame cells
¾ In nemerteans, protonephridia are found closely associated with the circulatory system
and they appear to be truly for excretion of metabolic wastes versus the primarily
osmoregulatory function of protonephridia in Platyhelminthes.
Acoelomates: Phyla Platyhelminthes & Nemertea
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Reproduction
¾ most forms are monoecious with great variation in reproductive strategy
¾ simple gonads
¾ asexual reproduction by fragmentation
¾ a few hermaphrodites
¾ fertilization is often external, but some show internal fertilization
¾ a few have oviparous reproduction; oviparous = eggs released from female with
development occurring outside the female body
Mechanism/Mode of Locomotion
¾ movement of cilia in epidermis
¾ contraction of longitudinal and circular muscles against parenchyma
¾ much of locomotion is dependent on movement over a track of mucous slime
Specialized Defenses/Toxin/Poisons
¾ toxins (eg. tetrodotoxin) produced by proboscis
Ecology & Adaptive Radiation
¾ nearly all marine with some in freshwater and soil; some live in gelatinous tubes
¾ about 900 species but not as abundant with less radiation than platyhelminthes
¾ most are active predators on annelids and other small invertebrates; a few are
scavengers
¾ some may only find food when the run into it while others may be able to track prey
over relatively long distances
¾ a few are specialized egg predators on brachyuran crab embyros
Social Organization
¾ none
Communication
¾ none (?)
Thermoregulation
¾ none
Phylum Nemertea (Rhynchocoela)
Class Enopla = proboscis is armed with stylets and mouth opens anterior to brain
Class Anopla = proboscis lacks stylets and mouth opens posterior or inferior to brain