Review Sheet Ch. 33 Invertebrates
1) Invertebrates include all animals except for Subphylum Vertebrata. Vertebrates are of special
interest because they are large and dominate the terrestrial environment. Invertebrates are a “catch all”
grouping for all animals that are not vertebrates. It is not a monophyletic group. Monophyletic means
that all species in a group share a common ancestor not shared by species outside the group.
2) Animal phyla are distinguished primarily by difference in body plan. Variations in body plan include
differences in cephalization, segmentation, coelom, and body symmetry. Most phyla have unique
cells and or structures not found in any other phyla (e.g. the mantle of molluscs). Some defining
characteristics may only be present at certain times during an individual’s lifespan (e.g. Echinoderms
have bilateral symmetry, but only as larvae).
3) The animal phyla that exist today vary in complexity. Some resemble their ancestors more than others.
Sponges (Phylum Porifora) probably look similar to their ancestors of 250 million years ago. In
contrast, dogs look very different from their ancestors of 250 million years ago (a synapsid reptile).
However, each phylum has been evolving along their own pathway for about the same amount of time,
and each has adapted to its own body plan and way of life in their own way.
4) Phylum Porifora (sponges) are more similar to their colonial choanoflagellate ancestors than any
other animal phylum.
Body plan
Sac perforated with holes; no body symmetry
Tissues
specialized cells but none of the tissues or organs that are found in other animal phyla.
Locomotion
Sessile as adult
Reproduction
Hermaphroditic; sperm fertilizes egg within a sponge, larva swims to a new location
and settles on substratum to develop into an adult
Gas exchange
Diffusion through body wall
Nutrition
Filter feeder: flagellated choanocytes create a water current that draws water
through sac-like body, strains out particle for food
5) Phylum Cnidaria (jellies, hydras, corals, sea anemone):
Body Plan
Sac with a central digestive cavity. Radial symmetry. Mouth ringed with tentacles.
Has oral and aboral ends. Polyp attaches to substratum at the aboral end, mouth and
tentacles point upwards. Medusa is free swimming and oral surface faces downward.
Tissues
Diploblastic (ectoderm and endoderm). Ectoderm produces not just epidermis and
nerve net, but also contractile fibers that function like muscles (in other phyla, muscles
form from mesoderm)
Locomotion
Polyp normally sessile; Medusa drifts or propels itself by contracting and expelling
water from its bell-like body
Gas exchange
Diffusion through body wall
Nutrition
Incomplete digestive tract; Predator; use cnidocytes (“nettle” cells) and nematocysts
(stinging cells) in tentacles to capture prey
6) Phylum Platyhelminthes (flatworms) are acoelomate and have a flat body. They are triploblastic and
have endoderm, ectoderm, and mesoderm. Because of their bodies are small and flat, they can
exchange gases with specialized organs. They have an incomplete digestive tract consisting of a
gastrovascular cavity. The gastrovascular cavity has a network of vessels projecting from it that
distribut nutrients to the tissues of the body.
Platyhelminthes classes
Turbellaria (planarians)
Free-living, mostly marine but some freshwater
Monogenea (flukes)
Parasites of fish
Trematoda (flukes)
Parasites, usually have intermediate hosts
Cestoda (tapeworms)
Parasites, made of proglottids
7) Turbellarians are free-living (non-parasitic) and usually marine. A few species, such as the group
called planarians, live in freshwater. Turbellarians have some cephalization, including eyespots that
can detect light and dark.
8) Flukes are found in two different classes. Both kinds are internal parasites. Trematodes usually have
two hosts.
9) Tapeworms are internal parasites that attach to the inside of their host’s digestive tract by the scolex at
their head. A tapeworm is made up of independent segments called proglottids. Proglottids are
mostly full of reproductive organs. The proglottids at the posterior end of the tapeworm are released in
the host’s feces. In some species that infect humans, the feces contaminates the food of livestock.
Tapeworm larva encyst in livestock muscles. When the muscles are eaten as meat a larva infects a new
host, and develops into a new adult tapeworm. Tapeworms lack a digestive tract and simply absorb
nutrients through their skin.
10) Phylum Rotifera are multicellular, and have organs and systems, but are also very small. The largest
are 0.2 mm and the smallest are microscopic. The have a complete digestive tract with mouth and
anus. They have pseudocoelom that is filled with fluid. This fluid filled cavity provides them with
support and is called a hydrostatic skeleton. The cavity also circulates nutrients. A rotifer has a
wheel organ. This organ is a ring of cilia that create a vortex that sucks water into the rotifer’s mouth.
11) Rotifers are usually parthenogenic. This means that females produce diploid eggs that develop
without needing to be fertilized by sperm. Some rotifers reproduce sexually during stressful conditions
(for example, if the pond dries up). In these cases, degenerate males are produced which at maturity
are incapable of feeding themselves and only survive long enough to fertilize a female’s eggs. The
zygotes produced are resistant to adverse conditions and begin developing when conditions improve.
12) Phylum Mollusca have 1) a muscular foot for movement; 2) a visceral mass that contains vital
organs; 3) a mantle, which is a fold of protective tissue that usually secretes a shell; 4) often also have
a radula, a hard, rasping tooth or beak-like appendage. Most are marine and have gills for gas
exchange. Most have significant cephalization.
Characteristics
Molluscan class
Oval-shaped, moves along substratum scraping of algae
Chitons
Terrestrial (no gills, use mantle as a lung); torsion (during development, visceral
Gastropods
(snails and slugs) mass twists 180o so that body has a more upright, less worm-like shape); most use
radula to eat plants, but some are carnivorous
No radula; no head; shell divided in two halves with the hinge on the dorsal surface;
Bivalves (clams
have an adductor muscle that opens and closes shells; foot may be used for digging
and oysters)
or anchoring; gills are used for gas exchange and for collecting food particles; some
are sessile, some are just sedentary
Fast predators with highly developed brains and nervous systems; radula is a beak for
Cephalopods
tearing prey; shell is reduced or absent; squids open their mantle cavities suck in
(squids, octopi,
water, then expel water out their siphon for jet propulsion
nautili)
13) Phylum Annelida is coelomate. The coelom is divided into segments separated by septa. Like
rotifers, annelids are supported by a hydrostatic skeleton from the fluid-filled segments.
Characteristics
Annelid class
Terrestrial; eat and extract nutrients from soil
Oligochaeta
(earthworms)
Polychaeta (polychaetes) Marine; have pairs of paddle-like appendages on each segment called
parapodia (used for movement and gas exchange); some burrow into the
seafloor, some live in tubes
Usually freshwater; some prey on invertebrates, some drink blood
Leaches
14) Phylum Nematoda are roundworms. They are unsegmented, usually dioecious, and have a complete
digestive tract. Their muscles are all longitudinal, so when they move they flail about. Like
arthropods, they secrete a cuticle, which is periodically shed are new cuticle secreted. Some are freeliving and others are parasitic (at least 50 human parasites, cause of trichinosis).
15) Phylum Arthropoda have 1) segmentation; 2) a secreted exoskeleton that is periodically shed and
replaced; 3) jointed appendages; 4) high degree of cephalization (good sight, olfaction, touch).
Characteristics
Arthropod class
Arachnida (spiders, scorpions,
Mostly terrestrial; Two segments (cephalothorax and abdomen);
ticks, mites), also horseshoe crabs
chelicerae (fang-like mouth appendages)
Merostomata (horseshoe crabs)
chelicerae, aquatic
Diplopoda (millipedes)
Millipedes Many segments; 2 pairs of legs per segment; eat dead
plant matter
Chilopoda (centipedes)
Centipedes Many segments; 1 pair of legs per segment; carnivorous;
venomous fanged appendages
Insects (flies, beetles, crickets,
Mostly terrestrial, rarely marine; have head, thorax, and abdomen
butterflies, etc.)
segments; have 3 pairs of legs (emerge from thorax); usually have
two pairs of wings (insects are the only flying invertebrates); have
tracheal system for gas exchange which consists of tubes that
perforate the insect’s body and allow ventilation of body tissues
Mostly marine (except pill bug); larger species have gills; many
Classes of Subphylun Crustacea
(crabs, lobsters, shrimp, copepods,
appendages with extensive specialization; smaller species like
barnacles, pill bugs or rolly-polies) copepod make up bulk of plankton
16) Along with vertebrates, insects are one of the dominant animal groups on land. There are more known
species of insects than all other known species combined. Insects are of great ecological significance: 1)
very important for pollination in flowering plants, which are the dominant autotroph on land; 2) important
link in most food webs (as predator and prey); 3) important to the life cycle of many parasites and
pathogens (as a vector of transmission).
16) Plankton are marine organisms that cannot swim faster than the current (e.g. copepods, unicellular
algae). Nekton are larger animals that can swim against the current (minnows, sharks, turtles, etc.).
17) Phylum Echinodermata are one of the two major deuterostome phyla. The other is Phylum
Chordata. Include sea stars, sea cucumbers, sea lilies, sand dollars, etc. All are slow-moving or
sessile. Adults have adapted to radial symmetry. Covered by a thin skin, with a spiny endoskeleton
underneath.
18) Echinoderms have a unique water vascular system that services a system of tube feet. A tube foot is
structurally like an eye dropper, with a muscular bulb and a tube extending out with a suction cup at
the end. When the bulb contracts, water is pumped into the tube extending the tube foot. When the
bulb relaxes the tube foot retracts. The tube feet are used for 1) movement; 2) feeding (can pull open a
bivalve shell); 3) gas exchange (feet function as gills).