Biology 40S
Animal Diversity – Part 1
The First Six Phyla – Answer Key
Questions to Secure your Understanding (because security is a good thing):
Phylum Porifera:
1) What is one feature of animals (sponges) in this phylum that does NOT appear in the
other phyla we have studied so far?
Several answers: asymmetrical, lack of nervous system, lack of true tissues and
organs.
2) In basic terms, how does a sponge obtain and digest its food?
It is a filter feeder. It obtains food that is swept through by collar cells. Nutrients are
absorbed by microvilli, and digestion is completed by amoeboid cells.
3) Sponges usually possess an osculum – what is the purpose of this structure?
It provides a path for water to leave the sponge – useful in waste removal.
4) Though they possess no circulatory or respiratory system, sponges are capable of
moving dissolved gases through their structure so that all cells obtain a steady
supply. What accomplishes this?
By using amoeboid cells to move dissolved gases.
5) Sponges are sessile. What does this mean? What other phyla include sessile
organisms?
It means they do not move (at maturity). Phylum Cnideria and Mollusca also include
such organisms (maybe Annelida – Christmas Tree Worms).
6) Compare and contrast sponge sexual and asexual reproduction.
Asexual reproduction involves NO gametes and includes regeneration, budding, and
spore formation. Sexual reproduction involves the release of sperm by one sponge,
which will impregnate another. Both lead to new sponges.
7) Describe, briefly, how sponges are further classified.
They are further classified by the type of spicule that makes up their skeleton. They
can have a chalk skeleton, a glass (silica) skeleton, or a skeleton made of protein
with no spicules.
Phylum Cnideria:
1) In terms of the layout of a cnidarian (body plan, symmetry, etc.), how do they differ
from animals in phylum porifera?
They are radially symmetrical (unlike the asymmetrical sponges), and they have a
sac plan, where there is a single digestive opening (a “manus”). They are also made
up of tissues, not just cells.
2) The gastrovascular cavity (as the name suggests), serves two major functions in
cniderians. What are those functions?
It provides an area for digestion to begin (gastro-) as well as a space for gas
exchange to occur (-vascular).
3) Which organism – sponges or jellyfish – would be expected to have a more welldeveloped nervous system?
Jellyfish, as Cniderians possess nerves, while sponges (Porifera) do NOT.
Biology 40S
Animal Diversity – Part 1
4) Cniderians typically cycle between two stages in their life cycle. What are these
stages, and what is the major difference between them?
They are the medusa stage and the polyp stage. The key difference between them is
that the polyp stage is sessile while the medusa stage is motile. Often, sexual
reproduction occurs during the medusa stage.
5) Sexual reproduction in cniderians usually occurs in which life cycle stage?
The medusa stage.
6) What are the representative cniderians we covered? How do they vary?
Jellyfish, corals, anemones, and Portuguese man-of-war. They differ in how long
they spend in the polyp or medusa stages, as well as whether or not they are solitary
or colonial. Portuguese man-of-war is unique in that it is made up of both medusa
and polyps in one mature organism.
7) Cniderians produce special cells called cnidocytes. What purpose do they serve, and
how, basically, do they work?
They provide a means of defense. When they are “tripped” (when something
touches them), a capsule within called a nematocyst shoots out a rope-like structure
to grasp prey, and/or barbs to sting and poison prey.
Phylum Platyhelminthes:
1) Platyhelminthes worms are acoelomate. What does this mean?
It means they do NOT possess a body cavity that surrounds their organs. The only
space inside of them would be the space within their digestive tract.
2) What do flatworms (Platyhelminthes) use to obtain their food from the
environment?
An extendable pharynx (tapeworms would use a scolex).
3) How is digestion in flatworms similar to digestion in cniderians?
It makes use of a gastrovascular cavity, and also involves a single opening
(“manus”).
4) Unlike the previous two phyla, flatworms possess an excretory system. Which
structures function to remove waste from flatworms?
Flame cells help to remove waste from flatworms. The wastes are moved out of
excretory pores.
5) How do flatworms exchange gases with the environment? Which other phyla use
this same method?
They allow gases to diffuse across their body wall. This is similar to the nematodes,
the annelids, and some molluscs (like snails, which allow gas to pass through the
mantle).
6) Of porifera, cnideria, and Platyhelminthes, which possess the most complex nervous
system? Explain.
Platyhelminthes. They posses ganglia (which cnideria did not have), as well as sense
organs like eyespots, in some cases.
7) Platyhelminthes are hermaphrodites. What does this mean? What other phyla
include hermaphroditic animals?
It means they produce both male and female gametes and reproductive structures.
Nematodes, molluscs, annelids (and even Cniderians) also include hermaphrodites.
Biology 40S
Animal Diversity – Part 1
8) When two flatworms engage in sexual reproduction, which worm becomes
impregnated?
Both.
9) How do tapeworms and flukes differ from planarians?
They are parasitic. They also lack eyespots.
10) What is a scolex? What organisms use them?
A structure made up of hooks and suckers used by tapeworms to adhere to and
attack hosts.
11) Why might it be easier to become infected by a fluke than a tapeworm?
Tapeworms have to be ingested (and can be noticeable because of that). Fluke
larvae can burrow through the skin (at follicles) to infect a host – that’s a lot simple
and easier!
Phylum Nematoda:
1) In terms of bodily structure, name two significant differences between flatworms
(Platyhelminthes) and roundworms (nematoda).
Nematodes have a pseudocoelom (fluid-filled cavity) surrounding their organs. They
have a true digestive tract (tube-within-a-tube plan).
2) Nematoda worms have one particular feature that is not seen in any of the other
phyla studied. What feature is that? What purpose(s) does that feature serve?
The pseudocoleom. It serves as a way to give the worm a hydrostatic skeleton – a
fluid skeleton that helps it with movement and maintains its shape/structure.
3) Compare the respiration of nematodes with the other two phyla of worms
(Platyhelminthes and annelida). Are there any differences?
No, not really. They all generally exchange gases across their body wall.
4) How does excretion in these worms differ from the organisms in the previous phyla?
They have a true anus (not a “manus”) to excrete solid waste.
5) How does movement in nematode worms differ from the flatworms?
Flatworms use cilia to move across a layer of mucus. Nematodes use longitudinal
muscle in their structure to move in whip-like motions.
6) Compare and contrast ascaris worms with trichinella worms. How are they similar
in their life cycle, and how do they differ?
Both mature in the intestines. Ascaris larvae will travel to the lungs and return to
the intestines by being coughed up and swallowed. Trichinella worms will “hide” in
muscle fibers before returning to the intestines.
7) How do filarial worms cause elephantiasis?
They block lymph vessels, causing a build-up of fluid in surrounding tissues.
Phylum Mollusca:
1) Name and briefly define the three major parts that comprise ALL molluscs.
Visceral mass – soft portion of the organism that houses the internal organs.
Foot – muscular region that is usually used for movement.
Mantle – a region surrounding and protecting the visceral mass. Secretes a shell in
some molluscs.
Biology 40S
Animal Diversity – Part 1
2) In terms of body layout (symmetry, plan, etc.), how are molluscs similar to the
nematodes? How are they different?
They have bilateral symmetry like nematodes, and also have a tube-within-a-tube
body plan. However, while nematodes have a pseudocoelom, molluscs have a TRUE
coelom (making them coelomates).
3) How does the feeding method of gastropods and cephalopods compare to that of the
bivalves?
Gastropods and cephalopods will usually use a radula (raspy, tongue-like structure)
to ingest food from the environment as they come across it (or hunt it, in some
cases). Bivalves are filter-feeders, “catching” food particles on their gills as water
flows through them.
4) All molluscs possess a separate anus and kidney-like nephridia. However,
cephalopods and bivalves aid their excretion with another structure. What is it?
They make use of a siphon to aid in excretion.
5) What other purposes does the structure from the previous questions serve for
cephalopods and bivalves?
For cephalopods, a siphon is also used for movement, spreading gametes (sperm or
egg), and, in some cases, for ejecting ink as a defense. Bivalves use a second siphon
to draw water into their bodies to aid in acquiring food.
6) Of the molluscs, which class is considered to have the MOST developed nervous
system?
Cephalopods (most developed brain in the phylum)
7) The foot is used for propulsion in all classes of molluscs except one. Which class
does NOT use it for propulsion, and what, instead, does it develop into?
Cephalopods again! Instead of propulsion (which is handled by the siphon), the foot
develops into arms and tentacles, which are often used to capture prey.
8) Respiration in molluscs is varied. What structure will marine molluscs use?
Terrestrial molluscs?
Marine molluscs generally use gills for respiration. Terrestrial molluscs often lack
gills and lungs, using gas exchange across the mantle instead. Other terrestrial
molluscs may possess lungs.
9) Are all molluscs motile? If not, which aren’t?
Some are, some aren’t. All classes (gastropods, cephalopods, and bivalves) have
motile members, but oysters are usually not motile.
10) Some molluscs contain an open circulatory system. Where is blood pumped in this
case?
It is pumped into spaces within the internal organs, rather than through close blood
vessels.
11) Only one group of molluscs is hermaphroditic, containing both male and female
reproductive structures. Which animals are these? Do they fertilize each other
internally or externally?
Gastropods (specifically snails). Fertilization is internal for these particular animals
(sperm is injected from one snail into another, where eggs will be fertilized).
Biology 40S
Animal Diversity – Part 1
Phylum Annelida:
1) Compared to the molluscs, what is the only new feature that annelids show in terms
of their body layout? Is this similar to or different from the other worm phyla?
Segmentation. This is also different from the other worms (nematodes,
Platyhelminthes) previously discussed.
2) Aside from an anus, what excretory structures do annelids possess?
Nephridia (aka “excretory tubes”).
3) How is circulation carried out in annelids? How does this differ from flatworms?
Circulation uses a heart and major blood vessels to carry oxygen and nutrients
throughout the body. This is different from flatworms, which use a gastrovascular
cavity to spread gases throughout the body.
4) In terms of movement, what structures do annelids possess in addition to the
muscle that nematodes did?
They also possess setae – fine hair-like structures that aid in movement. Some also
have parapodia, as well.
5) What are the three classes of annelids (provide the scientific name and the common
name)?
Class Polychaeta – marine worms
Class Oligochaeta – earthworms
Class Hirudinea - leeches
6) What external features distinguish polychaeta worms from the other 2 classes?
What functions do they serve?
Plenty of setae (which help worms to adhere to surfaces), as well as parapodia
(“feet”-like structures that helps the worm not only to move, but to pick up oxygen
from the water; parapodia are rich in blood vessels).
7) The three classes of annelids all obtain their food differently. Contrast the feeding
habits of the three classes.
Polychaeta can be predatory, hunting for their food. Some are filter feeders as well.
Oligochaeta tends to obtain nutrients from dead or decaying matter. Hirudinea are
ectoparasites that attack host organisms from the outside to “steal” nutrients.
8) How are oligochaeta worms such a perfect example of segmentation?
Parts of every major body system are repeated throughout the structure of the
worm:
 Each segment contains a digestive structure (usually intestine, crop, or
gizzard)
 Each segment contains branches of the circulatory system
 Each segment contains nerves and ganglia
 Each segment contains nephridia for excretion
 Each segment is physically divided, inside and outside of the worm, by a
septum.
9) What distinguishes leeches from all of the other parasitic worms we’ve talked about
so far?
They are ectoparasites. Other parasitic worms in the other phyla usually attack a
host from the inside (often from the intestines), but leeches attack from the outside.