5 Steps to a 5 AP Biology, 2014-2015 Edition
Taxonomy and Classification ❮ 163
- Platyhelminthes.These are flatworms,members of the acoelomateclub. They have
bilateral symmetryand a touch of cephalization.There are three main types of flat-
worms you should be familiar with:
- Flukes:parasitic flatworms that alternate between sexual and asexual reproduction
life cycles.
- Planarians:free-living carnivores that live in water.
- Tapeworms:parasitic flatworms whose adult form lives in vertebrates, including us
(humans).
- Rotifera.These are also members of the pseudocoelomateclub; they have specialized
organs,a full digestive tract, and are very tiny.
- Nematoda.These are roundworms,found in moist environments. They have a psuedo-
coelomatebody plan. Trichinosis, a disease found in humans, is caused by a roundworm
that infects meat products, usually pork. Humans ingesting infected meat can become
affected with this disease.
- Mollusca.These creatures are members of the protostomedivision and include such
species as snails, slugs, octopuses, and squids. They are coelomateswith a full digestive
system.Bivalves,such as clams and oysters, are mollusks that have hinged shells that
are divided into two parts.
- Annelida.These are segmented worms such as earthworms and leeches.
8. Arthropoda.This is the most heavily represented group on the planet. These creatures
aresegmented, contain a hard exoskeletonconstructed out of chitin,and have special-
ized appendages.Some well-known members include spiders, crustaceans, and insects.
One interesting tidbit about arthropods is that, like humans, some members of this
group, when born, are miniature versions of their adult selves that grow in size to
resemble adults. Others look completely different from adults and exist in a larva form
in their youth. At some point, the larvae undergo a metamorphosis and change to the
expected adult form.
- Echinodermata.These are sea stars.These coelomates are of the dueterostome body
plan. One neat characteristic of echinoderms is the presence of a water vascular
system,which is a series of tubes and canals within the organism, that plays a role in
ingestion of food, movement of the organism, and gas exchange.
- Chordata.This group includes invertebrates(animals lacking backbones), and verte-
brates(animals with backbones). Just in case you are asked to identify some vertebrates
on a multiple-choice question, here are some members—fish, amphibians, reptiles,
birds, and mammals. There are four features common to chordates you should know:
- Dorsal hollow nerve cord:forms the nervous system and becomes the brain and
spinal cord in some.
- Notochord:long support rod that is replaced by bone in most (mesodermal in
origin).
- Pharyngeal gill slits:slit-containing structure, which functions in respiration and
feeding, present only in the embryonic stage of most chordates.
- Tail:extension past the anus that is lost by birth in many species.
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