162 ❯ STEP 4. Review the Knowledge You Need to Score High
The first major branchpoint occurred after the development of multicellularity from
choanoflagellates. Off this branch of the tree emerged two divisions:- Parazoa:sponges; these organisms have no true tissues.
- Eumetazoa:all the other animals with true tissue.
After this split into parazoa and eumetazoa, the second major branchpoint in animal evo-
lutionary history occurred: the subdivision of eumetazoa into two further branches on the
basis of body symmetry. The eumetezoans were subdivided into - Radiata:those that have radial symmetry, which means that they have a single orienta-
tion. This can be a top, a bottom, or a front and back. This branch includes jellyfish,
corals, and hydras. - Bilateria:those that have bilateral symmetry, which means that they have a top and a
bottom (dorsal/ventral) as well as a head and a tail (anterior/posterior).
The next major split in the phylogenetic tree for animal development involved the split of
bilateral organisms into two further branches—one of which subdivides into two smaller
branches: - Acoelomates:animals with no blood vascular system and lacking a cavity between the
gut and outer body wall. An example of a member of this group is the flatworm. - Animalswitha vascular system and a body cavity.
- Pseudocoelomates:animals that have a fluid-filled body cavity that is notenclosed by
mesoderm. Roundworms are a member of this branch. - Coelomates:acoelomis a fluid-filled body cavity found between the body wall and
gut that has a lining. It comes from the mesoderm.
The final major branchpoint comes off from the coelomates. It branches into two more
divisions:
- Pseudocoelomates:animals that have a fluid-filled body cavity that is notenclosed by
- Protostomes:a bilateral animal whose first embryonic indentation eventually develops into
amouth.Prominent members of this society include annelids, arthropods,andmullusks. - Deuterostomes:a branch that includes chordatesandechinoderms.The first indentation
for their embryos eventually develops into the anus.
These two divisions differ in their embryonic developmental stages. As already mentioned,
the protostomes’ first embryonic indent develops into the mouth, whereas for the
dueterostome, it becomes the anus. Another difference is the angleof the cleavages that
occur during the early cleavage division of the embryo. A third difference is the tissue from
which the coelom divides.
That concludes the evolutionary development portion of this chapter. Now let’s take a
quick look at a few members of the various branches we mentioned above.- Porifera(sponges). These are simple creatures, which, for the most part, are able to
perform both male and female sexual functions. They have no “true tissue,” which
means that they do nothave organs, and their cells do not seem to be specialized in
function. - Cnidaria.These organisms are of radial symmetryand include jellyfish and coral ani-
mals, and they lackamesoderm.A cnidarian’s body is a digestive sac that can be one
of two types: a polyp or a medusa. A polyp(asexual) is cylinder shapedand lives
attachedto some surface (sea anemones). A medusa(sexual) is flatand roams the
waters looking for food (jellyfish). Cnidarians use tentacles to capture and eat prey.
- Porifera(sponges). These are simple creatures, which, for the most part, are able to