Encyclopedia of Evolution.pdf - Online Reading Center

food particles with whiplike structures. In the evolution of
animals, the following significant divergences occurred (see
figure). All of these divergences, and probably all animal
phyla, had evolved by the end of the Cambrian period.
Sponges and placozoans vs. true animals. Sponges (Phylum
Porifera) diverged from the Eumetazoa (true animals).
The sponges are among the animals most like colonial protists.
They have no true tissues. They resemble a colony of
single cells, each of which is mostly on its own to obtain
food. Each of the choanocyte cells has flagella that propel
the water and grab food particles from it. Choanocyte cells
closely resemble the cells of colonial choanoflagellates. The
sponge cells usually rest on a framework of spicules, which is
the tough matrix that remains when the cells die. One example
of such a matrix is the familiar kitchen sponge. Sponges
have some whole-organism characteristics that identify them
as animals rather than clusters of protists. The entire sponge
grows from one reproductive cell. Also, the choanocytes whip
their flagella in unison, creating an orderly flow of water in
through pores and out through a chimneylike structure at the
top. Finally, the sponge produces amoebocyte cells that patrol
the inner passages and eliminate invaders. The structural
complexity of the sponge is a good example of emergence,
as it results from the simple interactions of its component
cells. The placozoans (Phylum Placozoa) are even smaller and
simpler clusters of flagellated cells.
invertebrates, evolution of
The major invertebrate groups can be distinguished on the basis of tissues, symmetry, and development. Many groups have been omitted from this
diagram. Sponges have no true tissues; eumetazoans (“true animals”) have tissues. Cnidarians have radial symmetry; the others have bilateral symmetry.
The protostomes that have ecdysis (molting) evolved separately from those that do not. All of the vertebrates are within the chordate phylum.
Radial vs. bilateral animals. The true animals diverged
into the radially symmetrical cnidarians and ctenophores and
the bilaterally symmetrical animals. In bilateral symmetry,
there is a top and bottom, left and right, front and rear; in
radial symmetry, there is at most only top and bottom.
• Radially symmetrical animals. The cnidarians (Phylum
Cnidaria; formerly called coelenterates) include jellyfish,
sea anemones, and corals. They have two tissue layers, an
outer and an inner, with a noncellular material between
them (the “jelly” of jellyfish). The sac formed by the inner
layer serves as a digestive cavity. They have a network of
nerves but no brains. They capture food by means of tentacles
with stinging cells. They have tissues but no complex
organs. Cnidarians alternate between a form that attaches
to surfaces (the polyp) and the swimming sexual jellyfish
stage (the medusa). What cnidarians lack in individual complexity
they may make up for by forming complex colonial
clusters. Most corals, and some jellyfish, obtain much of
their food from single-celled algae that live inside their tissues
(see coevolution). The ctenophores, or comb-jellies,
share a common ancestor with cnidarians but, aside from
a superficial similarity to jellyfish, are different in structure.
• Bilaterally symmetrical animals. All bilateral animals share
a set of Hox genes that specify the front-to-back order of
body parts (see developmental evolution) and are
described below.