Body Tissues:
Another basic way in which animals’ body plans differ is in how their tissues are organized. You no-doubt recall that a
tissue consists of body cells of the same type that together perform a common function. Sponges (Phylum Porifera) and a few other animals lack true tissues, but all other animals have body tissues that develop from the
embryonic tissues that are produced during
gastrulation.
As you recall, during early development, the blastula folds inward to produce a gastrula. In the process, two layers of
embryonic tissues are formed, the inner
endoderm and the outer
ectoderm. These tissues are known as the
germ layers, because all body tissues are ultimately derived from the embryonic tissues. (“Germ,” as in “germinate,” means to sprout or grow.) The ectoderm forms the outer covering of the animal, and in most animals, the central nervous system. The endoderm forms most of the tissues associated with the digestive system.
Animals whose bodies develop from only these two germ layers are said to be
diploblastic. These include the jellies, sea anemones, corals and other members of the Phylum Cnidaria. The other phylum of diploblastic animals is the Phylum
Ctenophora; these animals are commonly referred to as “comb jellies.”
The cnidarians and ctenophorans are collectively referred to as the
Radiata, since almost all of them have radially-symmetrical bodies.
A jelly (Phylum Cnidaria) is diploblastic – its body develops from only
two embryonic germ tissue layers. Note the lack of complex internal
organs. These develop from mesodermal tissue in other animals.
A “comb jelly” (Phylum Ctenophora). Like cnidarians,
ctenophorans are diploblastic and lack complex internal organs.
Body Tissues
Linear Mode
