PHYLUM PORIFERA AMIMALS WITHOUT TISSUES Members of the kingdom
Animalia, which are all multicellular eukaryotes, are thought to
have evolved from single-celled eukaryotes (protistans). Among
the simplest of all animals are the members of the phylum
Porifera (sponges). Sponges are little more than loose
aggregations of cells with no true tissue organization. There is
some division of labor amongst the cells, but there are no
organs. The basic body form of all sponges consists of a sac-
like structure with an outer layer of epidermal cells, an inner
layer of flagellated choanocytes, and a middle mesohyl layer
containing amoeboid cells that produce skeletal structures of
various sorts. These layers are perforated by a large number of
small pores, which give the group the name Porifera (= pore
bearer). The cavity of the sac is called the spongocoel and it
has at least one opening to the outside, the osculum. Figure 1.
Cutaway diagram of a simple sponge. After Margulis & Schwartz,
1998. The phylum Porifera is commo