. Principles of modern biology. Biology. The Animal Kingdom - 629 THE PORIFERA (LITERALLY, PORE BEARERS) These primitive multicellular animals, which commonly are called the sponges (Fig. 32-5), constitute a relatively small (about. Fig. 32-5. Some sponges: Phylum Porifera. 5000 species) phylum. Almost all sponges live in the sea, many in shallow waters. How- ever, some live at depths extending down to four miles; and there is one widely distrib- uted fresh-water family. Most familiar sponges are drab in color; but some of the less familiar species are brightly colored in various hues of red,
. Principles of modern biology. Biology. The Animal Kingdom - 629 THE PORIFERA (LITERALLY, PORE BEARERS) These primitive multicellular animals, which commonly are called the sponges (Fig. 32-5), constitute a relatively small (about. Fig. 32-5. Some sponges: Phylum Porifera. 5000 species) phylum. Almost all sponges live in the sea, many in shallow waters. How- ever, some live at depths extending down to four miles; and there is one widely distrib- uted fresh-water family. Most familiar sponges are drab in color; but some of the less familiar species are brightly colored in various hues of red, orange-yellow, blue, and violet. All sponges display a relatively primitive structure. The body wall (Fig. 16-11), which surrounds the central cavity, consists of only two cell layers—an outer dermal layer and an inner gastral layer. These layers are sepa- rated bv a thick gelatinous matrix, which is strengthened bv numerous microscopic cal- careous or siliceous spicules, or by a network of delicate fibers, composed of a silklike pro- tein, spongin. Also a number of active amoe- boid cells, called amoebocytes, can be found wandering through the matrix. Some of the amoebocytes give rise to gametes, or to asex- ual buds, and thus they must be considered as potential germ cells. The gastral layer consists mainly of collar cells (Fig. 16-2). Individually these cells take small particles of organic food into gastric vacuoles, where all the digestive processes of the animal occur. The dermal layer, on the other hand, consists of a poorly differ- entiated epithelium. The sponges do not have any excretory or respiratory organs and there are no clearly recognizable nerve or muscle cells. Some of the amoebocytes, however, tend to aggregate around the pores, which lead into the gastral cavity. In fact, these amoebocytes possess a fair degree of contractility and collectively they act to regulate the size of the pore open- ings. In commercially sold natural sponges (Fig. 16-3), only the
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