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 spongin part of the matrix 

 remains after processing; and the calcareous 

 and siliceous (glass) sponges are never used 

 for cleaning purposes. 



The primitive status of the Porifera is in- 

 dicated by (1) the radial type of symmetry 

 (except in the few asymmetrical forms); (2) 

 the absence of well-defined tissues and or- 

 gans; (3) the absence of a general digestive 

 cavity (that is, digestion remains intracellular 

 within the gastric vacuoles of the individual 

 collar cells); and (4) the striking resemblance 

 of the collared gastral cells (choanocytes) to 

 the collared flagellates (choanoflagellates). 

 The dermal and gastral layers probably are 

 not homologous to the germ layers (ectoderm 

 and endoderm) of higher animals, since in- 

 vagination occurs from the animal pole of 

 the developing sponge blastula, rather than 

 from the vegetal pole, as in other multicellu- 

 lar animals. The free-swimming larva of the 

 sponge is called, in the blastula stage, an 

 amphiblastula, because only the cells in the 

 animal half are flagellated; and it is these 



