100 



ELEMENTS OF BIOLOGY 



contains supporting rods or spicules composed in some genera o£ 

 silicon, in others o£ a calcium salt, and in others o£ a peculiar tough 

 substance called spongin. Some sponges are composite individuals 

 consisting of a considerable number of subordinate individuals 

 with the central cavities continuous. The common bath sponge is a 

 composite individual. In its market form it is merely the spongin 



PORIFERA 



•'"^Maj^wT 



SPICULES 



INCUHKENT CANAL 



CENTRAL CAi'/ry 



OSCULUM iEXCURHENT) 



ECTODERM 



ENOODERM 



FIG. 32. A SIMPLE SPONGE (WALL CUTAWAr) 



network of such an individual from which the tissues of the ani- 

 mal have been removed. 



Phylu7n Coelenterata 

 Members of this phylum are also composed essentially of two 

 layers of cells enclosing a cavity, and are vase-shaped. The endoderm 

 lines the sac-shaped cavity which serves as a digestive organ. The 

 cavity has but a single opening, the mouth; the walls are not 

 perforated as in the Porifera. They, like the sponges, are radially 

 symmetrical and occur as single and as composite individuals. All 

 members of the phylum are equipped with a peculiar type of sting- 



