CHAPTER VII 



ADDITIONAL ANIMAL STUDIES 



A. Porifera (sponges) 



130. Sponges. — The sponges are animals more complex in 

 structure than the Protozoa, for they are composed of many cells ; 

 nevertheless, they are comparatively simple in structure since they 

 have no digestive, circulatory, respiratory, or nervous system, and 

 therefore each cell has to carry on practically all the necessary 

 nutritive functions. 



Sponges differ largely in the kind of skeletons that they possess. 

 In the common bath sponge (Fig. 123) this is composed of a tough, 

 horny material. When , ,__ 



sponges are ready for market, 

 only the horny skeleton re- 

 mains, the living cells hav- 

 ing been killed and removed. 

 The sponge skeleton shows 

 a large number of pores in 

 the outer surface, and for 

 this reason the name Porifera 

 (Latin = pore-bearing) is 

 given to this group of ani- 

 mals. The pores lead into 

 canals that run through the body, finally connecting with one or 

 more larger central cavities that lead outward, usually at the top. 

 In certain parts of these canals there are cells with cilia; their 

 action causes water to rush into the canals through the pores, 

 bringing food and oxygen to aU the cells of which the sponge is 



175 



Fig. 123. — Bath sponge. 



