68 



ANIMAL BIOLOGY 



is relatively slight. However, the Sponge dominates its tissues. 

 Just so long as the component cells of the Sponge remain associated, 

 they are not to be regarded as individuals, but as specialized 

 centers of action and reaction of the Sponge's body, by means of 

 which physiological division of labor is made possible. 



At first glance, indeed, one would not recognize a Sponge as 

 an animal at all. Thus the common Leucosolenia of the New 

 England coast appears to be a group of tiny tubes, or sacs, per- 



Osculum 



Flagellum 

 Collar - 



Jdim 



B 



Fig. 35. - - A, small colony of Leucosolenia; B, a collar cell; C, Grantia. 



manently attached at the base to a rock just below low-tide 

 mark. It is without the power of locomotion and does not visibly 

 respond in any way when touched. However, if it is examined 

 under a lens, it will be found that the whole surface of each sac-like 

 body is dotted with innumerable pores, or ostia, through which 

 water is being drawn into the gastral cavity and passed on out 

 through a sieve-like membrane at the top, or osculum. The name 

 of the phylum, Porifera, refers to the pores. (Fig. 35.) 



A section of the body wall of Leucosolenia shows that it is 

 supported by a skeleton, composed of a network of three-pronged 



