112 



THE SPONGES— PHYLUM PORIFERA 



Therefore, the sperm is much smaller than the egg, but, nevertheless, 

 carries just as much weight in determining heredity because it carries 

 just as much of the chromatin as is found in the egg. In the sponge the 

 sperms migrate to the egg and one of them will unite with it to form a 

 zygote. This begins to divide and forms a small larva with cilia that is 

 able to swim about in search of a good place to settle down and grow 



• Osculum 



p*m, »n. f**^ 



-Incurrent pore 



Collar cells 



Food particle 



Flagellum 



Fig. 8.3. Scypha, internal structure. A. Diagrammatic longitudinal section show- 

 ing the vase-like structure of the body. B. Detail of small portion of the body wall 

 showing course of the water through the pores from the outside to the cloacal cavity. 

 C. Collar cells. The spicules are not shown in any of the drawings. 



into an adult. By this method the sponges achieve wide distribution so 

 that the offspring will not congregate too close to their parents and be 

 in competition with them for the available food supply. Because both 

 sperms and eggs may be produced in the body of one sponge, we call the 

 sponges hermaphroditic animals as contrasted to the condition of higher 

 forms where the two types of gametes are produced in separate indi- 

 viduals which are male and female animals. 



