METAZOA PORIFERA. 7 



single individual with one central trunk into a creature 

 with overgrown walls to the body, with a radiating or 

 branching cavity, and with the digestive function of the 

 central trunk transferred to expanded portions of the 

 branches or canals near the exterior. The whole process 

 evidently hinges on the rapid growth of the mesoderm,, 

 because when this is thin the food supply is close to the 

 central cavity ; when this is thickened the pores must 

 become tubes ; when it is still thicker, the tubes must 

 lengthen. The food supply of the body is thus carried 

 away from the central cavity, and it is but natural that 

 the cells in the canals nearer the pores should get more^ 

 grow more and gradually make it unnecessary or impossi- 

 ble for the cells farther inward to get food. These last 

 must then necessarily suffer reduction and lose first the use 

 and then the habit of growing out collars and flagella, and 

 sink into the form of epithelial membrane cells. 



The function of the short canal leading into the 

 ampulla from the exterior is obviously to bring food of 

 microscopic size, and that of the continuation of the canal 

 beyond the ampulla is to carry away the excrements of 

 the ampullaceous cells. These cells are voracious feeders 

 and throw out a large amount of waste matter which is 

 carried into the great central cavity by the excurrent 

 canals, and thence it is transported to be ejected at the 

 cloacal opening above. 



We shall presently see how in other orders of sponges 

 the law of specialization by reduction has destroyed all 

 tendency to grow into symmetrical shapes, so that the 

 Silicea and Keratosa well deserve the designation of 

 amorphous or formless, so often bestowed upon them. 



This irregularity in form together with greater com- 

 plexity of structure is found in Leuconia aspera (No. 66) 

 which represents the group of Leucones, the most spe- 

 cialized of the calcareous sponges. 



