ELISHA MITCHELL SCIENTIFIC SOCIETY. 99 



outline. The bundle of spicules, formerly at the non- 

 ciliated pole, become distributed all over the body of the 

 little sponge. The attached larva, at first circular in out- 

 line, speedily grows irregular in shape, and becomes sur- 

 rounded by a thin ectodermal membrane. The subder- 

 mal cavities and canals appear as lacunae in the substance 

 of the larva, the surrounding cells becoming flattened to 

 form the epithelioid wall. The various canals and cavi- 

 ties, originally independent, open one into the other, and 

 to the exterior, by simple perforation of the intermediate 

 tissue. The oscula and pores are at first indistinguisha- 

 ble, and are scattered over the surface of the sponge, 

 with no attempt at arrangement. Even in the adult I can 

 see no morphological distinction between the pores and 

 oscula. The difference in size is the only difference, 

 and that loses its significance because of the occurrence of 

 apertures, which hold several intermediate positions in 

 this respect between pores and oscula. 



The flagellated chambers arise as independent struct- 

 ures, which subsequently acquire connection with the 

 canal system. 



There are in the larva, when it first attaches, a large 

 number of mesoderm cells, distinguishable from the rest 

 by their size and bulky shape. Such cells I may call 

 formative cells. They diminish greatly in number, and 

 grow smaller in size during the metamorphosis. The 

 formative cells contain as a rule several nuclei, and are 

 destined for the most part to split up into much smaller 

 cells. The particular w^ay in which the flagellated cham- 

 bers are formed in any larva depends on the behavior of 

 the formative cells. (All the cells of the larva, I may add, 

 are connected together by fine processes). In some larvae 

 the formative cells arrange themselves round a central cav- 

 ity (intercellular space) so as to form a hollow sphere. 

 Numbers of such spheres, consisting of comparatively large 



