DEVELOPMENT OF SPONGES. 



45 



pears which may be seen through the body- walls. At this 

 time the germ consists of two layers, the inner layer of cili- 

 ated cells (endoderm) forming a closed sac, enveloped in the 

 spiculiferous layer. Such are the observations of Metschni- 

 koff on the development of Sycon. According to the ob- 

 servations of Barrois, the larva or gastrula fixes itself by what 

 are destined to be the ectodermal cells, and which are the 

 round non-ciliated cells forming the posterior end (Fig. 31, 

 C) of the free-swimming gastrula. About this time the 

 mesoderm separates from the endoderm, either before or 

 just after the gastrula becomes stationary, according to the 

 group to which it belongs. 



When the young sponge becomes stationary it does not 

 differ from the gastrula, except that it becomes more or less 



Fig. 31. Development of a sponge (Sycon ciliatum). After Metsclmikoff. 



irregular in form. Then appear the food or digestive cavi- 

 ties in the endoderm, in Sycandra becoming radiating tubes 

 lined with ciliated, collared, monad-like cells ; or in Leucon 

 and Halichondria, and their allies, forming scattered pock- 

 ets, called " ampuUaceous sacs." Inmost sponges (except 

 some calcareous species) there is no general body-cavity in 

 the gastrula, nor in the young after the larva becomes sta- 

 tionary, according to Barrois. After the formation of the 

 ampullaceous sacs the pores open through the mesoderm 

 and connect the sacs and ciliated channels, as the case may 



