107 



sort of arch, with a hollow in the middle, surrounding which a 

 large number of very fine brown pigment corpuscles are collected. 

 The next change of importance is the disappearance of the cayity, 

 the upper or ciliated half of the body being much reduced in size. 

 Then the large round cells of the hinder part are united into a com- 

 pact mass, leaving only a single row. The ciliated cells are gradu- 

 ally withdrawn into the body cavity. Fig. 50, D, shows this process 

 going on. At this period also the larva becomes sessile, and now 

 begins the formation of the sponge spicules, which develop from 

 the non-ciliated round cells. Metschnikoff calls attention to the 

 fact that at this early stage the Sycon passes through a phase 

 which is persistent in the genus Sycyssa. The layer of ciliated 

 cells are gradually withdrawn into the body cavity, until a small 

 opening is left, surrounded with a circle of cilia. These cilia 

 finally disappear, and a few more spicules grow out, and meanwhile 

 the opening disappears. In the next stage (represented at D) a 

 considerable (gastrovascular) cavity appears, which may be seen 

 through the body-walls. At this time, by soaking the specimen 

 in acetic acid, the body of the sponge was seen to consist of 



enveloped in the spicule-generating layer (representing the ento- 

 derm). At this time no mouth-opening was formed, though three- 

 pointed spicules had appeared. 



It results from Metschnikoff s observations that the body of the 

 larval sponge is composed of two primary germ-layers, an "ento- 

 derm " and " ectoderm," the two germ layers about which we shall 



The observations of Carter, made on several additional species 

 both of sHicious and calcareous sponges, confirm the results of 

 Metschnikoff as to the later history of the larval sponge, and those 

 of Hagckel as to the mode of segmentation of the egg. Our Fig. 

 48, A (copied from Carter), shows the total segmentation of the 

 yolk in Halisarca lobularis into two portions ; these portions farther 



embryonic cells are produced. 



Carter observes that the embryos may be found at all seasons, 

 from March through the summer. These observations are not dif- 

 ficult to follow out. We have, by tearing apart a species of Sy- 



found the planula much as figured by Hseckel, Metschnikoff and 



