44 



ZOOLOGY. 



the formation of a two and afterward three-layered sac, cor- 

 resj^onding to the gastrula of the higher animals. In this 

 state (Fig. 30) the germ breaks out of the parent sponge into 

 the sea. Fig. 31 represents the development of the common 

 little calcareous sporge [Sycon, ciliatum), found between 

 tide-marks. A indicates the morula with the segmentation- 

 cavity (c), which afterward 

 disappears as at B. The 

 gastrula is represented at 

 C, and consists of ciliat- 

 ed and non-ciliated large 

 round cells ; the first series 

 forming a sort of arch, with 

 a hollow in the middle, 

 around which a large number of very fine brown pigment 

 corpuscles are collec1;ed. The next change of importance is 

 the disappearance of the cavity, 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 compact 

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

 gradually withdrawn into the 

 body-cavity. Fig. 31, D, shows 

 this process going on. At this 

 period also the larva becomes ses- 

 sile, and now begins the formation 

 of the sponge-spicules, which de- 

 velop from the non-ciliated round 

 cells. Metschnikotf calls atten- 

 tion to the fact that at this early 

 stage the Sycon passes through a 

 phase which is persistent in the 

 genus Sycyssa. The layer of cil- 

 iated cells are gradually withdrawn into the body-cavity, 

 until a small opening is left surrounded with a circle of 

 cilia. These cilia finally disappear, a few more spicules 

 grow out, and meanwhile the opening disappears. In the 

 next stage (represented at D) a considerable body-cavity ap- 



convenience to avoid circumlocutioQ. It may be tliat these conditions 

 will lie found to be essentially modified in different groups of animals. 



Fig. 30.— Gastrula of a Bpongc (%- 

 candra raphamii).—AtteT Schulze. 



