SYl'ATA VIVIPARA. 63 



first series gives rise to the five primary tentacles, and the second series corresponds to 

 those which in X. diyitata u'ivc rise to the radial water-canals. In S. diyittifn however, 

 there is a sixth outgrowth at the extreme left, the rudiment of the Polian vesicle, but in 

 S. viuipara this vesicle is not formed until after the closure of the hydrocoel ring. The 

 water-canal enters this ring at a point just between the fourth primary tentacle (counting 

 from before backwards) and the fourth secondary outgrowth (Fig. 23). In this particu- 

 lar my observations confirm Bury ('89) in opposition to the statements of Semon ('88). 

 Before the outgrowths of the hydrocoel are very evident, that part of it which lies 

 dorsally and in immediate connection with the water-canal bulges out, becomes thinner- 

 walled than the rest of the hydrocoel, and gradually separates from it, but before 

 the separation becomes marked, the outgrowth diminishes in size and with the 

 increasing growth of the water-canal disappears. This structure, I believe, is the 

 "anterior coelom " which Bury ('95) has shown to exist in the auricularia of 

 S. diyitata. It is very marked in some specimens of S. vivipara (Fig. 24), and I 

 see no reason to doubt Bury's interpretation. He does not make very clear what the 

 ultimate fate of this coelom is in X. diyitata, but leaves the impression that it is connected 

 with the subsequent formation of the madrepore plate, as Ludwig ('91) considers it to be 

 in Cucumaria. In S. vivipara however, there is seldom any trace of it left after the 

 hydrocoel ring closes, and there is no reason to suppose that it has any connection with 

 the much later madrepore openings. About the time of the appearance of the primary 

 tentacles, the larval anus, which was the original blastopore, closes entirely and the 

 digestive tract ends blindly. Accordingly we now have a regular elliptical larva, about 

 a third of a millimeter long, with the ventral ectoderm much thicker than the dorsal, 

 without ciliated bands, calcareous particles, or nervous system, a mouth on the anterior 

 ventral surface but no anus, a well-developed coelomic pouch on each side of the digestive 

 tract, and a hydrocoel with five primary tentacles and five secondary outgrowths, opening 

 to the exterior through the dorsal pore, by means of an adradial water-canal, upon 

 which may still be seen the vestige of an anterior coelom. 



6. THE DEVELOPMENT or THE PENTACTULA. 



In such larvae, the ectoderm of the ventral surface continues to thicken and before 

 long is sharply set off from the ectoderm of the rest of the body, which consists of a 

 single layer of cells. The thickened ectoderm forms a circular field around the mouth 

 though the latter does not lie at its center, but nearer to its anterior edge. This circular 

 disc gradually sinks below the level of the rest of the ectoderm, and the latter grows in 



