ECHINODERMA TAONTOGEN Y 



527 



3. The left enterocoel vesicle (3 in the figs.). This vesicle, again, has an out- 

 growth (7) on the left, which is 



4. The hydroccel vesicle. 



Simultaneously with the formation of the hydroecel rudiment, that of the water 

 pore appears dorsally, somewhat to the left of the median line, as an invagination 

 of the ectoderm, which grows towards the left enterocoel, and breaks through 

 into it. 



Sixth and seventh day. The outer form of the larva has been considerably 

 modified on the fifth day. The larval organ has increased in size, and its sloping 

 circular ridge projects considerably beyond the surface of the larval body. 



The rudiment of the hydrocoel has grown out further backward, but is still 

 anteriorly in open communication with the left enteroccel. Five outgrowths (Nos. 

 I-V in Fig. 435) now appear at its posterior edge ; these are the rudiments of the 

 five radial vessels. The water pore (dorsal pore, madreporite) still leads into the 



post 



FIG. 430. The same 

 specimen of Asterina gib- 

 bosa viewed from the 

 left and from the ventral 

 Side. 1, Larval organ with 

 ir> dorsal and ventral 

 lobes ; 2, larval mouth. 



FIG. 431. Asterina gibbosa, larva at the 

 beginning of the eighth day, from the left 

 side; the larval organ is very largely de- 

 veloped (after Ludwig). 



left enteroccel. "A channel develops on that wall of the hydrocoel which faces the 

 interior of the body, which soon closes to form a canal. " This canal runs towards 

 the point where the dorsal pore opens into the left enteroccel. One end of this 

 canal remains in open communication with the hydroccel, while the other enters 

 the enterocoel quite near the aperture of the dorsal pore. This canal is the stone 

 canal of the future Asteroid. The dorsal pore of the larva does not thus lead 

 direct into the stone canal, but enters it through the left enteroccel (Fig. 436). 

 Only at a later stage does the dorsal pore come into direct connection with the stone 

 canal. 



Formation of the hydro-enterocoel in other Asteroids. In the larva of Asterias 

 fulf/aris also, the entero-hydroccel arises in the form of two lateral diverticula of 

 the blind and somewhat swollen archenteron, whose wall has become thinner. The 

 two diverticula soon become constricted from the archenteron, and become distinct 

 vesicles. Each sends off an outgrowth towards the dorsal surface, a growth of the 

 ectoderm running in towards it. The two meet and fuse, become hollow, and form 

 the stone canal with the water poro. Thus in the young Bipinnaria larva of 

 Asterias vulgaris, the bilateral symmetry is so marked that a right as well as a 

 left stone canal attains development (Fig. 432). The right pore, however, soon 

 disappears, and the right canal somewhat later. 



