ECHINODERMA TA—ONTOGEN Y 



52^ 



3. The left enterocoel vesicle (3 in tlie tigs.). Tliis vesicle, again, lias an out- 

 growth (7) on the left, which is — 



4. The hydrocoel vesicle. 



Simultaneously witii the formation of tlie hydrocoel rudiment, that ol' the water 

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

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

 into it. 



Sixth and seventh day. — Tiie 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 considerabl}' 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 enterocoel. 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 



ytotit 



Fig. 430. — The same 

 specimen of Asterina gib- 

 t)osa viewed from the 

 left and from the ventral 

 side. 1, Larval organ with 

 its dorsal and ventral 

 lobes : 2, larval nioutli. 



A 



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

 interior of the body, which soon closes to form a canah" 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 hydrocoel, wdrile 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 oi Aster ias 

 vulgaris also, the entero-hydroccel arises in tlie form of two lateral diverticula of 

 the blind and somewhat swollen archenteron, wdiose 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 pore. 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. 



