158 



PHYLUM ECHINODERMATA. 



The mouth is formed as a funnel-shaped depression of the 

 vestibule, which passes through the hydrocoel ring and opens 

 into the enteron (Fig. 115, 8). On the shifting of the vestibule 



to the hind end, the oral plates 

 which, like the basals, have become 

 arranged in a circle, also shift 

 backwards. They come to lie in 

 the thin roof of the vestibule, and 

 when the latter ruptures and splits 

 into five lobes, each lobe contains 

 one oral plate. The stage we 

 have now reached is sometimes 

 called the cystid stage (Fig. 116, 6). 

 It is characterized by having a 

 mouth overhung by five oral 

 plates, an absence of arms, and an 

 anus which has been formed as a 

 lateral perforation through the 

 body wall outside the circle of the 

 orals. There are twenty-five ten- 

 tacles, which at first arose in five 

 groups, but now all spring sepa- 

 rately from the water - vascular 

 ring. 



As to internal changes, we may 

 mention that the mesenteries be- 

 tween the right and left posterior 

 coeloms, which, by shifting, have 

 some time before become aboral 

 and oral, break down so that the 

 perivisceral cavity is a continuous 

 cavity. Further, the anterior 

 coelom loses its walls and becomes 

 merged in the general body-cavity. 

 The result of this is that the 

 primary water-pore and the sand- 

 organ f jf fibrous strands in the stalk. G3iTldti ] ) w hich at the previous stage 



both opened into the anterior coelom, now open directly into 

 the' body-cavity. The primary water-pore and the anus lies in 

 the same interradius. Later each of the other interradii 



FIG. 115. Attached larva of Antedon 



The calcareous plates are not shown. 

 d dorsal, v ventral side. 1 right pos- 

 terior (aboral) coelom ; 2 stomach ; 

 3 left posterior ^oral) coelom ; 4 sac- 

 culi ; 5 vestibule ; 

 tacles, 15 in number derived 



