ECHINODERMATA 



451 



Hitherto we have learned of the intestinal canal of the 

 Antedon larva only as a closed sac. The mouth and oeso- 

 phagus do not arise until the formation — on the floor of the 

 vestibule in the middle of the 

 water- vascular ring — of a depression, 

 which fuses with the intestine (Fig. 

 220 m). The intestine therefore 

 does not even yet open directly to 

 the outside world, but into the 

 vestibule. Its interior at this time 

 is not empty, but filled with cells 

 (Burt) or with a kind of nutritive 

 yolk (Barrois). The entodermal 

 mass elongates backwards (basal- 

 wards) to form the intestine, and 

 winds spirally about the axial part 

 of the body cavity. Its end then 

 moves in the transverse mesentery, 

 at about the height of the upper 

 margin of the basal plates, up to 

 the body- wall (Fig. 220), with 

 which it fuses, subsequently break- 

 ing through to the exterior. The 

 anus comes to lie in the vicinity of 

 the water-vascular pore. Subse- 

 quently it is shifted to its final po- 

 sition on the ventral wall of the cup. 

 As in the rest of the Echinoderms, 

 the anus seems to have no dii^ect 

 relation to the blastopore. 



Having considered the internal 

 developmental processes, we turn 

 again to the external shape of the 

 larva, which in the meanwhile has 

 essentially changed. These changes 

 are partly due to the metamorpho- 

 sis of the hydrocoele. Each of the 

 five primary tentacles, which we have already seen to be 

 an evagination of the water- vascular ring, splits into three 



Fig. 223.— Diagram of a 

 pentacrinoid larva of Antedon 

 rosacea (after Thomson, from 

 Balfour's Comimmtlvc Em- 

 hryology), cd, centrodorsal 

 plate ; or, oralia ; 4, radialia ; 

 3, basalia ; 1, terminal plate. 



