522 



COMPARATIVE ANATOMY 



anterior (enterocoel) vesicle is tlie one whicli opens outward through the water 

 pore ; it, however, does not become the liydrocirl, liut the middle vesicle (which is 

 in comnuinieation with it) now represents the rudiment of the hydrocoel. It is 

 probable that this is ])roduced by consti'iction from the left anterior hydro-en teroco-l 

 vesicle. 



On the left side, the development of the hydro-enteroctel is now as follows. 

 The water pore leads into the posterior end of a left anterior enterocrel vesicle, 

 which, again, communicates, by means of a constricted jiortion, with the hydroccel 



Fig. 419.— Dorsal aspect of an Echinoid 



PlUteus, to illustrate the relations o( the hydr'o- 

 enterocoel (after Bury), unt, Anterior ; jmsf, pos- 

 terior ; sin, left ; rftj', right ; 1, larval oesophagus ; 

 2, left anterior enterocoel ; 3, ectodermal in- 

 vagination ; 4, rudiment of the liydrocffil ; 5, left 

 posterior enterocoel vesicle ; 0, stomach intes- 

 tine ; 7, right posterior enterocoel ; S, hj'dro- 

 pore ; 9, unpaired olorsal skeletal piece ; 10, right 

 anterior enterocoel. Tlie arms ari' not fully 

 represented. 



Fio. 4iO.— Rudiment of the Echinoid 

 in the Pluteus larva of Echinocyamus 

 pusillus (afttT Theel). Tlif I'luteus is seen 

 from the dorsal side, and only tlie left side 

 is completely drawn. 1, Arms of the 

 Pluteus ; 2, aperture of invagination of the 

 sac (3), whose floor will form the oral body 

 wall of the Echinoid ; 4, outgrowths of the 

 hydrocoel, which push this wall before them 

 and form the tirst ambulacral tentacles ; 

 skeletal rods of the Pluteus ; 6, hydrocoel ; 

 7, hydropore ; 9, portion of a skeletal piece 

 lying in the neighbourhood of the hydro- 

 pore, which will probably become tlie 

 madreporite ; s, stomach-intestine. 



vesicle. This latter is embraced posteriorly by the horse-shoe-shaiied left posterior 

 enteroeiil. The stone canal does not arise out of the water pore, but out of the 

 connecting piece between the left anterior enterocoel vesicle and the hydrocoel 

 vesicle, which becomes drawn out into a canal. The left anterior enterocoel 

 seems to become the madreporitic ampulla. 



(The above descrii)tion of the differentiation of the hydro-enteroctel must not be 

 considered as fully established. The observations are not quite continuous, and 

 do not all agree.) 



