522 



COMPARATIVE ANATOMY 



CHAP. 



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

 pore ; it, however, does not become the hydrocad, but the middle vesicle (which is 

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

 probable that this is produced by constriction from the left anterior hydro- en teroccel 

 vesicle. 



On the left side, the development of the hydro-enterocad is now as foUoAvs. 

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

 which, again, commnnicates, by means of a constricted portion, with the hydroccel 



ant 



2JoU 



Fig. 419.— Dorsal aspect of an Echinoid 

 Pluteus, to illustrate the relations of the liydro- 

 enteroccel (after Bury), "nt, Anterior; post,i)os- 

 terior ; sin, left ; dex, right ; 1, larval cesophagus ; 

 2, left anterior enterocoel ; 3, ectodermal in- 

 vagination ; 4, i-udiment of the hydroccel ; 5, left 

 posterior enterocoel vesicle ; 6, stomach intes- 

 tine ; 7, right posterior enterocoel ; 8, hydro- 

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

 anterior enteroccel. The arms are not fully 

 represented. 



^'^^ 



Fig. 420.— Rudiment of tlie Echinoid 

 In the Pluteus larva of Echinocyamus 

 pusillus (after Th^el). The Pluteus is seen 

 from the dorsal side, and only the left side 

 is completely drawn. 1, Arms of the 

 Pluteus ; 2, aperture of invagination of tlie 

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

 wall of the Echinoid ; 4, outgrowths of the 

 hydroccel, which push this wall before them 

 and form the first anibulacral tentacles ; 

 skeletal rods of the Pluteus ; 6, hydroccel ; 

 7, hydropore ; 9, portion of a skeletal piece 

 lying in the neighbourhood of the hydro- 

 pore, which will probably become the 

 niadreporite ; S, stomach-intestine. 



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

 enteroccel. 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 description of the differentiation of the hydro- enteroccel must not be 

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

 do not all agi-ee. ) 



