522 



COMPARATIVE ANATOMY 



CHAP. 



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

 pore ; it, however, does not become the hydrocoel, 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-enterocu-1 

 vesicle. . 



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

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

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



ler 



post 



FIG. 419. Dorsal aspect of an EcMnoid 

 Pluteus, to illustrate the relations of the hydro- 

 enterocoel (after Bury), ant, Anterior ; post, pos- 

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

 2, left anterior enterocoel ; 3, ectodermal in- 

 vagination ; 4, rudiment of the hydrocoel ; 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 the Echinoid 

 in the Pluteus larva of Echinocyamus 

 pusillus (after Theel). The Pluteus is seen 

 from the dorsal side, and only the left side 

 is completely drawn. 1, Arms of the 

 Pluteus ; 2, aperture of imagination 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 first 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 the 

 madreporite ; 8, stomach-intestine. 



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

 enteroca-1. 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 hydroccel 

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

 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 r >t all agree.) 



