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-en teroc< ! 

 vesicle. 



On the left side, the development of the hydro-en terocrel 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 hydroco-1 



afti 



pott 



FIG. 419. Dorsal aspect of an Echinoid 

 Pluteus, to illustrate the relations of the hydro- 

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

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

 2, left anterior enterocoel ; 3, ectodernial in- 

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

 posterior enteroccel vesicle ; (j, 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. 



FK;. 4-20. Rudiment of the Echinoid 

 in the Pluteus larva of Echinocyamus 

 pusillus (after The'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 imagination of the 

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

 wall of the Kchinoid ; 4, outgrowths of the 

 hydrocoel, which push this wall before them 

 and form the first ambulacral tentacles ; 

 skeletal rods of the Pluteus ; 6, hydrocoel ; 

 ", hydro pore ; 9, portion of a skeletal pit-re 

 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 

 enterocoel. 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 hytiro-enteroccel must not be 

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

 do not all agree.) 



