DEVELOPMENT OF COELOM 



609 



quickly assumes a crescentic form, and gives off five blunt out- 

 growths, which are the rudiments of the radial canals, and the 

 terminal tentacles. It remains in connexion with the anterior 

 coelom by a narrow neck, which later becomes the stone-canal. 

 That on the right side separates completely from the right 

 anterior coelom ; it remains small, and forms the madreporic 

 vesicle (Fig. 287, A 3 , r.hy) of the adult. In the Ophiopluteus 



l.hy V . 



Ipc ~- -~ >-pc Ipt^ H^ Lpc. 



Asteroid Ophiuroid Echinoid Holothuroid 



Fig. 287. Diagrams of the mode of formation and division of the coelom in Echino- 

 dermata. a.c, Anterior coelom ; coe, primitive coelomic rudiment ; int, intestine ; 

 l.a.c, left anterior coelom ; l.hy, left hydrocoel ; l.p.c, left posterior coelom : oes, 

 oesophagus ; p.c, posterior coelom ; r.a.c, right anterior coelom ; r.hy, right hydro- 

 coel ; r.p.c, right posterior coelom ; st, stomach ; stom, stomodaeum. 



and in the larva of Asterina gibhosa (v. infra) it occasionally 

 takes on a form similar to that of the hydrocoel ; from which 

 circumstance, as well as from the similarity in its mode of 

 origin, it is here regarded as a right hydrocoel, i.e. a rudi- 

 mentary fellow of the organ which develops into the water- 

 vascular system. 



In Auricularia (Fig. 287, D) the coelomic vesicle, after the 

 pore-canal is formed, divides into an anterior and a posterior 

 half. The posterior part then divides into right and left halves, 

 vol. 1 2 R 



