INVERTEBRArE MORPHOLOGY. 



of the Orinoids ia maiataining a circulation of the ccelomic 

 fluid. 



So far as is known, the portion of the ccelom which in the 

 embryo opens to the exterior by the water-pore and with which 

 the stone-canal communicates in the Asteroids and Echinoids 

 does not persist in the adult Holothurian, and consequently 

 there is no axial sinus, and it is doubtful if a structure com- 

 parable to the ovoid gland of other forms exists. Schizocoelic 

 sinuses corresponding to the perihsemal canals of the Echinoids 

 occur in their usual position between the nervous system and 

 the hydrocoel-canals, and consist of a ring accompanying the 

 nerve-ring and five radial canals which abut against the ring 

 at their oral ends but seem to be completely separated from 

 it by septa. A lacunar system is well developed, consisting 

 of a plexus in the walls of the intestine, the various branches 

 uniting to form a dorsal and a ventral intestinal vessel, which, 

 passing forwards, unite with a lacunar ring surrounding the 

 oesophagus at about the level of the hydrocoel-ring. From 

 this ring five radial lacunae extend backwards, lying in the 

 connective tissue between the radial perihsemal sinus and the 

 hydrocoel-canals, and giving branches to the tentacles and the 

 tube-feet. A lacuna also extends from the pericesophageal 

 lacunar ring to the reproductive organs arising from a thick- 

 ened portion of the ring, and this thickening has been re- 

 garded as the rudiment of the ovoid gland. 



The hydrocoel has the usual arrangement, consisting of a 

 ring (Fig. 269, c) surrounding the oesophagus behind the ring 

 of peripharyngeal ossicles, and having arising from it a stone- 

 canal which in the majority of forms hangs freely in the 

 ccelomic cavity, where it terminates in a madreporiform plate. 

 In the embryo it as usual opens upon the surface of the body, 

 and this condition is retained in many Elasipoda, in which the 

 canal opens upon the dorsal surface of the body, probably 

 indirectly through the intervention of an ampulla, as in other 

 forms. In the majority of forms, however, the connection with 

 the exterior becomes lost, the ampulla which is present in the 

 embryo disappearing, and occasionally a number of secondary 

 canals develop. A single Polian vesicle (e) is usually attached 

 to the ring, but in some oases the number of these structures 



