536 INVERTEBRATE MORPHOLOGT. 



upon a sievelike calcareous plate known as the madreporiform 

 tvhercle or madreporite. The hydrocoel in the adult communi- 

 cates with the left enterocoel by a tube, termed the stone- 

 canal from the deposition of calcareous matter which occa- 

 sionally takes place in its walls, and so indirectly opens to 

 the exterior through the madreporiform tubercle (see Fig. 

 265). The various departures from this arrangement which 

 occur will be more conveniently considered in connection 

 with the special descriptions of the various groups ; the con- 

 dition just mentioned may be provisionally accepted as rep- 

 resenting the typical arrangement. 



After the separation of the hj'droccel from the left entero- 

 coel, the latter and the enterocoel of the right side increase in 

 size and finally apply themselves closely to the inner surface 

 of the body-wall and to the outer surface of the digestive 

 tract, forming the peritoneal lining of these structures. Where 

 the two coelomic sacs meet there are formed, of course, two 

 partitions extending from the body-wall to the intestine, and 

 suspending that structure between them. These partitions 

 are the mesenteries, but before the embryo reaches the 

 adult stage one of these mesenteries disappears, the other 

 persisting in a more or less perfect form. The coiling of the 

 intestine, which occurs frequently in the adult forms, brings 

 about complications of the course of the mesentery, compli- 

 cations further increased in most cases by the formation of 

 other partitions which may traverse a greater or less portion 

 of the coelom either longitudinally or transversely. One of 

 the transverse partitions, most frequently present, separates 

 off more or less completely from the rest of the coelom, a por- 

 tion of it surrounding the pharyngeal region of the digestive 

 tract and hence termed the peripharyngeal cavity, while in 

 some cases a perianal cavity may similarly be formed. 



The hydrocoel, whose origin has been desci-ibed, develops 

 into a tubular ring (Fig. 248, cc) surrounding the oesophaous 

 quite close to the mouth. Upon this ring in the interradii 

 one or several saclike diverticula, termed Polian vesicles (n) 

 occur, and in one interradius a canal, the stone-canal (sc) 

 passes aborally to open into a thin-walled sac termed the 

 ampulla of the stone-caiml, which is in reality a portion of 



