viii ECHINODEEMATATHE CCELOMIC CAVITIES 437 



noidea, this is quite cut off from the body cavity. The membrane 

 which separates the body cavity from the pericesophageal sinus is 

 called, in those Echinoids which are provided with a masticatory frame- 

 work (Cidaroida, Diadematoida, and Clypeastroida), the lantern mem- 

 brane. This membrane entirely covers the lantern on the side turned 

 to the body cavity. 



In many Echinoids, this part of the ccelom protrudes externally 

 at the edge of the peristome, forming the outer gills : in others, 

 the lantern membrane bulges out into the body cavity, and forms 

 Stewart's organs. 



In the Holothurioidea and Echinoidea, the hind-gut is surrounded by 

 a small coelomic sinus, the perianal sinus. 



In the Echinoidea, Astcroidea, and Ophiuroidea, a part of the ccelom, 

 cut off from the rest, runs from the region of the madreporite inter- 

 radially to the circular canal of the water vascular system. This is 

 the axial sinus in which the stone canal runs. It contains also a 

 lymph gland, the so-called ovoid gland or axial organ. 



The axial sinus, in the Echinoids, is in open communication with 

 the ampulla which lies beneath the madreporite. Recent ontogenetic 

 researches have shown that this ampulla also is of enterocoelomic origin, 

 and is thus a section of the coelom. Since the stone canal opens into 

 the ampulla, an open communication exists at this point, and at this 

 alone, between a closed division of the coelom (the axial sinus) and a 

 section of the water vascular system (the stone canal). 



A. The Body Cavity. 



1 . Holothurioidea. The spacious body cavity of the Holothurioidea 

 is divided up by the mesentery which attaches the intestine to the 

 body wall. This mesentery may be described as having three parts 

 corresponding with the three sections of the intestine, viz. the dorsal 

 mesentery, belonging to the first section of the intestine which runs 

 backward ; the left dorsal mesentery, belonging to the second 

 section, which bends forward ; and the right ventral mesentery, 

 belonging to the third section, which runs backward to the cloaca. 

 All three parts of the mesentery lie interradially. 



The so-called ciliated urns or funnels (Fig. 368) are found only 

 in the Synaptidce. These are funnel-, cup-, or slipper-shaped organs, 

 each of which is attached to the body wall or the mesentery by a stalk, 

 and hangs down freely into the body cavity. They are specially 

 numerous to the right and left of the dorsal mesentery. In Chirodota, 

 many funnels have one common stalk, and so form trees of ciliated 

 funnels. 



The funnel consists of three layers, an outer endotheKal tessellated epithelium, a 

 middle and extremely thin layer of connective tissue, and an inner layer of columnar 

 epithelium, which lines the lumen and carries long cilia. Towards the stalk 

 the lumen is closed, but it is open towards the body cavity. 



