212 ECHINODERMATA. 



gut, within the first coil, there lies a canal or " siphon," open 

 at both ends. Its import is unknown. The spacious body- 

 cavity is lined by ciliated epithelium and contains a 

 "perivisceral" fluid, whose corpuscles seem to have a 

 respiratory brown pigment. When the fluid of a perfectly 

 fresh Sea-urchin is emptied out, the contained corpuscles 

 are said to unite in plasmodia, forming composite amoeboid 

 clots. 



The Water- Vascular System.— The madreporic plate com- 

 municates with a membranous stone-canal, which goes 

 vertically through the body, and leads into a circular vessel 

 near the upper end of the lantern. This gives off five inter- 

 radial transparent vesicles, and five radial vessels which 

 extend down the sides of the lantern and up each ambulacral 

 area. Finally, each radial vessel gives off' numerous lateral 

 branches, which communicate with the internal ampullae 

 and the external tube-feet. When the tube-feet are inade 

 tense with fluid, they extend beyond the limit of the spines, 

 and are attached to the surface of the rock over which the 

 sea-urchin slowly drags itself. The sucker at the tip of each 

 tube-foot bears small calcareous plates regularly arranged, 

 indeed there is hardly any part of an Echinoderm in which 

 lime may not be deposited. Before bending upwards 

 from the base of the lantern, each radial vessel gives off a 

 branch to two large tentacle-like tube-feet without attach- 

 ing discs. They lie round about the mouth, and are 

 sensitive. 



The Bloodv Vascular System is not readily traced, and there 

 is uncertainty as to its precise nature. Alongside the stone- 

 canal there lies an enigmatical structure, to which such 

 names as "plexiform organ," "ovoid gland," "dorsal organ," 

 and "heart" are given. It is connected superiorly with the five 

 genital organs, inferiorly with a circular vessel surrounding 

 the pharynx at the top of the lantern, within and beneath 

 the water-ring. This vasculla.r ring seems to be connected, 

 by branches at least, with the five pockets of the water-ring 

 to which the name " Polian vesicles " has been mistakenly 

 applied. A distinct vessel arises from the ring and runs 

 along the inner or ventral surface of the intestine, while 

 another on the opposite side seems to originate from 

 capillaries. It is likely enough that there may be radial 



