552 THE ANIMAL KINGDOM. 



ambulacrum anteriorly, in the middle, or posteriorly, and is penetrated by 

 processes from the two radial vessels of the bivium. The tube-feet may 

 be arranged in rows or scattered over the surface irregularly as in Dendro- 

 chirotae Sporadipoda, and they are frequently less numerous in the bivium. 

 They possess ampullae which project slightly from between the circular 

 muscles. In Elasipoda there are spaces branched or unbranched in the 

 integument connected with the feet and dorsal papillae ( = processes, Theel), 

 and true ampullae are rarely found and only in connection with the dorsal 

 processes. The stone-canal is generally single but may be multiple or 

 branched. In many Elasipoda it opens on the dorsal surface medianly by 

 usually one or rarely several pores close to the generative pore ; and when 

 it is retracted into the coelome, it does not hang freely but is partly im- 

 bedded in the integument. It hangs freely in the coelome or is attached 

 to the mesentery in other Holothurioidea. It may or may not be ter- 

 minated by a calcareous madreporic plate, but whether it is or not, it is 

 covered by coelomic epithelium. Numerous corpuscles occur in the water- 

 vascular system. The blood-vascular system consists of a series of spaces 

 in the connective tissue, not lined by an epithelium, and disposed as a 

 circular peripharyngeal plexus closely connected to the water-vascular 

 ring, but placed posteriorly to it, and a ventral and dorsal intestinal 

 vessel, the latter usually double except in Elasipoda. They are united by 

 anastomoses round the intestine. The ventral vessel has been observed to 

 contract from the centre to either end ; the dorsal is connected to a plexus 

 in the mesentery by which the left respiratory tree is loosely invested in 

 Aspidochirotae and Apod a Pnetunonophora. 



The mouth is placed centrally in a peristome surrounded by the circle 

 of tentacles. It is very frequently turned to the ventral surface ; always in 

 Elasipoda^ and to an especial degree in the family Psychropotidae. It is in 

 many instances terminal or even turned dorsally, and opens into a 

 pharynx. The digestive tract is straight in some Apneumona, but is 

 generally disposed in a descending, an ascending and a descending 

 section. The first section is supported by a mesentery which is attached 

 to the median dorsal line interradially, and corresponds to the dorsal line 

 of union between the right and left peritoneal vesicles of the larva, the 

 ventral union being absorbed. The ascending section is borne by a mes- 

 entery attached to the left dorsal interradius, and the second descending 

 by one attached to the right ventral interradius. The tract is therefore 

 slightly coiled. The pharynx is followed by the short ' stomach,' which 

 has well-developed muscular walls, and is separated by a constriction from 

 the intestine by far the longest part of the tract. The last portion or 

 rectum is dilated, forming the so-called cloaca, which has been observed to 

 contract rhythmically. Its walls are connected to the integument by radial 

 muscles. There are appended to it, except in Apneumcna and Elasipoda^ 



