HOLOTHUROIDEA. 253 



outside the longitudinal muscular bands to the apical pole, 

 where they terminate blindly in the integument near the anus. 

 There is no projecting terminal tentacle as in Asteroids and 

 Echinoids. The radial canals are five in number ; they are 

 absent only in Synaptidae. The circular vessel has two appen- 

 dages the polian vesicle and the stone-canal. The polian 

 vesicle (Fig. 181) is generally single and may be of large size; 

 it is usually attached to the circular canal in the left adult- 

 ventral interradius. Exceptionally, additional polian vesicles 

 are present, generally in the adult-ventral region of the body. 

 The stone-canal is usually single (always in Molpadiidae, Pela- 

 gothuridae and Elasipodidae), but in the Synaptidae, Aspido- 

 chirotae and Dendrochirotae it is occasionally multiple. When 

 it is single it lies in the dorsal mesentery ; when multiple, the 

 primary canal alone is in the mesentery, and the accessory stone- 

 canals which are very variable in number (2 to 160) project, 

 mostly from the dorsal half of the ring-canal and on either side 

 of the mesentery, freely into the body-cavity into which they 

 open (see below). Occasionally the primary stone-canal is in- 

 dependent of the dorsal mesentery, and projects into the body 

 cavity on the right-hand side (many Aspidochirotae). In a few 

 species (Thyone chilensis, Synapta beselii) the stone-canal is 

 branched, with a body-cavity opening at the end of each branch. 

 The wall of the stone-canal is without muscles and usually 

 contains calcareous deposits ; the internal lining consists of a 

 ciliated epithelium which on one side of the tube is composed 

 of much more columnar cells than on the other. The termina- 

 tion of the stone-canal presents the most remarkable variations. 

 In some it is attached to the body- wall and opens to the exterior 

 in the dorsal middle line just in front of the generative opening 

 either by a single pore (Pelagothuria and some Elasipodidae) 

 or by more than one pore (2-50 or more, many Elasipodidae). 

 In other cases (certain Elasipodidae and Molpadiidae) the stone- 

 canal ends blindly in the body wall in the dorsal middle line, 

 and opens, not to the exterior, but into the body-cavity by a 

 number of pores which perforate its walls just before its blind 

 end. In all other Holothurians it has lost its connexion to the 

 body wall altogether, and opens into the body-cavity by a large 

 number of pores placed upon its slightly swollen termination. 

 It is remarkable that species of the same genus may differ in the 



