HOLOTHUROIDEA. 237 



with fresh water by means of the rhythmic contractions of 

 the cloaca, and are respiratory, hydrostatic, and excretory. 

 The body fluid sometimes contains a red pigment like 

 haemoglobin. Arising from the base of the left respira- 

 tory tree, in some Holothurians there are remarkable 

 " Cuvierian organs," consisting of numerous tubes, in 

 most cases glandular. The Holothurian can eject these 

 tubes through the cloaca, the wall of which is apparently 

 ruptured in the process. The tubes are very viscid, and 

 seem to grow longer in the water; they will adhere 

 to almost everything but the Holothurian itself. Those 

 Holothurians in which the organs are well developed are 

 often called " cotton-spinners," on account of the dense 

 mass of viscid substance which they eject. A little fish, 

 Fierasfer, introduces itself — tail first — into the cloaca of 

 several Holothurians, and lives there as an innocent 

 commensal. 



The water vascular system shows many peculiarities. In what, by 

 analogy with the other classes, may be " described as the primitive 

 condition, there is a ring canal round the mouth communicating with 

 the exterior by a stone canal and a madreporite, with one or more 

 Polian vesicles hanging in the body cavity, and with five radial canals. 

 The radial canals, as in star-fishes and sea-urchins, are connected with 

 internal ampullae and external tube-feet. The anterior tube-feet are 

 greatly enlarged and modified to form the tentacles which encircle the 

 mouth. It is, however, only rarely that the water vascular system 

 exhibits this primitive condition. In most cases the stone canal loses 

 its original connection with the exterior and opens merely into the 

 body cavity ; often it is represented by numerous small canals, hanging 

 freely in the body cavity (Fig. 103, si. ). Certain of the tube-feet are 

 always modified to form tentacles, and they may, as in Synapta, be the 

 only representatives of the tube-feet. In regard to the function and 

 degree of development of these, there is indeed much variation. 



The blood vascular system is well developed, but here as elsewhere 

 the vessels are lacunar and without endothelial linings, and tend to 

 form ramifying networks over the surface of the organs. The arrange- 

 ment of the vessels is in essence the same as in sea-urchins. 



The sexes are usually separate. The reproductive organs 

 do not exhibit radial symmetry, and are branched tubes 

 which open within or just outside the circle of tentacles. 

 They and other internal organs of Holothurians are often 

 very brightly coloured. The larva is, in most cases, 

 what is known as an Auricularia. Sometimes, how- 

 ever, the larval stage is skipped, as in Cucumaria crocea 



