256 PHYLUM ECHINODERMA. 



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, 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 ampullce and 

 external tube-feet. The anterior tube-feet are greatly enlarged and 

 modified to form the tentacles which encircle the mouth. It is, how- 

 ever, 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. 121, st. ). 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 consists of a circum-oesophageal ring and 

 five radial vessels, besides vessels to the alimentary canal and the 

 gonads. The system is in great part lacunar. There is also a pseud- 

 haemal system. 



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. 

 Like other internal organs of Holothurians they 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 

 and Psolus ephippiger, where the eggs and young are 

 attached to the back of the mother. In Cucumaria lavigata 

 there is an invaginated brood-pouch ; in Synapta vivipara 

 and some others the body cavity serves as a brood-pouch. 



The calcareous plates of Holothurians are found as far 

 back as Carboniferous strata. 



As " trepang " or beche-de-mer," the Holothurians of 

 the Pacific form an important article of commerce, being 

 regarded as a delicacy by the Chinese. 



