270 PHYLUM ECHINODERMA. 



organs," which emit white conical bodies from the cloaca 

 when the animal is irritated. The bodies remain adherent 

 by their bases, are greatly elongated by internal fluid 

 pressure into sticky tubes which break off. They will 

 adhere to almost everything but the Holothurian itself. 

 Those Holothurians, e.g. Holothuria ntgra, 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 starfishes 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, 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. 139, sf.). Certain of the tube-feet are always modified to 

 form tentacles, and these 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-cesophageal ring and 

 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 reproducti-ve 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, however, the 

 larval stage is skipped, as in Cttcumaria crocea and Psolus 

 ephippifer^ where the eggs and young are attached to the 

 back of the mother. In C. curata the eggs and young are 

 sheltered on the ventral surface ; in C. parva in a shallow 

 ventral insinking ; in C. Icevigata there is an invaginated 

 ventral brood-pouch ; in Chiridota contorta the young are 

 sheltered in the genital tubules ; in Synapta vivipara and 



