308 PHYLUM ECHINODERMA 



ten, or more tentacles, the food canal coils to the opposite 

 pole. There it expands in a cloacal chamber sometimes 

 contractile, and from this are given off in many forms a 

 pair of much-branched " respiratory trees," which extend 

 forward in the body cavity. These " trees " are supplied 

 with water by means of the rhythmic contractions of the 

 cloaca. They are respiratory, hydrostatic, and excretory. 

 The body fluid sometimes contains a red pigment like 

 haemoglobin. Arising from the base of the respiratory trees 

 in some Holothurians there are the remarkable " Cuvierian 

 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 nigra, 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 ampulla? 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. 167, 5/.). 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 diversity. 



The blood vascular system consists of a circum-oesophageal ring and 

 vessels to the aUmentary canal and the gonads. The system is in great 

 part lacunar. There is also a pseud-h^mal 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 



