SEA-CUCUMBERS 463 



intestine, which dilates near its termination into a section which 

 is known as the cloaca, though it can scarcely be said to correspond 

 to the similarly-named cavity found in many of the vertebrate 

 animals (see pp. 69, 146, 200, 240, 261). 



The circulatory organs consist, as in a star-fish or sea-urchin, 

 of a blood system and a lymph system. The former is chiefly 

 made up of a ring round the beginning of the gut, of five radial 

 vessels, and of branches to the digestive and some of the other 

 internal organs. The lymph system consists of the spacious 

 body-cavity filled with fluid, in which float both colourless and 

 red corpuscles, reminding us of the blood of a typical vertebrate 

 (see p. 38). 



Breathing is no doubt partly effected, as in other cases, by the 

 tentacles and tube-feet, but there are also other structures which 

 probably have to do with the same function, in the form of two 

 large branching respiratory trees that open into the cloaca. The 

 branches of these organs are beset with innumerable minute ciliated 

 funnels, by which the body-cavity is placed in communication with 

 the exterior. It is extremely likely that these trees also have 

 to do with getting rid of the nitrogenous waste of the body, and 

 if so, they are excretory as well as respiratory organs. 



The water -vascular system is constructed on the same plan 

 as in a star-fish. There is a ring round the beginning of the gut 

 from which a radial vessel runs along each ambulacrum to give off 

 the double row of tube-feet. Branches of these vessels supply 

 the tentacles, which are to be regarded as much-modified tube-feet, 

 and there is a stone-canal with a madreporite which, instead of 

 opening to the exterior, hangs down into the body-cavity. In 

 a very young Holothurian, the stone-canal opens directly to the 

 exterior, as in a star-fish or sea-urchin, but this connection is lost 

 in the typical forms, though there are some deep-sea species 

 in which it persists throughout life. 



The nervous system is of the type already described for other 

 subdivisions of the class (p. 454). Sense Organs are chiefly repre- 

 sented by the tentacles and tube-feet, which no doubt have to do 

 with the sense of touch. 



Within the limits of the class there are numerous variations 

 in many respects. Certain deep-sea forms present a well-marked 

 bilateral symmetry, and the body is produced into pairs of pro- 

 jecting processes. In such forms, a well-marked flattened lower 



