ECHINODERMATA. 



175 



its entire length. The whole intestine is generally found distended 

 with sand, in which may be detected the debris of corals, algse, 

 fuci, and other marine substances. 



(216.) In the structure of the respiratory apparatus, the Holothu- 

 ridse differ materially from the rest of the Echinodermata, and in 

 fact from all other animals. In the Asterida and Echinida, the 

 reader will remember that respiration was effected by the free 

 admission of sea-water into the interior of the animal, which, thus 

 penetrating to every part of the body, rendered the existence of 

 special respiratory organs unnecessary. In the Holothuria like- 

 wise the aeration of the circulating fluid is provided for by allow- 

 ing the surrounding element freely to enter into the internal parts 

 of the creature ; but in this case, instead of bathing the surfaces of 

 the viscera, the water is confined in a peculiar system of ramifying 

 canals, forming a structure of great beauty, and, from its singularity, 

 extremely interesting in a physiological point of view. We have 

 seen that the intestinal canal terminates in a membranous recep- 

 tacle or cloaca {Jig. 74, e,) contained within the cavity of the 



Fig. 74. 



