488 COMPARATIVE ANATOMY CHAP. 



the body, and more particularly the branchial tentacles on 



the petaloids, cf. p. 433. 

 3. The accessory intestine, in which, at least in regular Echinoids, 



a streaming of water takes place which does not interfere 



with the digestive processes going on in the principal 



intestine, cf. p. 481. 

 (V) Asteroidea. 



1. The papulss, cf. p. 439. 



2. The ambulacral feet. 

 (e) Ophiuroidea. 



1. The bursse (respiratory and genital chambers). 



2. The ambulacral tentacles. 

 (/) Crinoidea. 



1. The ambulacral tentacles. 



:'. The anal tube (anal cone), which alternately takes in and 

 gives out water. 



XVI. The Cuvierian Organs of the Holothurioidea (Fig. 383, 22, 



p. 477). 



In certain Holothurioidea, peculiar accessory structures are found 

 connected with the terminal portion of the respiratory trees ; these 

 are known as the Cuvierian organs. They occur chiefly in the Aspi- 

 dochirotce (especially in the genera Holothuria and Miillerw) ; in other 

 Holothurioidea they only occur in isolated cases (Mdpadia cliilen*i*. 

 Cucumaria frondosa). The Cuvierian organs are usually very numerous, 

 even as many as a hundred occurring in some of the species provided 

 with them. Although, as already mentioned, they are usually found 

 in the terminal portion of the respiratory trees, they may shift higher 

 up the principal trunk, and may even pass over on to the branches. 



It is not improbable that they represent morphologically trans- 

 formed branches of these trees, the structure of their walls agreeing 

 in general plan with that of these latter. 



Two kinds of Cuvierian organs are distinguished (1) glandular, 

 and (2) non-glandular. 



The Cuvierian organs of the glandular kind are long tubes, the 

 very narrow axial canals of which open into the terminal section of 

 the respiratory tree. Each of these axial canals has a spiral course, 

 and is lined by a unilaminar epithelium. This epithelium is followed 

 externally by a very thick layer of connective tissue, which projects 

 into the axial canal in the form of a spiral fold. The next layer 

 consists (1) of isolated circular muscle fibres, and (2) of external 

 longitudinal muscle fibres gathered into small bundles. Outside of 

 the muscle layer there is another layer of connective tissue, which, 

 on the side of the body cavity, is covered by a peculiarly developed 

 glandular layer ; this no doubt represents the modified endothelium 

 of the body cavity. 



