150 Descriptions of Preparations. 



water-vascular ring anteriorly. The commencement of the intes- 

 tine proper is seen to be connected with the convex portion of the 

 second convolution of the tube by a considerable number of vessels 

 passing" between the segments of the ventral vessel in connection 

 with each portion of the tract. The first convolution of the in- 

 testine has its concavity looking forwards, its convex aspect being 

 in relation with the posterior extremity of the animaFs body ; the 

 second convolution or the first ascending segment of intestine 

 reaches about as far forwards as the middle of the body, where 

 it turns backwards to end in the 'cloaca.^ This term may be 

 applied to the terminal segment of the intestine, inasmuch as, 

 though it does not receive the duct of the generative organs, it 

 does receive those which lead from or into the so-called respiratory 

 trees, the functions of which, there is good reason to think, are as 

 much depuratory or renal as respiratory. These organs are seen 

 to take origin on either side of the cloaca as hollow stems carrying 

 somewhat scanty ramifications, and reaching a considerable but 

 varying distance forwards in various specimens into the cavity of 

 the body. They are both attached to the body-walls by mesen- 

 teries, which are, however, reduced by extreme fenestration to mere 

 series of filaments inserted along the left and right borders respect- 

 ively of the ventral trivium. In the Dendrochirotae there is not, as 

 in the Aspidochirotae, any connection between the respiratory tree 

 of the left side and the pseud-haemal plexus developed upon the 

 vessel in connection with the dorsal surface of the intestinal tube ; 

 and in this sense, though not in that in which the word has been 

 applied to them, the Dendrochirotae may be called ' Adetopneu- 

 mones.' The arborescent form of the tentacles of the Dendro- 

 chirotae may, by exposing a greater surface to aeration than the 

 short shield-shaped tentacles of the Aspidochirotae, compensate for 

 this less perfect evolution of the internal aerating apparatus ; and 

 with the greater evolution of the tentacular apparatus and its 

 division into delicate twigs, we may connect again the evolution 

 of the special system of retractor muscles already noticed. Accord- 

 ingly as these respiratory trees are absent or present, the class 

 Holothurioidea is divided into the two orders of Apneumona and 

 Pneumonophora; the Apneumona, Synaptidae, having, like such 

 of the Gephyraean Vermes, Sipunculidae, as are similarly devoid 

 of respiratory appendages to their cloacae, certain ciliated infundi- 



