AND TREPANGS. 187 



progresses at a perfectly symmetrical rate, and always in refer- 

 ence to right and left. To such a degree, moreover, is this 

 carried in certain Trepangs, that the repetitions are reduced to 

 nothing so far as concerns the reproductive and blood circulating 

 organs, there being but a single one of each ; and the parts of 

 such as the digestive organ and the water circulating systems 

 are simplified in a manner that leaves them under such peculiar 

 relations to each other, that, until within a very few years, most 

 naturalists have been in the habit of looking upon them as pos- 

 sessing more of the characters of those of a certain group of 

 worms ; or rather, they united these worms with the vermiform 

 Trepangs. How far they were justified in doing this I shall 

 bring up for discussion hereafter. At present 1 shall produce 

 for illustration one of our native Trepangs, in which, so far as 

 the group to which it belongs is concerned, the process in ques- 

 tion attains the climax of perfection. 



Fig. 114- 



Fi"-. 114. Caudina arenata. Strap. Natural size. A longitudinal, semi- 

 diaramic view of a common Trepang of our coast. /, i 1 , the four-pronged, 

 anchor-shaped feelers of the head; f,f*, the stave-like, calcareous, forked pieces 

 of the buccal ring ; g, the anterior end of the intestine ; g\ the first bend of the 

 same ; </ 2 , the second bend of the same ; <7 3 , the posterior or cloacal region of 

 the intestine ; g*, posterior aperture of the same ; rt, rt l , rft, the respiratory 

 [vv^ cUUc,. T.^.K 



