210 



LECTURE X. 



caeca which are transformed into a distinct respiratory organ, and 

 which presents the form of two long and beautiful arborescent tubes. 

 The complex circulating system in the Holothuria is in great part 

 represented in this diagram {^fi(j' 100.) in connection with the equally 

 extensive system of chylaqueous sinuses and canals which regulate 



the protrusion and 

 retraction of the nu- 

 merous tubular feet. 

 The most important 

 part of the circu- 

 lating system is the 

 trunk (e), which runs 

 along the free border 

 of the intestine, and 

 which is character- 

 ised by the short 

 and wide vessel (di- 

 vided at f,f^ homo- 

 logous with the heart 

 in the Echinus, and 

 which connects the 

 corresponding ves- 

 sels of the two prin- 

 cipal folds of the 

 intestine. The in- 

 testinal capillaries 

 reunite, performing 

 at the same time the 

 office of absorbents 

 and conveying the 

 chyle to the great 

 intestinal vein (y), 

 from which proceed 

 the singular and 

 beautiful respiratory 

 plexuses {h, K), 

 which are submit- 

 ted to the influence 

 of the sea water by 

 contact with the 

 branchial trees («, n). 

 The aerated blood 

 is conveyed to a great mesenteric trunk {i, i), or branchial vein, from 



Holothuria. (CLXVIT,) 



