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 Ilolothuria is in great part 

 represented in this diagram (^^. 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 y^/) 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 (^), 

 from which proceed 

 the singular and 

 beautiful respiratory 

 plexuses (A, h), 

 w^hich are submit- 

 ted to the influence 

 of the sea water by 

 contact with the 

 branchial trees (w, 72). 



Holothuria. (CLXVIT.) x Jii j 



The aerated blood 

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



i 



