EMBKYOLOGY OF HOLOTHUEIA. 



207 



of the arterial system, in communication with which is the contractile 

 vesicle (/) ; and this he looks upon as a reservoir for the nutritive fluid. 

 From the circular vessel various arteries are given off: large branches 

 pass into the tentacula around the mouth (i) ; so that these organs, be- 

 sides being instruments of touch, from the extent of surface that they 

 present and their great vascularity, are most probably important 

 auxiliaries in respiration. Five other large arteries, derived from the 

 same source (k Tc, Z), pass backwards to supply the integuments of the 

 body, and also to communicate by small cross branches with the little 

 vesicular organs connected with the locomotive suckers, which, in the 

 opinion of Delle Chiaje, are distended with the same blood as that which 



Fig. 103. 



Embryology of Holothuria. 



circulates through the rest of the body. The descending arteries, thus 

 destined to supply the integument and distend the prehensile suckers, 

 run in the centre of each of the five longitudinal fasciculi of the mus- 

 cular tunic of the skin as far as the cloaca, and exhibit in their distri- 

 bution a remarkable exception to the usual arrangement of the arterial 

 system, which is generally found to divide and subdivide continually 

 into smaller and still smaller canals: in the case before us there 

 would seem to be no diminution in the size of the main trunks as they 

 approach their termination ; and the cross branches given off in their 

 course, instead of ramifying, all end in the minute ambulacral vesicles, 

 to the injection of which they would appear to be subservient. 



