ECHINODERMATA. 



177 



are indicated in the annexed diagram {fig. 75) by the letters 

 c, e, p, p, </, q ; these communicate with each other not only by 

 the intervention of numerous Fig. 75. 



anastomosing branches, (d, d,) 

 but likewise by means of de- 

 licate vascular plexuses (a) 

 passing between them. All 

 these veins terminate in two 

 large venous canals (o), which 

 convey the blood and nutri- 

 ment absorbed from the in- 

 testine to a vascular circle 

 (g) 9 placed around the com- 

 mencement of the oesopha- 

 gus, which corresponds with 

 the circular vessel around 

 the mouth of the Echinus. 

 This circle Delle Chiaje re- 

 gards as the centre of the 

 arterial system, in communi- 

 cation with which is the con- 

 tractile vesicle (/), which 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 (z), so that these organs, besides being instruments of touch, 

 from the extent of surface that they present, and their great vas- 

 cularity, are most probably important auxiliaries in respiration. 

 Five other large arteries, derived from the same source, (&, fc, /,) 

 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 circulates through the rest of the body. The descend- 

 ing arteries, thus destined to supply the integument and distend 

 the prehensile suckers, run in the centre of each of the five lon- 

 gitudinal fasciculi of the muscular tunic of the skin as far as the 

 cloaca, and exhibit in their distribution 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, but, in the case before us, there would seem to be 



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