ECHIXODERMATA. 



have appeared obscure or complicated in our description of Echi- 

 nus and Holothuria will receive elucidation from the diagram- 

 matic form in which all the organs connected with the circulation of 

 the blood are represented in the adjoined figure. The intestinal 

 vein (m) may be traced along the entire length of the alimentary 

 canal ; commencing near the anal extremity of the bowel, it fol- 

 lows all its convolutions, and receives from every part the minute 

 vessels which ramify over the intestinal walls. These venous 

 ramifications undoubtedly perform the office assigned to the lac- 

 teals of higher animals, and imbibe the nutritive particles furnished 

 by digestion, which, of course, are conveyed into the great venous 

 trunk (m). Arrived opposite to the termination of the oesopha- 

 gus, the intestinal vein divides into two vessels : one performing 

 the office of a branchial artery, by conveying a part of the blood 

 to the respiratory organs in the neighbourhood of the mouth ; the 

 other, which we may call the aorta, distributing the remainder to 

 all parts of the tegumentary system. The branchial vessel (M) 

 runs from the bifurcation of the intestinal vein to the base of the 

 oral tentacles, where it forms a vascular circle around the com- 

 mencement of the oesophagus, analogous to that which we have seen 

 in Holothuria ; and in connexion with this circular vessel we find 

 the " ampulla Poliana " (A), which Delle Chiaje conceives to be 

 here, as in other cases, a receptacle for the circulating fluid. From 

 the vascular circle around the mouth, vessels are given off, to ramify 

 minutely through the substance of the tentacula (a), so that these 

 appendages may be considered as respiratory organs, like those of 

 Holothuria. The other vessels derived from the oral circle have 

 not been traced ; but we may conclude from analogy that arteries 

 supplying the mouth and alimentary canal are furnished from this 

 source. 



The aorta (o) is the other large vessel derived from the intes- 

 tinal vein, and is seen to pass in a flexuous course from its origin 

 to the posterior extremity of the body, following the median line, 

 and giving off transverse branches on both sides opposite to every 

 ring of the muscular integument. At the commencement of the 

 aorta is a dilated vesicle (/), which may be looked upon as a heart 

 (auricle, Delle Chiaje). The vesicle alluded to is of a conical 

 form, the apex of the cone being directed towards the tail 

 of the animal ; and, from the impossibility of making mercury 

 pass from the aorta through this organ in the direction of the 

 intestinal vein, it is probable that it contains an apparatus 



