212 ECHINODEEMATA. 



have the multiplied organs of the other families exhibiting so much 

 simplicity of arrangement, that whatever may have appeared obscure 

 or complicated in our description of Echinus and Holothuria will receive 

 elucidation from the diagrammatic form in which all the vessels con- 

 nected with the circulation of the blood are represented in fig. 105. 

 The intestinal vein (m) may be readily traced along the entire length 

 of the alimentary canal : commencing near the anal extremity of the 

 bowel, it follows 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 lacteals 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 oesophagus, 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 neigh- 

 bourhood of the mouth ; the other, which we may call the aorta, dis- 

 tributing the remainder to all parts of the tegumentary system. The 

 branchial vessel (n) runs from the bifurcation of the intestinal vein to 

 the base of the oral tentacles, where it forms a vascular circle around 

 the commencement of the oesophagus, analogous to that which we have 

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

 the " ampulla Poliana" (&), 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. 



(551.) 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 (1), 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 di- 

 rection of the intestinal vein, it is probable that it contains an appa- 

 ratus of valves so disposed as to prevent any retrograde motion of the 

 blood. At the termination of the aorta there appears to be a second 

 enlargement, to which the name of ventricle has been given, and which 

 is perhaps also capable of contraction, so as to assist in the propulsion of 

 the circulating fluid. The blood of these animals is of a purple colour in 

 the veins, but red in the arterial vessels. 



