DENTALIUM. 229 



I have not detected auricles on each side of the heart, nor 

 near it, as might be expected from the symmetry of the 

 branchiae ; there are certainly minute points on each side of 

 that organ, but I object to call them auricles, and rather 

 think they denote the valvular appendages of the heart, to pre- 

 vent regurgitation into the branchial veins. The blood of the 

 posterior part of the body is brcmght to the branchial artery 

 which runs at the inner base of the branchiae, by two longi- 

 tudinal veins, which pass between the branchiae on their 

 convex surface, receiving tributaries ; I could not trace those of 

 the anterior part. The arterial blood is then distributed into 

 the ramifications of the branchiae, and after aeration is passed 

 by each principal vein, which coasts the edges of those organs 

 at their dichotomous points, to the heart, which throws out a 

 posterior and anterior short trunk, both of which bifurcate 

 into two smaller arteries, which supply veins infusing a re- 

 newed vitality into all parts of the body, from whence the 

 blood is again returned to the arterial centre. Under the 

 microscope the blood of the tributary and superficial veins 

 appears to be in some individuals of a pale pink colour, and 

 in others of a purplish pale red cast. 



The branchiae are two symmetrical, sublateral, and some- 

 what post-centrally situated, dark greenish-brown, elongated, 

 suboval organs, having their bases fixed on and hanging from 

 the concave surface of the animal, with their points vertically 

 parallel to the bases ; the two branchiae are united at their 

 inner surfaces by a bridle of branchial strands arranged sym- 

 metrically. 



The heart in the testaceous Gasteropoda, spiral and other- 

 wise, is always placed at the posterior end of the branchial 

 cavity, or, in other words, is fixed at that extremity of the 

 branchiae furthest from the entry of the aerating fluid *. This 



* M. Milne-Edwards has placed a few genera, as Bulla, Pleurobranchus, 

 and Aplysia, in his division Opisthobranchiata, as having the heart anterior 

 to the branchiae. Without closely considering the merits of his arrange- 

 ment, which perhaps is fine-drawn and not constant, we may say, in respect 

 of the case between M. Deshayes and myself, that it does not bear upon it, 

 as, in whatever way the question is looked at, Dentalium is a strict proso- 

 branchiate animal, with the branchiae anterior to the heart. 



