334 Mr. W. Clark on the Animal of Dentalium Tarentinuni. 



corresponding ingress and egress of the water seen*. I have not 

 detected auricles on each side of the hearty 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 demur to call 

 them auricles, and rather think they denote the valvular appen- 

 dages of the heart to prevent regurgitation into the branchial 

 veins. The blood of the posterior part of the body is brought 

 to the branchial artery which runs at the inner base of the 

 branchiae, by two longitudinal 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 

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

 blood is again returned to the arterial centre. Under the micro- 

 scope 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. I have preparations to illustrate this order 

 of the organs. 



The branchiae are two symmetrical, sublateral, and somewhat 

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

 organs, having their bases fixed on and hanging from the con- 

 cave 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 symmetrically. 



The heart in the testaceous Gasteropoda, spiral and otherwise, 

 is alvvays placed at the posterior end of the branchial cavity, or, 

 in other words, is fixed at that extremity of the hvdMc\i\2d furthest 

 from the entry of the aerating fluid : this statement of position 

 is of importance in coming to a conclusion as to the mode of 

 entry of the water. But if the position of the organs of Denta- 

 lium is examined under the view of the water approaching the 

 branchiae under the mantle, as in the ordinary Gasteropoda, they 

 will be found to be the reverse of what I have stated to be the 

 usual natural position ; the heart will be found at the anterioi' 

 end of the branchial cavity instead of at the posterior, and near- 

 est to the entrance of the water instead of furthest from it : here 

 is a subversion of the order of nature in respect to the position 

 of these essential organs : how are they to be placed in harmony 

 with her laws ? The solution of this question is simple : we have 



* Lamarck in the last ed. ' Aiiim. sans Vert.,' Milne Edwai'ds's, 3rd vol. 

 p. 13, says, "Car, apres les animaux vertebres, la nature n'ofFre, dans aucun 



