276 Mr. W. Clark on the Chitonidse. 



in the animal kingdom : this is not stated in an objective sense, 

 but as a curious fact, though he gives no reason for this aberra- 

 tion of the usual structure. Enough has now been said to de- 

 monstrate the little value of the much insisted on dorsal rectilinear 

 position of the motive power of the circulation in Chiton, in com- 

 parison with the Annelida. 



As to the objection to the allocation of these animals with the 

 Mollusca on account of the symmetry of the reproductive organs, 

 we think they are of small importance, even if double : and who 

 can say that the symmetrical Patelloida have not in this respect 

 a similar structure ? but these points are doubtful. M. Deshayes 

 says, " Quoique nous ayons fait des anatomies minutieuses d'Os- 

 cabrions, il nous a ete impossible de trouver la seconde issue des 

 organesde la generation;" and M. Cuvier observes, they became 

 so attenuated that he confessed he could not trace them. Our 

 own researches lead to doubts of these appendages being oviducts; 

 at the same time we admit, they may prove to exercise those 

 functions: they are situate in the immediate vicinity of the heart 

 and auricles, and may be glands to secrete a liquor for those or- 

 gans, or the fecundating pouches of the peculiar hermaphroditism 

 of this tribe, in which latter case, the true issue for the ova will 

 probably be found between the rectum and the posterior part of 

 the ovarian sac. 



It has been said that the body is subannulate : in a hundred 

 dissections we could not see much trace of such contiguration, or 

 breaks in it to correspond with the segmental arrangement of the 

 valves ; only slight marks, the effect of pressm-e, were observed. 

 The connection of the Chitons with the Crustacea is, as I think, 

 so very slight and remote as to require no further notice. 



Having cursorily disposed of certain objections, we will proceed 

 to state our own views, and in their course, allude to other objec- 

 tions and discrepancies. Though the Chitons are in closer 

 alhance with the Bivalves, anatomically, by the arrangement of 

 the circulatory apparatus, symmetry of the branchiae, and in 

 the absence of tentacula and eyes, than by the external hard 

 parts, still in them there are points of coherence which are not 

 without their value; for instance, in Pholas dadylus, its immediate 

 predecessor, in our method, though the bivalve portion is not 

 broken into regular segments, there are certain testaceous pieces, 

 commonly, though perhaps incorrectly, called accessories, in 

 number six, including the principal valves. We also find in the 

 Chitons a subsymmetrical division into eight segments of what 

 I consider essentially an integral patelloid cone, and as much 

 accessorial as those of Pholas ; indeed both in one and the other, 

 these component parts are equally necessary and essential. I 

 admit that no great stress ought to be laid on the contrasted 



