Mr. W. Clark on the Chitonidae. 3r7" 



points ; nevertheless, in conjunction with other decided anatomical 

 analogies, they have theit weight in the balance. Oui- view of 

 the natural position of Chiton is after Dentalium, with which it 

 has marked affinities, and in immediate contact with the Patelloid 

 group, in which we regard, in almost every respect, Fissurella as 

 the point of comparison, as in it is seen the same form of the cone, 

 though entire instead of broken, the same parity of the branchiae, 

 a similar posterior anal debouchure, and the attenuated mantle, 

 gradually thickening, in both genera, to a tumid coriaceous 

 margin, which in Fissurella can scarcely be withdrawn within 

 the shell, studded alike in both with papillae and rugosities, and 

 the same marginal fringes. The nervous masses in the two 

 accord closely : the only exception is the striking, but really un- 

 important, division of the cone of the Chitons into segments; but 

 this incident may be accounted for on very simple grounds. We 

 consider the fracture of the shell not a character denoting an 

 affinity with the Annelida, as the body of the animal has no cor- 

 responding articulations, but simply an aid to facilitate locomotion. 

 The foot of the strict symmetrical testaceous Gasteropoda is 

 generally nearly concurrent with the length of the body, and 

 forms its base, from which, by a gradually increasing cone, the 

 animal becomes at maturity fixed to the summit of the shell by 

 a powerful muscle : this structure united to an entire cone, and 

 combined, as in the Bivalves, with the depressing efi^cts of a 

 complete hermaphroditism, that of Venits sine concubitu, almost 

 extinguishes the locomotive functions ; and we see throughout 

 the patelloid tribe, all of which have a foot co-extensive with the 

 body, an almost entire fixation to the same spot; indeed we 

 believe that some of the Patella pass their existence in the 

 depressions of the rock on which they are cast as embryos, and 

 the same apathy exists in Pileopsis and Calyptroea. But nature 

 has judged fit to give the Chitons additional motive powers, for 

 when fixed on the area of a smooth pebble, they will travel off it, 

 whilst the Patellce are immoveable: this is owing to the integrity 

 of the cone, and the enormous muscle which fixes the animal to 

 its apex: this structure does not permit the body sufficient flexi- 

 bility for much progression, and almost confines it to a vertical 

 elevation and depression ; but the Chitons, by the segmental con- 

 dition of their shells, have accorded to them sufficient flexibility 

 to obtain a vermicular motion, and its consequence, a greater 

 facility of march. The Trachelipoda have infinitely superior 

 attributes for motion, by the foot being fixed to a small portion 

 of the body by an elastic cylindrical pedicle that affords a perfect 

 pliability, and the acquisition of the necessary undulatory quality 

 to effect a comparatively facile progression. 



The above remarks perhaps furnish us with the proper value 



