592 HISTORY OF SYSTEMS. 



unite them in one and the same order, to which lie gives tlie 

 name of Prosobranches. 



As to the Chitons, it is difficult to say where tliey can be 

 placed most naturally in the system. Cuvier and Lamarck 

 have concluded them to be affined to the Patellse, — a deci- 

 sion in which concologists in general have acquiesced ; but 

 Blainville, being of opinion that they are not mollusks forms 

 a class of them amongst annulose animals. To justify this 

 view, he reminds us of the peculiar disposition of the valves 

 of the Chitonidse, which prompts a comparison with the seg- 

 mented character of the Annelides ; and of the position of 

 the vent opposite to the mouth, which is likewise an annulose 

 and not a molluscan structure. Milne-Edwards adds that the 

 reproductive organs, while they differ essentially from those 

 of Gasteropods, resemble those that are found in the Anne- 

 lides. In the Gasteropods these organs are always unjDaired 

 and asymmetrical as well interiorly as in the position of their 

 external orifices ; but in the Chitons they are arranged alike 

 on each side of the mesial line, with a couple of orifices simi- 

 lar to those of the Crustacea. The disposition of the circu- 

 lating apparatus tends equally to alienate the Chitons from 

 the Gasteropods, and to approximate them to the articulated 

 animals, for the heart simulates a dorsal vessel, and has a 

 structure very different from that of any normal Gasteropod. 

 In fact, every thing in the organisation of the Chitonidae 

 aj)pears to indicate a tendency to a bilateral distribution of 

 the organs regulated by a straight line ; while in the Gastero- 

 poda the body as a whole, and in its parts, seems to have been 

 modelled on a curved line. Thus the opinion of Blainville 

 would appear to be more correct than that of Cuvier, as to 

 the rank of the Chitons ; but to solve the question it is first 

 necessary to know the phases of these animals in their de- 

 velopment: for it is their embryotic condition that can alone 

 inform us if they are descendants from the Mollusca which 

 have received some features of the annelidan race ; or if they 

 rather descend from the Annelides and ape only in part the 

 habit of the Mollusca. Yet, however this question may be 

 answered, it seems impossible to retain the Chitons au}^ 

 longer amongst normal Gasteropods : and were it decided to 

 attach them to the Mollusca, it would be necessary to form 

 with them an order apart, or rather to detach them as a small 

 satellitious group, pendant from the typical body, without 

 just being a constituent portion of it. Mac Leay would call 

 it an osculant group ; but Milne-Edwards, more fanciful than 

 he, would liken it to a satellite that occupies a thinly- 

 studded field in space, removed far away from the closely 



