594 HISTORY OF SYSTEMS. 



ton.* Hence M. Blanchard infers that to this sub-class 

 must be assigned the first or highest rank amongst the gas- 

 teropod mollusca a conclusion to which Milne -Edwards had 

 also come from embryogenic considerations. You may rea- 

 sonably falter to follow them in these views when you call to 

 recollection the many affinities that bind the pectinibranchial 

 Zoophaga with the tetrabranchiate Cephalopods ; and there 

 are structural peculiarities in the circulating system, and in 

 the form of the gills, that would lead us to believe that the 

 Opistobranches cannot, in a natural system, be far dissoci- 

 ated from the Bivalves. To assert that the centralisation of 

 the nervous system disproves their right to this lower posi- 

 tion is to assume the question at issue, for the importance 

 justly attachable to that character must depend on its being 

 correlative with an equal elevation in other systems, and 

 on its coincidence with the animal's general economy and 

 habits.f 



Of the Mollusca which enter into the composition of the 

 Opistobranches, none have attracted greater attention than 

 the Nudibranches. Linnaeus knew imperfectly six or seven 

 species only of this order ; Miiller added several from the 

 shores of Denmark to the number ; and Colonel Montagu 

 augmented the catalogue with some English supplies. His 

 example was followed by Dr. Leach in England, by Profes- 

 sor Jameson in Scotland, and by Dr. Fleming in Zetland ; 

 but Miiller had more ardent followers in Nordmann, Sars, 

 Loven and Kolicker, who began to mark their progress by 

 discoveries in the structure and development of the animals. 

 Cuvier, however, had long before worked the same field, 

 and with his usual superiority and success ; for his anatomies 



* " On taking a review of the nervous system of Eolis, we are at once 

 struck with the high grade of development, and with the symmetrical ar- 

 rangement that obtains in it ; the heterogangliate character applicable to 

 many gasteropodous mollusks being, so far as our researches have led us, inap- 

 plicable to this more elevated being. The nervous centres are closely concen- 

 trated around the oesophagus, and there exists a sufficient correspondence 

 between them and the same organs in the Cephalopoda to enable us confi- 

 dently to compare them ; indeed we have every reason to think that we 

 recognise in them the homologues of the principal masses of the nervous 

 centres of the vertebrata." Ann. and Mag. N. Hist. Ser. 2. iii. 191. 



f In relation to the position of the Chitonidse M. Emile Blanchard has 

 made a similar remark : (e Mais les caracteres les plus gene'raux n'etant 

 presque jamais absolus, on peut se me*prendre facilement, si Ton s'en tient 

 a la consideration d'un seul fait, et non pas a tout 1'ensemble de 1'organisa- 

 tion." Ann. des Sc. Nat. ix. (1848), 184. Blanchard's very interesting 

 papers on this order are contained in this and in the eleventh volumes. They 

 are not yet completed. From the disposition of their nervous ganglia, 

 M. Blanchard restores the Chitonidse to the place usually assigned them 

 near the Patella and Haliotis. Du. Syst. Nerv. chez les Invertebres, p. 9. 



