100 CHARACTER OF MOLLUSCA. 



magnificent work of M. Poli aided me no further than by 

 some descriptions and some anatomies useful to my end, and 

 these were confined to the multivalves and bivalves. I have 

 verified all the facts which that able anatomist has furnished 

 me ; and, as I think, have determined with more accuracy 

 the functions of some of the organs. I have also sought to 

 characterize the animals to wlrich the principal forms of 

 shells belong, and to classify these in accordance with the 

 organization of their inhabitants, leaving the ulterior divi- 

 sions of tliem into genera and subgenera to those who devote 

 themselves in particular to this work." 



According to Cuvier there appear to be four general plans 

 or models, if we may so speak, after which all animals seem 

 to have been created. The variations superinduced on these 

 plans may at a first glance seem considerable, and have, in- 

 deed, properly given rise to certain divisions in each of them ; 

 but by whatever name we designate these — classes, or orders, . 

 or legions — a close analysis of their organism demonstrates 

 that the variation has been the result not of any change in 

 the essence of the type, but of a slight or graduated modi- 

 fication of some one of the organic systems, — of the motive 

 organs, of the nervous or circulating apparatus, — to which 

 some parts, not inconsistent with the model, have been added 

 or subtracted. Now, under the second of these four plans 

 (sub -kingdoms) are reduced all the creatures to which the 

 name of mollusca is assigned. The essential character lies 

 in the peculiar arrangement of their nervous system, which 

 consists not of a medullary ganglionated chord, but of some 

 ganglions scattered, as it were, irregularly through the body, 

 and from which nerves issue to its various parts. Hence 

 the mollusca are less symmetrical in general than any other 

 class of beings ;* hence also the poverty of their instinctive 

 faculties, and the deficiency of their muscular activity, for 

 in these respects they are decidedly inferior to insects and 

 annulose worms, and scarcely superior to the radiated ani- 

 mals. They have no skeleton nor vertebral column, so that 

 the muscles are inserted to the skin, which forms a soft en- 

 velope contractile in every direction, and in or upon which 

 are generated, in very many species, hard calcareous plates 

 called shells — analogous, in the opinion of Cuvier, to the 



* " Le corps tend toujours h se composer de parties binaires placees 

 symetriquement des deux cotes d'un plan median ; mais cette symetrie 

 n'est jamais complete, et ce plan, au lieu de se developpcr suivant une ligne 

 droite, tend a decrire une courbe. II en resulte que d'ordinaire la bouche 

 et I'anus sent plus ou moins rapproches, et que le corps, considere dans son 

 ensemble, presente en general un aspect plus ou moins difforme." — Milne 

 Edwards, Elem. de Zoologie, iv. 237. 



