182 STRUCTURE OF THE MOLLUSCA. 



the pulmonary arteries are distributed; an alimentary canal 

 with two apertures, and consisting of an oesophagus, one or 

 several stomachs, and an elongated intestine ; a nervous 

 system composed of a cerebriform ganglion, placed under or 

 surrounding the cesopbagus, and communicating with the 

 ganglia of the various functions. The organs of sense are 

 generally little developed, those of touch or general sensa- 

 tion however being more perfect. In some, generation is 

 eflfected by mutual impregnation, each individual of the 

 species being hermaphrodite ; in others, the sexes are sepa- 

 rate, some individuals being male, others female ; in others, 

 each individual is bisexual, and produces eggs or young 

 without communicating with another individual. Some are 

 viviparous, but by far the greater number oviparous. 



Those Mollusca which are most highly organized approach 

 more nearly in structure to the Vertebrata than any other 

 invertebrate animals. Yet there is a wide separation be- 

 tween these two great divisions ; for the mode of structure 

 of the most imperfect fish differs essentially from that of the 

 most perfect molluscous animal. 



Many systems or methods of arrangement have been pro- 

 posed by authors for classifying the Mollusca. Although 

 zoology has made vast progress of late years, it is not yet 

 so far advanced as to possess general classifications of the 

 different series of animals admitted to be correct by all or 

 most of those engaged in the examination of some of the 

 various groups. The system of Linnaeus, once generally 

 adopted, was soon found to be insufficient, it not having 

 been based upon an extensive and accurate knowledge of 

 the structure and relations of animals. That of Cuvier, not 

 many years ago nearly as general as that of Linnaeus for- 

 merly was, is also daily undergoing modifications. In each 

 particular branch, as of birds, for example, or reptiles, every 

 author has a system of arrangement of his own. This want 

 of agreement is the necessary result of the imperfect state 

 of our knowledge, and must continue until natural objects 

 be completely investigated and understood. Any attempt 

 to bind the public to any particular system in any depart- 



