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ratus is necessarily adapted to the different media they inhabit, the corres- 

 ponding conditions of the respiratory organs, together with certain modifica- 

 tions in their arrangement, have been selected as characters for the primary 

 distribution of the class into Orders ; before speaking, however, of the sys- 

 tematic arrangement of the Gastropods, a brief account should be given of 

 their structure and general economy. 



The Jiead, as in the higher orders of animals, forms the anterior extremity 

 of the body, and is mostly rounded and prominent. It is furnished with 

 from two to six tentacles, or feelers, and the eyes, never exceeding two in 

 number, are situated sometimes at the base of the tentacles, in some in- 

 stances at the summit, and often at some intermediate part. The tentacles 

 are rather sensitive to the touch, and in some genera the mollusk pos- 

 sesses the faculty of withdrawing them by inversion, an action which 

 Lamarck describes as being accomplished by the aid of a nerve reaching 

 internally to the summit. 



The nervous system of the Gastropods is represented by three ranges of 

 cords or ganglions, termed the cerebral, as relating to the head, the pallial, 

 as relating to the mantle, and the branchial, as relating to the gills ; the 

 principal of these, the cerebral ganglions, seated in the head and vicinity of 

 the oesophagus, or gullet, and called on that account by M. Deshayes the 

 oesophageal circle, was termed by Lamarck the brain ; but their relation 

 with that organ in the vertebrate animals is one of very remote analogy. 



The respiratory apparatus of the class consists, in the water-breathing 

 kinds, of two or more branchiae or gills, sometimes exposed, but mostly con- 

 cealed ; in the air-breathing kinds it consists of a net-bag or lung; and in 

 those which are amphibious it presents a modification of both. 



As the breathing organs of the Gastropods are necessarily modified to 

 the different media they respire, so are the mouth and alimentary organs 

 adapted to the diversified nature of the food they devour. Most of the terres- 

 trial kinds are herbivorous, feeding upon leaves ; of the large proportion of 

 marine species some few are fucivorous, feeding upon seed-weed, whilst the 

 rest are carnivorous, devouring many of their own nature and other living 

 organized matter, besides offal in all stages of putrifaction. The mouth 

 of the herbivorous kind is furnished with a horny armature on the upper 

 lip only ; in the carnivorous species, the mouth is furnished with a rasping 

 plate or tongue, or a pair of dentated jaws, or a flexible retractile trunk, 

 susceptible of elongation or concealment within the body, the extremity of 

 which is cleft and supplied with numerous small recurved teeth capable of 

 considerable execution. The common Whelk of our market has a retractile 

 trunk ; and the circular hole which is sometimes found drilled in bivalve 

 shells is supposed to be due to the agency of this destructive organ. 



Of the muscles it is only necessary for the present purpose to speak of 

 such as serve for the attachment of the shell. In the simple univalves, the 



