CLASSIFICATION OF THE GASTEKOPODA. 



531 



acquainted with the general features of an organization which is more 

 or less common to all the members of the extensive class under consider- 

 ation. We must now, however, enter upon a more enlarged survey of 

 the GASTEROPODA, and divide them into such groups as will facilitate 

 our further investigations concerning their structure and habits. The 

 most convenient character by which the different orders composing the 

 class are distinguished has been found to be derived from the nature 

 and arrangement of the respiratory apparatus, which of course varies 

 both in construction and position, according to the circumstances under 

 which particular tribes or families are destined to exist. 



(1403.) We have already found that terrestrial species, such as the 

 Snail, breathe air, which is alternately drawn into and expelled from a 

 cavity lined with a vascular network ; and these, from the resemblance 

 between such a mode of breathing and that of animals possessed of 

 proper lungs, have been formed into an order distinguished by the name 

 of PULMOBRANCHIATA. Nevertheless all the pulmobranchiate Gastero- 

 poda are not terrestrial ; our fresh waters abound with various species 

 that respire air by a similar -p- ^QQ 



contrivance, and are conse- 

 quently obliged, in order to 

 breathe, to come continually 

 to the surface of the shallow 

 pools wherein they are found. 

 The Planorbis and Limnceus 

 are examples of this mode of 

 respiration, and are met with 

 in every ditch, where they 

 voraciously devour the sub- 

 aquatic vegetables upon which 

 they feed. 



(1404.) It is at once evi- 

 dent that in marine Gaste- 

 ropods another mode of aera- 

 ting the blood must be re- 

 sorted to, and branchiae, of 

 some description or other, 

 substituted for a pulmonary 

 cavity. 



(1405.) The branchiaB given 

 for this purpose are variously 

 constructed sometimes ap- 

 pearing as extensively branch - 

 ed and arborescent append- 

 ages to the skin, or else they form broad and thin lamellae attached to 

 the exterior of the body ; but more frequently the respiratory apparatus 



Doris. 



