GASTEROPODA. 403 



arises from the general muscular mass composing the foot and re- 

 tractile apparatus provided for drawing the snail into its shell : the 

 long slip of muscular fibres so derived, accompanied by the optic 

 nerve (/), traverses the interior of the cylindrical tentacle quite to 

 its extremity, where it is attached ; and thus, as the reader will 

 easily conceive, is quite competent to cause its inversion. The 

 lower feeler (d) is represented in the figure as partly retracted by 

 the action of its appropriate muscle k ; while the corresponding 

 one (a), being completely turned inside out, is fully withdrawn 

 and securely packed among the viscera. 



One circumstance connected with the contrivance above describ- 

 ed cannot but excite attention ; and this is the peculiar arrange- 

 ment of the tentacular nerves, whereby they are adapted to 

 changes of position so extensive: the optic nerve (jf), for ex- 

 ample, must not be stretched even when the eye-bearing tentacula 

 are protruded to the uttermost ; and in order to provide for this, 

 when the feelers are not extended, the nerves become thrown into 

 close folds (A), and lodged within the cavity of the body. 



(440.) From the above somewhat lengthened account of the ana- 

 tomy of the snail, the reader will at least have been able to become 

 acquainted with the general features of an organization which is 

 more or less common to all the members of the extensive class 

 under consideration. 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 circum- 

 stances under which particular tribes or families are destined to 

 exist. 



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 net-work ; 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 PULMOBRANOHIATA. Ne- 

 vertheless, all the pulmobranchiate Gasteropoda are not terres- 

 trial ; our fresh waters abound with various species that respire 

 air by a similar contrivance, and are consequently obliged, in 



2 D 2 



