ANATOMY OF THE COMMON SNAIL. 523 



calcareous laminse, the exterior of the mantle forms a broad edge 

 variously sculptured in different species : but along its central part the 

 shelly plates, generally eight in number, are partially imbedded in its 

 substance ; being, no doubt, secreted by the surface whereunto they are 

 attached. These mollusks, notwithstanding the singularity of their 

 covering, which almost reminds us of the armour of many ARTICULATA, 

 in their internal anatomy conform exactly to the type of structure 

 common to the Gasteropod orders, and offer no peculiarities of organi- 

 zation worthy of special notice. 



(1376.) Feeble and languid as are the sluggish movements of these 

 creatures, they nevertheless present to the eye of the anatomist a type 

 of organization considerably superior to any that we have had an oppor- 

 tunity of considering in such forms of the HETEROGA^GLIATA as have 

 been described in the preceding chapters. From the superiority of their 

 mode of progression, it is evident that they are adapted to enjoy a less 

 limited intercourse with external objects than even the most highly 

 gifted of the burrowing CONCHIFERA ; and accordingly we find in them 

 a nervous system exhibiting a more complete development, senses of a 

 higher character, and, in the organization of their internal viscera, a 

 complexity of parts such as has not heretofore fallen under our notice 

 every indication, in fact, that they are animals of a higher grade and 

 more elaborate structure. The GASTEROPODA, for instance, exhibit a 

 distinct head, in which is lodged a supra-ossophageal ganglion of large 

 proportionate size ; and upon the head are found retractile instruments 

 of sensation of peculiar structure, and not unfrequently perfectly-formed 

 organs of vision. 



(1377.) Let us, however, select one species for particular description; 

 and after having become acquainted with the details of its anatomy, we 

 shall be better prepared to examine such modifications of the various 

 organs as are found in other orders destined to exist under different 

 circumstances. 



(1378.) The common Snails (Helix) are well known as far as relates 

 to their external appearance ; and, insignificant as they might be thought 

 by those unacquainted with their habits, they not unfrequently become 

 formidable pests to the horticulturist, from the ravages caused by their 

 voracity. On examining a Snail more attentively, we find its body par- 

 tially enclosed in a thick muscular envelope composed of transverse and 

 longitudinal fibres, which, being unsupported by any skeleton, allows 

 the shape of the animal to vary at pleasure, as it is shortened or elon- 

 gated by the contractions of the muscles composing it. The foot, or 

 ventral disk, is equally composed of an interlacement of muscular fibres, 

 and not only forms an extensive sucker, but, by the successive action of 

 various portions of its substance, a slow and gliding progressive motion 

 is produced. 



(1379.) From the head of the Snail when its body is expanded, as when 



