GASTEROPODA. 395 



and accordingly we find in them a nervous system exhibiting a 

 more complete developement, 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 indica- 

 tion, 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-oesophageal 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. 



Let us, however, select one species for particular description ; 

 and, after having become acquainted with the details of its ana- 

 tomy, 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. 



(430.) 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 un- 

 frequently become formidable pests to the horticulturist, from the 

 ravages caused by their voracity. On examining a snail more 

 attentively we find its body partially 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 elongated by the con- 

 tractions of the muscles composing it. The foot, or ventral disc 

 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. 



From the head of the snail when its body is expanded, as when 

 in the act of seeking food, four tentacula are protruded, (Jig. 

 195, c, a) which, besides being exquisitely sensitive organs of touch, 

 carry at the extremities of the superior pair two minute but per- 

 fect eyes. When the creature is at rest, the tentacula as well as 

 the eyes are retracted into the visceral cavity by a mechanism 

 hereafter to be noticed. A large proportion of the viscera is en- 

 closed in a turbinated calcareous shell, of sufficient capacity to 

 allow the whole body of the animal to be withdrawn from observa- 

 tion and lodged in its interior. 



The mouth is situated upon the under-part of the head, and, 

 when widely opened, exhibits a cutting instrument of singular 



