THE SNAIL. 203 



thing is one-sided, and as if the body had been trodden upon 

 and flattened out unequally, in such away as to force the whole 

 right half up toward the back. 



In some kinds of oysters, e. g\, Gryphaea, the left valve is so 

 deep, and so much curved upon itself, in fact, partially rolled up 

 spirally, as to leave but little of the body to be covered by the 

 right valve. In this respect they approximate the form of the 

 shell of the Snail, and the included body is likewise modified 

 so as to conform to this configuration. 



In the Snail, (Helix,) the one-sided development of the body 

 is carried to the highest pitch of asymmetry that is observable 

 among Mollusca. It commences its career with a perfectly 



ft 1 fh k pv in 1 ow 1 ac 1 sff 



ac ao n/j ma st og n o g c m 



Fig. 122. 



symmetrical body, but ere long the right side outgrows the 

 other, and finally the whole organism has the spiral conformation 

 represented here (figs. 122, 123). All that is usually seen of a 



Fig. 122. Helix albolabris. Say. Diagramic representation of the common 

 Snail. 2 diarn. ac, ac 1 , the abdominal cavity ; sh, the shell ; t, the larger pair of 

 feelers, with an eye {e) at the tip of each ; <l, the smaller pair of feelers ; /, 

 mouth; st, stomach; in, intestine; in 1 , posterior opening of in; sg, the superior 

 ganglion of the head ; g, the inferior, or sub-oesophageal ganglion ; c, the nervous 

 collar ; n, ng, the foot nerves ; og, the oesophageal, or gullet nerves ; h, the auricle, 

 and A 1 the ventricle of the heart ; ao, the aorta, or main artery ; pv, vein from the 

 lung, or pulmonic vein ; ov, the ovary, or egg-bearing organ ; ov 1 , the oviduct, 

 or emptying conduit of ov ; o, exterior aperture of ov 1 ; r, the fertilizing gland, 

 or male element of the reproductive organs ; ma, the matrix. Original. 



