TAYLOR : niOLOGY OF TUn, MOLLUSCA. 269 



The study of the variation of the animal and its shell is very 

 important, for in these variations we see incipient species, and they are 

 the visible tokens of the progress of evolution in the changes they are 

 undergoing, unless they are atavic and thus reflect some past stages 

 in the history of the species. Many naturalists view the shell merely 

 as an exquisitely formed and beautifully marked object; but to the 

 thoughtful student each species bears an impress upon its shell which 

 unmistakably denotes its habits and mode of life. 



A region like the Channel Isles, markedly deficient in limestone, is 

 exemplified by the shells of this species being small, thin, and trans- 

 parent, and sometimes weighing not more than four grains, while from 

 a suitable region shells are sometimes found weighing 120 grains or 

 even more ; and it is fully recognized that every well-marked district 

 necessarily imparts to the shells and other forms of life permanently 

 resident there a certain general character or facies, which will be in 

 harmony with the geological, climatal, or other conditions to which 

 they are exposed, and the more widely diverse in character the dis- 

 tricts are, the more divergent are the shells found therein. 



To facilitate the study of variation, it is found necessary to classify 

 the various modifications to which the shell is subject, distinguishing 

 by special names those of the most striking character, and in thi^ way 

 many beautiful forms have become known to science. 



The Internal structure of this snail is full of interest, and I will 

 now briefly summarise the more salient features of its organizatioii, 

 and describe some of their physiological functions. 



The Nervous system, upon which all sensibility and motion are 

 dependent, is composed of several paired medullary masses, or ganglia, 

 which are closely adherent to each other, and surround the throat, 

 the cerebral ganglia, or brain, being formed by paired masses of nerve 

 substance above the gullet, from which all voluntary muscles and 

 sense organs are innervated. 



The organs for the sense of Orientation and the perception of sounds 

 or vibrations exist as a pair of sacs placed upon the pedal ganglia and 

 surrounded by a network of nerves which connect with the brain. 

 Within the sacs are large numbers of minute oval bodies which dur- 

 ing life are in incessant motion or oscillation, due to the action of the 

 cilia lining the sac, but in addition there are some long sensory hairs 

 which are connected with and convey the impressions of sound or 

 direction to the brain. 



Snails are generally thought to be mute, and really are so, but 

 under certain circumstances, as when crawling over a thin sheet of 

 glass or other suitable vibratory substance, they can produce musical 

 sounds analogous to those produced by. drawing a moist finger along 



