CH. XIII] HELIX 285 



is easier to dissect, as a type of the Mollusca. In Lower Canada the 

 genus Helix is not very abundant, and the largest species, Helix 

 albolabrus, is rather small for convenient dissection. Limnaea 

 stagnalis, the large river-snail, is however common and easy to obtain, 

 and its structure is similar in its main outlines to that of Helix. 



The shell is coiled into a spiral form ; the body contained in it con- 

 sists of a visceral hump, coiled like the shell and closely adhering 

 to it, and of a portion which we call the head, neck, and foot, which 

 can be drawn within the opening of the shell if the animal is alarmed, 

 but which under ordinary circumstances is quite outside it. The 

 snail is devoid of anything in the nature of legs, an important 

 character of the Mollusca as contrasted with the Arthropoda, but 



9 



FIG. 127. Helix pomatia. Side view of shell and animal expanded. From 

 Hatschek and Cori. 



I. Mouth. 2. Anterior tentacles. 3. Eye tentacles. 4. Edge of mantle 

 5. Respiratory pore. 6. Anus. 7. Apex of shell. 8. Foot. 



9. Reproductive aperture. 



the part of the body next the ground is a flat muscular surface 

 called the foot. By means of wave-like contractions of the muscular 

 fibres of this organ the snail moves along, always preparing the ground 

 for itself by depositing a layer of slime on it. This slime is poured 

 forth from a gland which opens in front of the foot, just beneath the 

 mouth (14, Fig. 130). The foot is one of the most important organs 

 of the Mollusca; it takes different shapes in the different groups but 

 always assists locomotion. In the pond-mussel, for instance, it is 

 shaped like a wedge, in order to force a path through the soft mud 

 at the bottom of the ponds in which the animal lives. The different 

 shapes which the foot assumes afford the chief basis for the classi- 

 fication of Mollusca. The motion of the foot has been analysed by 

 Prof. Parker. The wave-like contractions which pass backwards over 



