126 THE VITAL FUNCTIONS. 



tive positions ; and Fig. 262 represents one of 

 the teeth detached from the rest. It is with these 

 teeth that the leech pierces the skin of the 

 animals whose blood it sucks ; and as soon as 

 the wound is inflicted, the teeth, being moveable 

 at their base, fall back, leaving the opening of 

 the mouth free for sucking. The wound thus 

 made is of a peculiar form ; being composed of 

 three lines, radiating from a centre, where the 

 three teeth had penetrated. 



Most of the Mollusca which inhabit univalve 

 shells are provided with a tubular organ, of a 

 cylindric or conical shape, capable of elongation 

 and contraction, by circular and longitudinal 

 muscular fibres, and serving the purpose of a 

 proboscis, or organ of prehension for seizing and 

 conveying food into the mouth. These tubes 

 are of great size in the JBuccinum, the Murex^ 

 and the Voluta ; as also in the Doris, which, 

 though it has no shell, is likewise a gasteropode. 

 In those mollusca of this order which have not a 

 proboscis, as the Limax, or slug, the Helix, or 

 snail, and the Aplysia, or sea-hare, the mouth 

 is furnished with broad lips, and is supported by 

 an internal cartilage, having several tooth -like 



270 projections, which assist in laying hold 

 of the substances taken as food. That 

 of the snail is represented in Fig. 270. 



All the Sepice, or cuttle fish tribe, are fur- 

 nished, at the entrance of the mouth, with two 



