112 THE VITAL FUNCTIONS. 



(g), wliich is tlie glossa, or tongue ; and the aperture 

 seen at its root is the passage into the oesophagus. 

 The sheath is merely the elongated labium, of 

 which the base is seen at l, in Fig. 268 ; but is 

 represented in its whole length in Fig. 269, where 

 the groove for containing the filaments above de- 

 scribed, is apparent. 



In the mouths of the Annelida we often meet 

 with hard bodies, which serve the purposes of jaws 

 and of teeth. The retractile proboscis of the Aphro- 

 dite, or sea-mouse, is furnished with four teeth of 

 this description. The Leech has, immediately 

 within its lips, three semi-circular teeth, with round 

 and sharp cutting edges : they are delineated in 

 Fig. 261, (p. 92), in their relative 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. 



Among the Mollusca, the great majority of those 

 inhabiting bivalve shells are either immoveably 

 fixed to submarine rocks, or embedded in sand or 

 other deposits near the shores of the ocean ; and 

 they derive their nourishment only from the minute 

 organic particles which are suspended in sea water, 

 and are brought to them by currents, either natural 

 or artificially created by the vibrations of cilia, and 

 directed by them towards the entrance of the ali- 

 mentary canal. The mouth, although furnished 



