52 



COMPARATIVE ZOOLOGY. 



A great advance is shown by the Sea-urchin, whose 

 mouth is provided with five sharp teeth, set in as many 

 jaws, and capable of being projected so as to grasp, as well 

 as to masticate, its food (Figs. 214, 28). 



In Mollusks having a single shell, as the Snail, the chief 

 organ of prehension is a strap-like tongue, covered with 

 minute recurved teeth, or spines, with which the animal 



rasps its food, while the upper lip 

 is armed with a sharp, horny 

 plate (Fig. 29). In many marine 

 species, as the Whelk, the tongue 

 is situated at the end of a retrac- 

 tile proboscis, qr muscular tube. 

 In the Cuttle-fish, we see the sud- 

 den development of an elaborate 

 system of prehensile organs. Be- 

 sides a spinous tongue, it has a 

 pair of hard mandibles, resem- 

 bling the beak of a Parrot, and 

 working vertically ; and around 

 the mouth are eight or ten pow- 



PIG. W'-SnctonrB on the Tentacles f j furnished with llUtner- 



of a Cuttle-fish : a, hollow axis of 



the arm, containing nerve and ar- QUS CUD-Hke SUCkei'S. So perfect is 

 tery; c, cellular tissue; d, radiat- 11 , 



ing fibres; h, raised margin of the adllCSlOU OI tllCSC SUCKCrS, that 

 the disk around the aperture/, g, ., . v u 



which contains a retractile mem- it IS easier to tear away a 111110 



brane, or "piston," i. 



t() 



i The Earth-worm swallows earthy 

 matter and decaying leaves, which 

 it secures with its lips, the up- 

 per one being prolonged. Other 

 worms (as Nereis) are so construct- 

 ed that the gullet, which is fre- 

 quently armed with teeth and for- 

 ceps, can be turned inside out, to FIG. IT. -Nereis -head, with ex- 



i tended proboscis: J. jaws T. 



form a proboSCIS for Seizing prey. tentacles; //, head ; E\ eyes. 



