118 THE VITAL FUNCTIONS. 



Of those instruments of prehension which are 

 not portions of the mouth itself, and which form 

 a series of variously constructed organs, extend- 

 ing from the tentacula of the polypus to the 

 proboscis of the elephant, and to the human 

 arm and hand, some account has already been 

 given in the history of the mechanical functions ; 

 butj in a great number of instances, prehension 

 is performed by the mouth, or the parts which 

 are extended from it, and may be considered as 

 its appendices. The prehensile power of the 

 mouth is derived principally from the mecha- 

 nical form and action of the jaws, which open to 

 receive, and close to detain the bodies intended 

 as food; and to this latter purpose, the teeth, 

 when the mouth is furnished with them, likewise 

 materially contribute ; although their primary 

 and more usual office is the mechanical division 

 of the food by means of mastication, an action 

 in which the jaws, in their turn, co-operate. 

 Another principal purpose effected by the jaws 

 is that of giving mechanical power to the 

 muscles, which, by acting upon the sides of the 

 cavity of the mouth, tend to compress and 

 propel the contained food. We find, accord- 

 ingly, that all animals of a highly developed 

 structure are provided with jaws. 



Among the animals which are ranked in the 

 class of Zoophytes, the highest degrees of deve- 

 lopement are exhibited by the Echinodermata, 



