90 THE VITAL FUNCTIONS. 



propel tlifi contained food. We find, accordingly, that all 

 animals of a highly developed structure arc provided with 



jaws. 



Among the animals which are ranked in the class of Zoo- 

 phytes the highest degrees of development arc exhibited 

 by the Kchinodermata, and in them we fmd a remarka- 

 l)le jM'rfection in tlu; organs of mastication. The mouth of 

 the Echinus is surroumled hy a frame-work of shell, con- 

 sisting of five converging pieces, each armed with a long 

 tooth; and for the movement of each part there are provided 

 sei)arate muscles, of whicli the anatomy has been minutely 

 described by Cuvier. In the shells of the echini that arc 

 cast on the shore, this calcareous frame is usually found en- 

 tire in the inside of the outer case; and Aristotle having 

 noticed its resemblance to a lantern, it has often gone by the 

 whimsical name of the lantern of Jiristotlc. 



In all articulated animals which subsist on solid aliment, 

 the apparatus for the prehension and mastication of the food, 

 situated in the mouth, is exceedingly complicated, and ad- 

 mits of great diversity in the dillcrent tril)es; and, indeed, 

 the number and variety of the parts of which it consists is 

 so great, as hardly to admit of being comprehended in any 

 general description. In most insects, also, their minuteness 

 is an additional obstacle to the accurate observation of their 

 anatomy, and of the mechanism of their action. The re- 

 searches, however, of Savigny,* and other modern entomo- 

 logists, have gone far to prove, that, amidst the infinite va- 

 riations observable in the form and arrangement of the se- 

 veral parts of these organs, there is still ])reserved, in the 

 general plan of their construction, a degree of uniformity 

 quite as great as that which has been remarked in the fabric 

 of the vertebrated classes. Not only may we recognise, in 

 every instance, the same elements of structure, but we may 

 also trace regular chains of gradation, connecting forms ap- 



• Sec l)is '♦Thcoric cks Organcs de la bouche des Aniniaux invcrtcbrcs et 

 articiiles," which forms the first part of the "Mcnioircs sur Ics Animaux 

 sans vcrtt'brcs." Paris, 181G. 



