214 POPULAR HISTORY OF THE AQUARIUM. 



the tyxncal structure of any group leing given, the different 

 habits of its component species or minor groups are provided 

 for, not hy the creation of neio organs or the destruction of 

 others, hut ly the modification in form, structure, or place, 

 of organs typically lelo7iging to the group.'' 



We learn, then, that there is a normal or theoretic num- 

 ber of parts which would compose a perfect or beaic ideal 

 crustacean, but which are not all distinguishable in any 

 single species. Some are fully developed in one and alto- 

 gether wanting in another form ; some are soldered toge- 

 ther in one order so as to be indistinguishable as separate 

 parts, while in another order they are distinct. 



" In order to give a general idea" of the manner in which 

 the various segments and appendages are modified in diffe- 

 rent species, Mr. Bell remarks, " that the ocular peduncles 

 are the only appendages which are never devoted to any 

 but their normal objects. The antennse are, as has been 

 before observed, sometimes modified into locomotive organs. 

 The cephalic appendages about the mouth, the mandibles, 

 and mamillse, are sometimes rudimentary, at other times they 

 are modified into mere organs of apprehension. The tho- 

 racic members are sometimes locomotive organs, at others 

 they subserve the nutritive function : the remaining thoracic 



