CLASSIFICATION OF CRUSTACEA. 5 



above the mouth, from which cords pass forward and backward, and 

 subdivide, in order to supply the posterior segments and their members; 

 and in this particular, the great characteristic which is laid down as 

 dividing the Articulata from the Mollusca, fails of being distinctive, 

 and we find essentially the nervous system of a molluscan with a 

 crustacean structure. 



The higher have large biliary glands ; the lower often no distinct 

 glandular masses of this kind. 



Again, as already observed, while in the higher orders the species 

 have a series of limbs for locomotion and prehension, the members are 

 reduced in the lowest to a single pair, or even this is wanting, a 

 long head with its mouth fitted for suction, and a long tail, making 

 up the body. 



What is, then, a Crustacean ? No definition can be wholly satis- 

 factory. As in other cases, the question must be answered by defining 

 the essential characters of the typical form, and then the relations of 

 the extreme divisions are to be distinguished by gradations of structure, 

 rather than by complete conformity to the type. We observe, how- 

 ever, that there are some invariable characters. All have a straight 

 intestine, without convolutions; all pass through a series of meta- 

 morphoses in development; all undergo exuviation; all have the 

 head and thorax combined essentially in a single cephalothorax ; all 

 are aquatic in their mode of respiration, the surface of certain gill-like 

 organs, or of some or all parts of the body, serving in aeration. 



While, therefore, Crustacea, in their typical forms, have the nervous 

 system of other Articulata, and are thus widely removed from Mol- 

 lusca, they graduate into species that have nearly the nervous 

 system of the latter division. Yet the articulated body, the structure 

 of the mouth, the jointed appendages, the character of the intestine, 

 and the process of exuviation, are decisive characteristics, with few 

 exceptions ; and in the exceptions, the species are elongated and 

 resemble worms rather than Mollusca. 



Again, while related to Insects in the nervous system, they are 

 separated from them by the existence of branchiae, or, if branchiae are 

 absent, by the fact that the surface of the body performs the function 

 of aeration. In other words, while Crustacea are aquatic Articulata, 

 Insects are essentially sub-aerial species. Moreover, the process of 

 exuviation, the structure of the heart, the coalescence of the head and 

 thorax, and the large number of jointed limbs in the typical forms, 



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