CLASSIFICATION OF CRUSTACEA. 13 



nerves of the body without other ganglions in their course ; 5, the 

 absence of branchiae, or any organs especially fitted for the purpose, in 

 most, species, and in cases where branchia-like appendages exist, they 

 being only an adaptation to this function of some portion of a tho- 

 racic leg. 



The species differ among themselves in number of segments, which 

 in a few instances is largely multiplied, and in others, reduced to 

 four or five, or even less ; in the size of the anterior shell or carapax, 

 which may be confined to the head, or be so enlarged as to enclose 

 like a shell, the whole body; in the number of legs, which varies 

 between a single pair, or even none, and fifty pairs or more ; in the 

 number of organs devoted to the mouth, from a single pair of mandi- 

 bles to mandibles with three pairs of maxillae or maxillipeds which 

 may either be regular jaws, as in the higher Crustacea; or, may 

 be imbedded, the basal joints of a series of legs acting as jaws; or, 

 may project and form a moveable trunk, with slender, spiculiform 

 organs for mandibles. 



Of these differences, the last mentioned is of the widest importance. 

 The trunk-form or sucker mouth characterizes a large number of 

 species, which constitute a natural group, among the Entomostraca; 

 and through these species, the class of Crustacea declines into the 

 more degraded class of worms. 



The mouth with jaws formed by the bases of a series of pairs of 

 legs, affords a less important distinction. The aspect and structure in 

 such species are peculiar, as observed in the Limuli, where this kind of 

 mouth is in perfection ; but, the mandibles are as much a pair of legs 

 in the Cypris, and, indeed, they are tne largest and strongest pair in 

 these species ; moreover, in the Cyclops, the jointed or pediform por- 

 tion of the mandibles and maxillae is largely developed, as already 

 remarked. The Limuli are, therefore, but an example of the same 

 principle, more perfectly carried out. Still, this may be a sufficient 

 ground for placing these species in a separate subdivision of the 

 Entomostraca, although not authorizing a wide separation from the 

 Order. 



The Entomostraca are, therefore, distributed here into three groups, 

 as follows : — 



Sub-order I. Gnathostomata (from ym&>«, jaw, and mp*, mouth). 

 The mouth with regular jaws, and not forming a moveable trunk. 



4 



