1406 CRUSTACEA. 



in its characteristics. It is, however, more remote in habit from the 

 Tetradecapods, than from the lowest Decapods, and is properly a dis- 

 tinct group. Unlike the Decapods and Tetradecapods, there are nor- 

 mally but six annuli devoted to the senses and mouth in the highest 

 of the species, and but five in others, the mouth including a pair of 

 mandibles, and either one or two pairs of maxillae (or maxillipeds). 

 This is an abrupt step below the Tetradecapods. We exclude from 

 these mouth organs the prehensile legs, called maxillipeds by some 

 authors, as they are not more entitled to the name than the prehen- 

 sile legs in Tanais, and many other Tetradecapods, There is an 

 exception to the general principle in a few species. A genus of Cyp- 

 roids has three pairs of maxillae; but this may be viewed as an 

 example of the variations which the type admits of, rather than as an 

 essential feature of it, possibly a result of the process of obsolescence 

 which marks a low grade, as in the Mysidse, whose abdomen by losing 

 its appendages, approximates in this respect to the Brachyural struc- 

 ture, though, in fact, far enough remote. 



The species of the Entomostracan type show their inferiority to either 

 of the preceding in the absence of a series of abdominal appendages, 

 and also in having the appendages of the eighth, ninth, tenth, and 

 eleventh' normal rings, when present, natatory in form. 



The range of size is very great, and this is a mark of their low 

 grade, for in this respect they approach the. Radiata, whose limits of 

 size are remarkably wide. Nearly all of the species, and those which, 

 by their activity, show that they possess the typical structure in its 

 highest perfection, are minute, not averaging over a line in length, or 

 perhaps more nearly three-fourths of a line. 



Taking this as the true expression of the mean normal size of the 

 type, the three primary types will vary in this respect as 24 (two 

 inches) : 6 : 1. 



The size in this third type, reaches its maximum in the Limuli ; 

 and these are unwieldy species, whose very habits show that vegeta- 

 tive growth has given them a body beyond the successful control of 

 its weak system, that is, a larger frame than it has power to wield 

 with convenience or defend, for it is at the mercy even of the waves 

 upon a beach. 



This type has its highest representatives among the Cyclopoids, 

 which remind us of the Mysis group of the higher Crustacea. In 

 these, the cephalic part includes six out of the fourteen cephalotho- 



