/. D> Dana on the Classification of Crustacea. 23 



of the Tetradecapods. There is thus another analogy between 

 this group and the Anomoura. 



The Trilobita probably belong with this second type, rather 

 than the Entomostracan. Yet they show an abberrant character 

 in two important points. First, the segments of the body are 

 multiplied much beyond the normal number, as in the Phyllopoda 

 among the Entomostraca ; and Agassiz has remarked upon this 

 as evidence of that larval analogy which characterizes in many 

 cases the earlier forms of animal life. In the second place, the 

 size of the body far transcends the ordinary Isopodan limit. 

 This might be considered a mark of superiority ; but it is more 

 probably the reverse. It is an enlargement beyond the normal 

 and most effective size, due to the same principle of vegetative 

 growth, which accords with the inordinate multiplication of seg- 

 ments in the body. 



The third primary type (the Entomostracan) includes a much 

 wider variety of structure than either of the preceding, and is 

 less persistent in its characteristics. It is, however, more remote 

 in habit from the Tetradecapods, than from the lowest Decapods, 

 and is properly a distinct group. Unlike the Decapods and Te- 

 tradecapods, there are normally 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 prehensile legs in 

 Tanais, and many other Tetradecapods. There is an exception 

 to the general principle in a few species. A genus of Cyproids 

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

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

 essential feature of it, — possibly a result of the process of obso- 

 lescence which marks a low grade, as in the Mysidae, whose 

 abdomen by losing its appendages, approximates in this respect 

 to the Brachyural structure, 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 Eadiata, 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 avera- 

 ging over a line in length, or perhaps more nearly three-fourths 

 of a line. 



