CLASSIFICATION OF CRUSTACEA. 14Q5 



short compact bodies, and abbreviated abdomen of the Isopods, are 

 proofs of their superiority of grade. The abdominal appendages are 

 simply branchial, and in the higher species are naked or non-ciliated 

 lamellae. The transitions to a lower grade are seen in the elongation 

 of these abdominal lamellae, their becoming ciliated, and the abdomen 

 being also more elongated and flexible; then in the abdominal lamellae 

 becoming elongated natatory appendages, and the abdomen taking a 

 length usually not less than that of the thorax, as in the Amphipods, 

 in which the branchiae are appendages to the thoracic legs. And 

 while this elongation goes on posteriorly, there is also anteriorly an 

 enlargement of the antennae, which in the Amphipoda are usually 

 long organs. There are thus two secondary types of structure among 

 the Tetradecapods, as among the Decapods ; a transition group be- 

 tween, analogous to the Anomoura, partakes of some of the characters 

 of both types, without being a distinct type itself. These are our 

 Anisopoda. The species graduate from the Isopod degree of perfec- 

 tion to the Bopyri, the lowest of the Tetradecapods. There is thus 

 another analogy between this group and the Anomoura. 



The Trilobita probably belong with the second type, rather than 

 the third. Yet they show an aberrant character in two important 

 points. First, the segments of the body 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 segments 

 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 



* Prof. Guyot very happily names the three great periods of geological history — usu- 

 ally denominated the Palaeozoic, Secondary, and Tertiary, or, by Agassiz, the age of 

 Fishes, that of Reptiles, and that of Mammals, — the Vegetative, the Motorial, and the 

 Sensorial epochs; — the first, being the period characterized prominently by vegeta- 

 tive growth in animal life ; the second, by the increased development of the muscular 

 system, as exemplified by the enormous reptiles of the epoch ; the third, by the develop- 

 ment of the higher functions of the brain, exhibited in the appearance of mammals. 



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