342 LECTURE XV. 



form abdominal feet ; it possesses neither the squamous tail nor the 

 complex stomach of the mature creature. 



The cumulative evidence of the metamorphoses of Crustacea can 

 no longer be rejected ; but their modifications in different genera, 

 and the number of the exceptions to the law, like that presented by 

 the Astacus JluviatiUs, are yet to be determined. Here, therefore, 

 is an ample field open to the researches of the original observer; a 

 field which must be diligently and extensively cultivated before it can 

 yield the fruits of true generalisations as to the extent, the nature, 

 and varieties of the metamorphoses in the class of articulate animals 

 which support their bodies on jointed limbs and breathe by gills. 



Sufficient has been observed to show, that if certain stages of the 

 development of a higher Crustacean were arrested, and growth alone 

 proceeded with, an animal would result having the characters of the 

 Crustacea of an inferior order. The Crab is "anomourous" before 

 it becomes brachyurous ; at an earlier period it is "macrourous," and 

 it is "edriophthalmous" before it becomes " podoplithalmous ;" all 

 which stages typify the successive forms of the Crustacea, as they 

 were introduced into this planet. The entomostracous characters 

 were never overpassed by the Crustacea anterior to the coal measures, 

 and the type of the Macroura did not begin to be departed from 

 until the pei'iod of the deposition of the chalk. All the decapod 

 Crustacea are, at first, macrourous, or manifest the Oolitic type ; and 

 all Brachyura pass through the anomourous or cretaceous type 

 before tlie proper bi'achyurous or tertiary character is finally ac- 

 quired. But these resemblances are general, and by no means exact in 

 specific details. No extinct species could be reproduced by arresting 

 the development of any known existing species of Crustacea ; and 

 every species of every period was created most perfect in relation 

 to the circumstances and sphere in which it was destined to exist. 



Class CRUSTACEA. 



Body articulated, with articulated limbs : head with antenna3 : 

 branchial respiratory organs. Sexes distinct. Metamorphosis in 

 most ; in none resulting in fixed individuals. 



Subclass ENTOMOSTRACA. 



Body with more or fewer segments than fourteen ; integument 

 chitinous. Eyes sessile. 



Order Trilobites. 

 Abdominal segments trilobed : limbs rudimentary, lamelliform, 

 probably branchial : sessile compound eyes in most. 



