CRUSTACEA. 297 



the larval state, — which I have argued to be the typical one*, — of 

 the Ejiizoa and Cirripedia ; in which view we may regard the 

 Crustacea as representing those larvae on a gigantic scale, and so 

 retaining the typical character with the faculty of locomotion. 



We thus arrive at a class of articulated animals in which some or 

 all of the annular segments of the skeleton of the body are constantly 

 provided with articulated limbs or appendages, in which all the species 

 are free and locomotive, and are provided with distinct respiratory- 

 organs. These animals are still, however, aquatic ; only a part of the 

 class can support themselves and move with their jointed limbs on 

 dry land ; the highest act of locomotion is that of climbing reeds or 

 trees, which a few species of the present class are enabled to effect 

 by long prehensile claws. But the breathing organs in all the species 

 are organised for aquatic respiration ; in other words, are branchiae j; 

 and it is the combination of branchiae with jointed limbs and distinct 

 sexes which constitutes the essential character of the class Crustacea. 



The name of this class refers to the modification of the external 

 tegument by which it acquires due hardness for protecting the rock- 

 dwelling marine species from the concussion of the surrounding 

 elements and from the attacks of enemies, and for forming the level's 

 and points of resistance in the act of supporting the body and moving 

 along the firm ground. The animal basis of this external skeleton is 

 the same peculiar tissue as in the two foregoing classes, viz., chitine, 

 which is insoluble in caustic potash, and is charred, not consumed, by 

 fire. In the crab and lobster tribes the integument is hardened by 

 the addition of earthy particles, consisting of the carbonate with a 

 small and varying proportion of the phosphate of lime. This crust is 

 coloured by a pigmental substance, diffused more or less irregularly- 

 through its outer surface : its basis being a vascular organised mem- 

 brane, or corium. The crust exhibits a minutely tubular structure f, 

 is covered by a thin chitinous layer composed of hexagonal cells |, 

 and it is lined by a thin fibrous membrane, which plays an important 

 part in the moulting process. The tubercles and bristles which beset 

 many parts of the shell are prolongations of it, or its basis — chitine. 

 In the smaller Crustacea the tegument retains a flexible pergameneous 

 character, and is usually so transparent as to allow the internal 

 coloured parts to appear through it. 



Whatever be the consistence of the external integument or skeleton, 

 it is always disposed in a series of segments, either actually separate 

 and moveable on each other, or confluent in a variable extent and 



* A character indicative of type is not to be confounded with the sum of cha- 

 racters determinative of class. 



t XXIV. p. 420. X CCXXXl. vol. ii. p. 393. 



