ANNULOSA : CRUSTACEA. 221 



the absence of a certain number of the most posterior rings of 

 the body." According to Dana, however, the abortion of 

 segments, with their appendages, almost always takes place at 

 the posterior end of the cephalothorax. 



In no single example can a general view be obtained of the 

 different segments and their appendages in the Crustacea. 

 " Indeed, the only segment that may be said to be persistent, 

 is that which supports the mandibles, for the eyes may be 

 wanting, and the antennae, though less liable to changes than 

 the remaining appendages, are nevertheless subject to very 

 extraordinary modifications, and have to perform functions 

 equally various. Being essentially and typically organs of 

 touch, hearing, and perhaps of smell, in the highest Decapods, 

 they become converted into burrowing organs in the Scyllaridce, 

 organs of prehension in the Merostomata, claspers for the male 

 in the Cydopoidea, and organs of attachment in the Cirripedia. 

 Not to multiply instances, we have presented to us in ,the 

 Crustacea, probably the best zoological illustration of a class, 

 constructed on a common type, retaining its general character- 

 istics, but capable of endless modification of its parts, so as 

 to suit the extreme requirements of every separate species " 

 (H. Woodward). 



Taking the common Lobster (fig. 95) as a good and readily 

 obtainable type of the Crustacea, the body is at once seen to 

 be composed of two parts, familiarly called the "head" and 

 the " tail," the latter being jointed and flexible. The so-called 

 " head " is really composed of both the head, properly so 

 called, and the thorax, which have coalesced so as to form 

 a single mass, technically called the 

 ' cephalothorax." The so-called 

 " tail," on the other hand, is truly the 

 "abdomen." The various appendages .....^ 

 of the animal are arranged along the ? Ik V 

 lower surface of the body, and consist -/ 

 of the feelers, jaws, claws, legs, &c. s * 



The entire body, with the articu- Fig. 82. Theoretical figure iiius- 



, , . J . , -, tratmg the composition or the 



lated appendages, IS enclosed in a tegumentary skeleton of the 



strong chitinous "shell," or exo- S^t^^^T^ 



Skeleton, and the Cephalothorax IS Tergal pieces; e e Epimeral 



covered by a great cephalic shield g e e S ^ n //^it4a1 

 or plate, which is termed the " cara- pieces ; / p insertion of the 



F ,, * extremities. 



pace. 



Each segment of the body may be regarded as essen- 

 tially composed of a convex upper plate, termed the "ter- 

 gum," which is closed below by a flatter plate, called the 



