ANNULOSA: CRUSTACEA. 193 



and others in which the number of somites is apparently less. 

 In these latter cases, however, the apparent diminution in the 

 number of segments is really due to some having been fused 

 together, as is shown by the number of appendages, since each 

 pair of appendages indicates a separate somite. In other cases, 

 however, in which the number of somites is really less than the 

 normal, this is due to an arrest of development. According to 

 Milne-Edwards : 



"In the embryo these segments are formed in succession 

 from before backwards, so that, when their evolution is checked, 

 the later, rather than the earlier, rings are those which are 

 wanting \ and, in fact, it is generally easy to see in those speci- 

 mens of full-grown Crustaceous animals whose bodies present 

 fewer than twenty-one segments, that the anomaly depends on 

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

 the body." 



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 ex- 

 traordinary modifications, and have to perform functions equally 

 various. Being essentially and typically organs of touch, hear- 

 ing, and perhaps of smell, in the highest Decapods, they 

 become converted into burrowing organs in the ScyUarida^ 

 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 Crus- 

 tacea probably the best zoological illustration of a class, con- 

 structed on a common type, retaining its general characteris- 

 tics, 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 as a good and readily obtain- 

 able type of the Crustacea, the body is at once seen to be com- 

 posed 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 ap- 

 pendages of the animal are arranged along the lower surface of 

 the body, and consist of the feelers, jaws, claws, legs, &c. 

 The entire body, with the articulated appendages, is enclosed 



VOL. I. N 



