ANNULOSA : CRUSTACEA. 173 



' 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 specimens of full-grown Crustaceous animals whose 

 bodies present fewer than twenty-one segments, that the ano- 

 maly 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 antenna3, 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 Deca- 

 pods, they become converted into burrowing organs in the 

 Scyllaridce, organs of prehension ia the Merostomata, claspers 

 for the male in the Cyclopoidea, 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 

 characteristics, but capable of endless modification of its parts, 

 so as to suit the extreme requirements of every separate spe- 

 cies.' (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 

 in a strong chitinous 'shell,' or exoskeleton, and the cephalo- 

 thorax is covered by a great cephalic shield or plate, which 

 is termed the ' carapace.' 



Each segment of the body may be regarded as essentially 

 composed of a convex upper plate, termed the ' tergum,' which 

 is closed below by a natter plate, called the ' sternum,' the line 

 where the two unite being produced downwards and outwards, 

 into a plate, which is called the ' pleuron,' or ' pleura ' (Jig. 49, 2) . 



Strictly speaking, the composition of the typical somite is considerably 



