ANNULOSA: CRUSTACEA. 2/7 



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 latter, 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." According to Dana, however, the abortion of seg- 

 ments 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 Cyclopoidea, and organs of at- 

 tachment in the Cirripedia. Not to 

 multiply instances, we have presented 

 to us in the Crustacea probably the , ....^ 

 best zoological illustration of a class, A Ik v Jj^ 

 constructed on a common type, retain- -^ f 



ing its general characteristics, but ca- s s 



pable of endless modification of its F %4S^^S5S of 



partS, SO aS tO Suit the extreme re- the tegumentary skeleton of 

 L f the Crustacea (after Milne-Ed- 



quirements of every separate species wards). D, Dorsal arc : / t 



(V( Wnrrlwnrrl\ Tergal pieces ; ee Epimeral 



\\. Woodward). pieces. V, Ventral arc: ** 



Taking the Common Lobster (fig. Sternal pieces ;//Episternal 



136) as a good and readily obtainable St^ Insertion of the 

 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, pro- 

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



