The Crayfish. 33 



value of a segment. In the same way the body ends anteriorly 

 in the projecting rostrum. The crayfish, enclosed as it is in this 

 rigid " skin," could not grow were it not for the fact that the 

 shell is shed at intervals, a new cuticle having been in the 

 mean time formed beneath the old one. 



Each segment of the crayfish's body is provided with a 

 pair of appendages, which are not invariably locomotive, but 

 perform, as will be seen, various functions. It will be con- 

 venient to commence with one of the abdominal swimmerets 

 which shows the limb, it is believed, in its simplest form. 



The third abdominal appendage is shown in Fig. 15, E. It 

 consists of a basal portion, with two branches arising from this ; 

 the basal part is termed the protopodite, its two distal branches, 

 according to their position with reference to the body, the 

 exopodite and endopodite. The limb is thus a biramous 

 structure, shaped like a Y. The protopodite really consists 

 of two separate calcareous pieces separated by a soft tract ; 

 it is, in fact, two-jointed. To these two joints the names 

 coxopodite and basipodite are applied. The endopodite consists 

 of a large terminal joint and a shorter annulated joint; the 

 exopodite, which is smaller, has the same divisions, but the 

 annulated part is proportionately longer. This description 

 applies, not only to the limb from which it has been drawn, but 

 also to the fourth and fifth of the abdominal limbs. The first 

 two abdominal appendages not only differ from this, but differ 

 from each other according to the sex of the individual. In the 

 female the second pair are the same as the limb which has just 

 been described, but the first pair are very rudimentary structures ; 

 they have merely a short protopodite and a feeble flagellum, the 

 whole hmb varying much in length, as is common with rudi- 

 mentary organs. In the male, the first pair of abdominal 

 limbs is a stiff scroll-like structure, shown in the figure 

 (Fig. 15, A). The boundaries between protopodite, exopo- 

 dite, and endopodite cannot be distinguished. In this sex 

 the second pair of Hmbs is particularly strong ; the basal piece 

 of the endopodite is much larger, and ensheathes (Fig. 15, 

 C a, b) the small flagellum. We have, finally, to consider the 

 tail swimmerets, or uropoda, as they are often termed. These 



D 



