Chap. ix. j SKELETON OF CRAYFISH. 301 



to be reckoned among active agents of offence, as when 

 they are developed on the two terminal joints of the 

 great chelae or forceps, the last of which is movable on 

 the last but one. The protective power of the hard 

 exoskeleton is, inversely, spoken to by the softness of 

 the hinder end of the body of the hermit-crab, which 

 lives in an empty snail-shell, and protrudes only the 

 anterior portion of its body. 



The larvae of various insects have often protective 

 spines or warts ; those of Crustacea, in the Zoea-stage, 

 have more or less long, anterior, dorsal, and lateral 

 spines. 



The several parts of which the skeleton is made 

 up may be conveniently studied in one of the ab- 

 dominal segments of a crayfish. We here see that 

 there is a continuous ring, the convex dorsal region 

 (t erg 11 m) of which is continued into two lateral 

 (pleiiral) regions, which hang down on either side ; 

 beneath is a flattened ventral region (sternum), to 

 which are articulated two jointed appendages; the 

 piece between the articulation of the appendage and 

 the pleuron of either side, is known as the epiineron. 

 The appendage, when completely developed, consists 

 of a two-jointed basal protopodite, with which are 

 articulated an inner and an outer branch, which are 

 known respectively as endopodite and exopodite ; 

 with these a third piece, epipodite, is sometimes con- 

 nected. In the crayfish the appendages of the abdo- 

 men are either flattened to serve as s\viinmerets, or 

 modified to act as accessory reproductive organs (see 

 page 496) ; the last pair of appendages are greatly 

 flattened out, and, with the last segment (telsoii), 

 which is, in all known cases but that of Scyllarus, 

 without appendages, forms the tail-fin. The four 

 hindermost pairs of thoracic appendages are con- 

 verted into walking limbs by a considerable in- 

 crease iii the size and strength of the endopodite, 



