XII] ASTACUS 181 



which a series of very short sclerites are situated, which quite 

 evidently correspond to the sternites of the abdomen. Hence we 

 conclude and the conclusion is confirmed by a study of the allies 

 of the crayfish that the thorax is composed of a number of segments 

 like those constituting the abdomen which have secondarily become 

 fused with one another through the thickening of the arthrodial 

 membranes. The appendages above-mentioned differ in many 

 respects from those of the abdomen. To begin with they are not 

 forked, they consist of a limited number of exceedingly stout joints, 

 and they are very much longer than those attached to the abdomen. 

 It is usual to speak of them as legs, distinguishing the appendages 

 of the abdomen asswimmerets. If we select for careful examination 

 the last leg we find that it consists of 7 segments which are named, 

 in the order proceeding from the base to the apex of the limb, coxo- 

 podite, basipodite, ischiopodite, meropodite, carpopodite, 

 propodite, dactylopodite. Many of these names are suggested by 

 fanciful analogies with the divisions of the human limb as the prefixes- 

 cox- (Lat. coxa thigh), dactyl- (Gr. SaKTvAos finger) and others testify. 

 It is indeed becoming usual to employ the English words wrist, hand 

 and finger, for carpopodite, propodite and dactylopodite respectively, 

 but it must be remembered that there is no real correspondence 

 between the parts of the arthropodan and the human limb. From 

 a comparison with the more anterior limbs of the crayfish it appears 

 that the coxopodite and basipodite represent the protopodite of the 

 swimmeret and the remaining five joints correspond to the endopodite. 

 The dactylopodite or "finger" is a sharp-pointed segment. On the 

 coxopodite of the last leg there is found in the male a small round 

 opening. This is the aperture of the male genital duct and 

 through it the male germ cells are shed into the tube formed by the 

 first two appendages of the abdomen. Proceeding forwards in our 

 examination of the appendages we find that the next leg is in all 

 respects similar to the one described, but the three preceding legs 

 are distinguished by the fact that each terminates in a claw. 



This claw is due to a simple modification of the two terminal 

 joints. The hand (propodite) is prolonged into a stiff spine which 

 with the finger (dactylopodite) constitutes a pair of pincers. Of the 

 three legs terminating thus, the first is by far the largest and strongest 

 and has the joints constituting the claw very much enlarged. It is 

 termed the chela, and it is the organ by which the crayfish obtains 

 its food and defends itself against its enemies. 



In order to give the chela rather more rigidity than the other 



