182 ARTHROPOD A [CH. 



legs possess, the basipodite and ischiopodite are fused into one 

 segment: this is effected simply by the thickening of the cuticle 

 covering the arthrodial membrane between the two segments men- 

 tioned so as to destroy its flexibility. The crayfish feeds on any 

 small animals that it can catch, it seizes water-snails for instance 

 dragging them out of their shells and tearing them to pieces with 

 its chelae, and also greedily seizes on any dead fish it may come 

 across. 



The chela is the first of the long leg-like appendages of the 

 animal, and as of these there are in all 5 pairs, the crayfish is said 

 to be a Ten-footed Crustacean and to belong to the order De capo da 

 (Gr. 8e/ca ten, TTOVS foot). 



The remaining 8 legs are used to enable the animal to creep 

 cautiously forward, the abdomen being supported by the vigorous 

 action of the swimmerets. 



On the coxopodite of the third claw-bearing leg there is in the 

 female a small round opening which is the aperture of the oviduct. 



In front of the chela there are still to be found three pairs of 

 appendages attached to the thorax but these are very much shorter 

 than the "legs" and diminish rapidly in size as we proceed forward, 

 so that the hindermost entirely conceals the rest. These appendages 

 .are termed maxillipedes which literally means foot-jaws (Lat. 

 maxilla jaw, pes foot) ; this name was given to them because they were 

 supposed to be intermediate in character between the other limbs 

 and the jaws. This is true, but all the jaws of the crayfish are, as 

 we shall see directly, modified limbs, hence the term foot-jaw is not 

 distinctive. The maxillipedes might with propriety be termed 

 "secondary jaws" for the corresponding limbs in the lower Crustacea 

 have no resemblance to jaws and hence we conclude that the maxilli- 

 pedes have only recently been modified in this direction, and may 

 be expected to show still the first stages of the change. This indeed 

 proves to be the case. If we examine the third and hindermost 

 maxillipede we can see that it consists of exactly the same segments 

 as the other thoracic limbs, but that the ischiopodite and basipodite 

 are each prolonged inwardly into a sharp edge, so that when the two 

 limbs of opposite sides are brought into contact in the middle line, 

 the sharp edges mentioned above act like the cutting parts of a pair 

 of nut-crackers. The meropodite and the "wrist," "hand" and 

 "finger" are smaller in proportion to the ischiopodite than is the 

 case with the corresponding segments of the limbs behind, so that 

 we see that the first step towards converting a leg into a jaw is to 



