184 ARTHROPOD A [CH. 



springing from the basipodite. We thus possess in the third maxilli- 

 pede an exactly intermediate link between the swimmeret and the 

 "leg," for it resembles the former in its forked character and the latter 

 in the segments into which the endopodite is divided. Since in the 

 lower Crustacea the forked type of limb is the rule we have every 

 reason for supposing that the "legs" once had an exopodite which 

 they have lost and indeed in one family of Shrimps vestigial exo- 

 podites are found, and so we regard the third maxillipede as the 

 typical limb of the crayfish, from which by modification all the other 

 types have been derived. The use of the exopodites will be pointed 

 out when the respiration of the crayfish is described. 



Turning now to the head region we find that it is covered by the 

 carapace, an unjointed shield, which closely invests it above and at 

 the sides and is produced in front into a pointed spine called the 

 rostrum (rs, Fig. 70). The shape of this spine is a useful mark 

 in discriminating one species from another. At its base on either 

 side is a curved indentation the eye-socket or orbit, from which 

 the eye springs. The shield which covers the back of the thorax 

 and forms the branchiostegites is a direct prolongation of the 

 covering of the head. When we examine the corresponding parts 

 of the body in the lower Crustacea we find that from the head there 

 projects backward a large free fold of skin covering the thorax on the 

 back and sides but not in any way adherent to it, and the segments 

 of the thorax are freely movable like those of the abdomen. Hence 

 we conclude that in the crayfish a similar outgrowth of the head 

 region exists but that it has become adherent to the thorax along 

 the mid-dorsal line whilst still projecting freely at the sides. 



On the under side of the head there are two sterna in front of 

 the mouth the first excessively narrow, the second broader and 

 termed the epistome (e, Fig. 71). These sterna are immovably 

 fixed to one another and to the upper lip. This last is termed the 

 labrum (/6, Fig. 71) ; it is a curved bar overhanging the mouth 

 which has the shape of a longitudinal slit. Behind the mouth are 

 two small lobes termed the metastoma (mt, Fig. 71) and at the sides 

 of the mouth the head bears three pairs of appendages which are 

 modified so as to form jaws, making with the maxillipedes six pairs 

 of jaws altogether. The hindermost of these is called the second 

 maxilla or simply the maxilla. It is a thin and blade-like limb not 

 unlike the first maxillipede in front of which it lies. The two 

 segments of the protopodite, the basipodite and the coxopodite are 

 each produced into gnathobases fringed with small spines 3 the 



