ARTHROPODA. 87 



thin, forming on each side a plate sloping steeply downwards, 

 which constitutes the inner wall of the gill-chamber. Grooves 

 converging upwards mark the boundaries of the segments. The 

 thoracic sterna (st) are narrow elongated plates, lying between 

 the bases of the appendages, and, except the last, closely united 

 together. The last four of these appendages are elongated 

 walking-legs (11-14), the two first of which are chelate, i.e., with 

 pincers, and possess no exopodite. The two-jointed protopodite 

 is continuous with a five-jointed endopodite, the last joint but 

 one of which, in the two first, is produced distally alongside the 

 last joint, which works against it to form a claw. The broad 

 proximal joint of the protopodite bears in the first three walking- 

 legs a membranous plate, the epipodite (ep., I, Fig. 27), which 

 projects into the gill-chamber, and on which gill-filaments are 

 arranged in a plume-like way. The male genital pore (g.o) is 

 placed on the inner side of the proximal joint of the protopodite 

 of the last walking-leg. The female genital pore is found in a 

 similar position on the second walking-leg. 



The fourth pair of thoracic appendages are the large j "weeps 

 (10). These are similar in structure to the chelate walking-legs, 

 but their claws are very much larger. The first three pairs of 

 thoracic appendages are relatively small and flattened, and are 

 termed foot-jaws (maxillipedes). They are directed forwards and 

 work against one another from side to side, their inner margins 

 being provided with stout setae. By turning a crayfish on its 

 back the last pair (27, H) will readily be seen, and upon removing 

 or turning back these the smaller second pair (27, G) will come 

 into sight. These similarly conceal the still smaller first pair 

 (27, F). They possess all the typical regions, but there is con- 

 siderable variation in detail as may be gathered from Fig. 27. 



(3) The head (Figs. 25 and 28, B) is probably made up of five 

 segments closely united, and bears five pairs of appendages. That 

 part of the carapace in front of the cervical groove represents the 

 terga. It is produced in front into the rostrum, on either side of 

 which is a deep notch, in which the stalked eye is placed. This 

 region is much broader than the corresponding thoracic one, and 

 bounds the front of each gill-chamber. The pleural region is 

 rudimentary, and represented by the edge of the carapace, while 

 the epimera are narrow and the sterna represented by several 

 small median pieces. On the sternal surface, between the 3rd 



