XI 



PHYLUM ARTHROPOD A 



503 



Ctft.TH, 



Crustacean series, than the normal anterior appendages. They are 

 therefore more properly to be looked upon as articulated processes 

 of the prostomium, developed in 

 connection with the need for an 

 increased range of vision. The prob- 

 ability of the antennules being also 

 prostomial structures has already 

 been referred to : assuming this to 

 be the case, it will be seen that the 

 body of the Crayfish consists of a 

 prostomium, eighteen metameres, 

 and a telson which is probably com- 

 posed of an anal segment, pins a 

 post-anal plate. The prostomium 

 bears eye-stalks and antennules : the 

 first four metameres are fused with 

 the prostomium to form the head, 

 and bear the antenna?, mandibles, 

 first maxillae, and second maxilla? : 

 the next eight metameres (5th 12th) 

 constitute the thorax, and bear the 

 three pairs of maxillipeds and the 

 five pairs of legs : the remaining six 

 metameres (13th-18th), together 

 with the telson, constitute the abdo- 

 men, and bear five pairs of pleopods 

 and one of uropods. 



The articulation of the various 

 podomeres of the appendages is on 

 the same plan as that of the ab- 

 dominal segments (p. 498). The 

 podomeres are, it must be remem- 

 bered, rigid tubes : they are con- 

 nected with one another by flexible 

 articular membranes (Fig.398, art. m.), 

 but at two points the adjacent ends 

 of the tubes come into contact with 

 one another and are articulated by 

 peg and-socket joints (h.), the two 

 joints being at opposite ends of a 

 diameter which forms the axis of 

 articulation. The two podomeres 

 can, therefore, be moved upon one 

 .another in a plane at right angles 



to the axis of articulation and in no other direction, the joints 

 being pure hinge-joints. As a rule, the range of movement 

 is from the perpendicular to a tolerably extensive flexion on 



n-.g 



it 



/I 



FIG. 398. Portion of leg of Astacus, 



with the exoskeleton partly re- 

 moved, showing articulations and 

 muscles. art. m. articular mem- 

 brane ; en. % 5, podomeres of endi >- 

 podite ; ext. extensor muscles ; i. 

 flexors ; h. hinge. 



