PHYLUM ARTHROPODA 



503 



en. 4- 



tacean series, than the normal anterior appendages. They are 



'fore more properly to be looked upon as articulated processes 



he prostomium, developed in 



sction with the need for an 



ased range of vision. The prob- 

 umoy of the ant ennui es being also 

 >rostomial structures has already 

 en referred to : assuming this to 

 the case, it will be seen that the 

 y of the Crayfish consists of a 

 tomium, eighteen rnetameres, 

 and a telson which is probably com- 

 posed of an anal segment, plus 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 maxilla3, and second maxillae : 

 the next eight metameres (5th-l 2th) 

 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- 

 n, and bear five pairs of pleopods 

 d 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 

 articubi r membranes (Fig.398, art. w.), 

 but at two points the adjacent ends 

 of the tubes come into contact with 

 one another and are articulated by 

 peg and-socket joints (&.), 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 



wit 

 anc 



FIG. 30S. Portion of leg of Astacus, 

 with the exoskeleton partly re- 

 moved, showing articulations and 

 muscles, art. -m. articular mem- 

 brane ; en. 2 5, podomeres of eiido- 

 podite ; ext. extensor muscles ; 1. 

 flexors ; h. hinge. 



