368 THE CRAYFISH CHAP. 



that the antennules are also prostomial and not meta- 

 meric structures : 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 composed of an anal segment plus a post-anal 

 extension. The prostomium bears eye-stalks and anten- 

 nules : the first four metameres are fused with the 

 prostomium to form the head, and bear the antennae, 

 mandibles, first maxillae, and second maxillae : the next 

 eight metameres (5th-i2th) constitute the thorax, and 

 bear the three pairs of maxillipeds and the five pairs 

 of legs: the remaining six metameres (i3th-i8th), 

 together with the anal segment, constitute the abodmen, 

 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 abdominal 

 segments (p. 362). The podomeres are, it must be 

 remembered, rigid tubes : they are connected with one 

 another by flexible articular membranes (Fig. 92, 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 articula- 

 tion and in no other direction, the joints being pure 

 hinge- joints (p. 57). As a rule the range of movement is 

 from the perpendicular to a tolerably extensive flexion 

 on one side the articulations are single- jointed, like 

 our own elbows and knees. The whole limb is, however, 

 capable of universal movement, owing to the fact that 

 the axes of articulation vary in direction in successive 

 joints : the first joint of a limb bending, for instance, 

 up and down, the next obliquely, the next backwards 

 and forwards, and so on. In some cases e.g., the 

 pleopods peg-and-socket joints are absent, the articula- 



