368 THE CRAYFISH CHAP. 



nules are also prostomial and not metameric 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 antennules : 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) consti- 

 tute the thorax, and bear the three pairs of maxillipeds and 

 the five pairs of legs : the remaining six metameres (i3th 

 1 8th), together with the anal segment, constitute the 

 abdomen, and bear five pairs of pleopods and one of 

 uropods. 



The articulation of the various podomeres of the append- 

 ages 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 (/), 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 (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 articu- 

 lation 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 



