326 THE CRAYFISH LESS. 



maxillae : the next eight metameres (fifth twelfth) consti- 

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

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

 eighteenth), 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. 320). The podomeres are, it must be remembered, rigid 

 tubes : they are connected with one another by flexible 

 articular membranes (Fig. 85, 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 

 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 backwards and forwards, the next obliquely, and so on. 

 In some cases, e.g., in the pleopods, peg-and-socket joints are 

 absent, the articulation being formed merely by an annular 

 articular membrane, movement being therefore possible in 

 any plane. 



Sections show the body-wall to consist of a layer of deric 

 epithelium (Fig. 83, Der. Epthni] secreting a thick cuticle 

 (6V), a layer of connective tissue forming the dermis 

 (Derw\ and a very thick layer of large and complicated 



