XI 



PHYLUM ARTHROPODA 



533 



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 podo- 

 meres 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 move- 

 ment 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 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 mem- 

 brane, and movement being therefore 

 possible in any plane. 



Body-wall. The exoskeleton is pro- 

 duced into spines of varying form and 

 size, and many parts of it bear tufts or 

 fringes of setae, which also exhibit a 

 wide variation in size and form. It is 

 composed of a thick laminated chitinous 

 membrane (Fig. 441, cu.), more or less 

 impregnated with lime-salts, and is shed 

 periodically once a year during adult 

 life. Beneath it is the epidermis (ep. 



C.I 



composed of a single layer of cells from FIG. 441. vertical section of skin 



and exoskeleton of Lobster. 

 c.t. connective-tissue ; cu. cuticle ; 

 ep. epidermis ; s. seta. (After 



which the chitin is secreted, and under- 

 lain by a layer of connective -tissue (c.t.) 

 to which the muscles are attached. 



The muscular system, like the exoskeleton, shows a great 

 advance in complexity over that of Apus. In the abdomen (Fig. 442) 

 the muscles are of great size, and are divisible into a smaller dorsal 

 and a larger ventral set. The dorsal muscles (d. m.) are paired 

 longitudinal bands, divided into myomeres, and inserted by con- 

 nective-tissue into the anterior border of each segment : anteriorly 

 they are traceable into the thorax, where they arise from the side- 

 walls of that region. When these muscles contract, they draw the 

 anterior edge of each tergum under the posterior edge of its pre- 

 decessor, and thus extend or straighten the abdomen. 



The ventral muscles are extraordinarily complex. Omitting details, 

 there is on each side a wavy longitudinal band of muscle (c. m.), 

 nearly circular in section, which sends off a slip (ex.) to be inserted 

 into each segment above the hinge (h.) : the contraction of this 

 muscle must obviously tend to approximate the terga, and so aid 

 the dorsal muscles in extending the abdomen. Around this central 



