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, CM.),, more or less 

 impregnated with lime-salts, and is shed 

 periodically once a year during adult 

 life. Beneath it is the epidermis (ep.) 

 composed of a single layer of cells from 

 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 



c.t 



FIG. 441. Vertical section of skin 

 and exoskeleton of Lobster. 

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

 ep. epidermis ; s. seta. (After 

 Uerstaecker.) 



