504 



ZOOLOGY 



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 ax< 

 articulation vary in direction in successive joints: the first joint 

 of a limb bending, for instance, up and down, the next back- 

 wards and forwards, the next obliquely, and so on. In some < 

 e.g. the pleopods, peg-and-socket joints are absent, the articulation 

 being formed merely by an annular articular membrane and move- 

 ment being therefore possible in any plane. 



Body-wall. The exoskeleton is produced 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. 399, cu.), more or less impregnated with lime-salts, and is 



shed periodically once a year during 

 adult life. Beneath it is the epidermis 

 (ep.) t composed of a single layer of 

 cells, from which the chitin is secreted, 

 and underlaid 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. 400) 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 prede< 

 and thus extend or straighten the abdomen. 



The trntrd/ muscles are extraordinarily complex. Omitting de- 

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

 nearly circular in section, which sends ott" a slip (ex.) to be ii.-- 

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

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

 the dorsal muscles in extending the abdomen. Around thisw 

 muscle is wrapped, in each segment, a band of muscle (cnv. //>.) in 

 the form of ;i loop, the outer limb of which turns forwards and is 

 1 into a sternum, while the inner limb turns backwards and 

 is inserted into another and more posterior sternum. The con- 

 >n of thi- /'/ produces an approximation of 



FIG. 899. Vertical section of skin 

 and exoskeleton of Lobster. 

 c.t. connective tissue ; cu. cuticle ; 

 seta. (After 



'(./>. epidermis ; 

 Gerstaecker.) 



