504 



ZOOLOGY 



SECT. 



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 back- 

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

 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 setse, 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.), 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 predecessor, 

 and thus extend or straighten the abdomen. 



The ventral muscles are extraordinarily complex. Omitting de- 

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

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

 into each segment above the hinge (/?-.) : 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 

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

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

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

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

 traction of this enveloping muscle produces an approximation of 



c.t 



FIG. 399. Vertical section of skin 

 and exoskeleton of Lobster. 

 c.t. connective tissue ; cu. cuticle ; 

 ep. epidei'mis ; s. seta. (After 

 Gerstaecker.) 



