546 



ZOOLOGY 



(Fig. 432, 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 (li.), 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 uni- 

 versal movement, owing to the fact that the axes of articula- 

 tion vary in direction in successive joints : the first joint of a limb 

 bending, for instance, up and down, the next backwards and for- 

 wards, 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 and movement 

 being therefore possible in any plane. 



Body- wall. — The exoskeleton is produced into spines of vary- 

 ing form and size, and many parts of it bear tufts or fringes of 



setae, which also exhibit a wide varia- 

 tion in size and form. It is composed 

 of a thick laminated chitinous mem- 

 brane (Fig. 433, cu.), more or less im- 

 pregnated 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- 

 laid 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. 434) 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 connective-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. 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 



Pig. 433. —Vertical section of skin 

 and exoskeleton of Lobster. 

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

 ep. epidermis ; s. seta. (After 

 Gerstaecker.) 



