THE AMERICAN LOBSTER. 101 



have seen in the large cheliped of the lobster, in a definite plane. It involves only the 

 nerves and blood vessels of the soft tissues, and is provoked by a vigorous muscular 

 contraction, which occurs whenever the nerve of the leg is stimulated violently, whether 

 bya mechanical stimulus, as by snipping oil' the terminal joints, or by electricity, heat, 

 or chemical action. The nervous mechanism is reflex, and thus beyond the control of 

 the animal. Autotomy occurs when the whole of the dorsal and cephalic regions of 

 the body, including the supra oesophageal ganglion or brain, is removed. The reflex 

 nerve center is found to lie in the thoracic ganglionic mass of the crab, or ventral nerve- 

 chain of the Macrura. 



The second compound joint is moved by two muscles, a flexor and an extensor, 

 of which the last only is essential to autotomy. Fracture of the limb w as produced so 

 long as the extensor muscle and its tendon were unimpaired, but when these were 

 sectioned autotomy was suppressed. 



The mechanism of the crustacean limb has been explained by Milne Edwards 

 (58, vol. 1, p. 152). The leg consists, as we have already seen in Decapods, of seven 

 joints, each of which is a lever of the third order. Any two joints are articulated like 

 a hinge, touching at only two points, and are capable of simple extension and flexion 

 only. The whole limb, however, is capable of executing complicated movements, since 

 the axes of articulation of the several segments are not parallel, but nearly at right 

 angles to each other. Each segment possseses two or more cuticular tendons at its 

 proximal extremity, upon which its flexor and extensor muscles are inserted, the fibers 

 of the latter being fixed upon the inner surface of the next proximal joint. 



Fredericq has shown that the distal extremity of the second joint, or basipodite, is 

 separated from the third joint, or ischiopodite, by a diaphragm, perforated near its 

 center only, for the passage of the nerves and blood vessels; and Andrews (3) has 

 pointed out that li in the spider crab, Libinia canaliculata, there extends from the plane 

 of rupture" a distinct membranous fold, "from the epidermis to the central nerve and 

 blood vessels." With the rupture of the limb the outer half of the membrane is torn 

 away, leaving a clean stump. This double membrane possibly represents, as Andrews 

 suggests, the invagination of the body wall, like that seen at an ordinary movable 

 joint. This membrane has thus experienced a complete change of function, and has 

 become modified so as to prevent excessive hemorrhage. 



In order that autotomy may occur it is necessary that the peripheral portion of 

 the limb should offer a greater resistance than the traction of the extensor muscle is 

 able to overcome, allowing the traction of the muscle to be equivalent to a weight of 

 250 grams. Ordinarily the sides of the carapace, the hard parts of an adjoining leg 

 or the clutch of an euemy afford the necessary resistance. 



If the compound — second and third — joint of the cheliped of the lobster be examined 

 a fine hair line is seen leading from the small spur next to the articular facet on the 

 under side, round the anterior border to the upper side of the joint. It then bends 

 forward and abruptly backward, crossing the small proximal end of the joint, to near its 

 pointof departure (x, cutsl2, 13, plate D). There are incomplete grooves in front of this 

 line (cut 12, a, &,) and a more oblique one behind it (cut 12, c). On the upper side of 

 the second joint of the small walking legs of the lobster a delicate hair line is also seen, 

 which turns abruptly forward at the anterior border of the appendage and joins the 

 arthrodial membrane. This groove looks as if it might mark the plane of rupture in 



