THE AMERICAN LOBSTER. 
101 
have seen in tlie large clieliped 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 
by a mechanical stimulus, as by snipping off 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- 
cliain 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 was 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 Ave have already seen in Decapods, of seven 
joints, each of Avhich is a lever of the third order. Any tAvo joints are articulated like 
a hinge, touching at only tAvo points, and are capable of simple extension and flexion 
only. The whole limb, hoAvever, 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 iscliiopodite, by a diaphragm, perforated near its 
center only, for the passage of the nerves and blood vessels; and Andrews ( 3 ) has 
pointed out that “ 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 Avail, 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 enemy afford the necessary resistance. 
If the compound — second and third — joint of the clieliped 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, cuts 12, 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 
