282 
BULLETIN OF THE BUREAU OF FISHERIES. 
having been found to lie in the corresponding ganglion of the cord, but if the animal is 
anaesthetized it will “forget” to shoot its daw. We have seen that the basis has lost 
its muscles, and that the ischium possesses two extensors only; in order that autotumy 
should normally occur it would seem to be necessary that the part of the limb distal 
to the breaking plane should offer a greater resistance than the traction of the small 
extensors of the ischium is able to overcome; ordinarily the clutch of an enemy furnishes 
the opposing force required, but since the action is purely reflex, “accidental” disjunc- 
tion of a limb which happens to be suddenly opposed in its movements may occasionally 
happen. The probable relations of autotomv to the interlocking mechanism of the 
coxa and ischium are described in chapter vii. 
While no tendons cross the breaking joint in the adult lobster, Emmel (97) has shown 
that this is not the case in the larvae, in which he has discovered a transitory muscle of 
considerable interest; this muscle originates on the inner wall of the basis, crosses what 
is then a free joint, and is inserted upon the inner side of the ischium. It acts as a flexor 
during the first four stages of life, begins to dwindle in the fifth stage, and is reduced to 
a mass of degenerate tissue in the sixth. It has been maintained that in the lobster 
the breaking plane does not represent a lost joint (see no. 255), but that a fusion has 
taken place between the third and fourth segments, a statement which is not easily 
understood. Thanks to the peculiar interlock of spurs on the first three podomeres, 
it is easy to follow the changes which these segments undergo from the fourth stage 
onward without difficulty (seech, vii, p. 259), and if any further evidence were needed 
to show that the breaking joint, which is functional up to the fourth stage, corresponds 
to the articulation of the second and third segments, it would seem to be furnished by 
Emmel’s discovery of a missing flexor muscle at this point. 
While autotomy does not normally occur before the fourth stage, the limbs are 
often snapped off at the joint destined to become the breaking plane. Lobsterlings 
occasionally cast a claw at the articulation between the second and third segments which 
has the appearance of a free joint; fusion is not completed until the fifth stage, from 
which time onward autotomy in its typical form becomes a common occurrence. 
An interesting adjustment to prevent excessive loss of blood in the stump of the 
refiexly amputated limb has been described by Emmel (97). We have seen, in referring 
to his account in another place (ch. vi, p 245), that as the venous sinus crosses the 
breaking plane it is divided into two channels by a septum in which are lodged the two 
arteries and two nerves of the limb; on the proximal side of the joint the septum gives 
off two folds, which are swung out by blood pressure after the break occurs and acting 
as valves to the small openings exposed, check the bleeding at once. It would appear 
from Emmel’s work that the severed arteries must immediately contract so that their 
blood is discharged proximally to the folds or valves which he describes. Whether a 
similar adjustment to prevent excessive loss of blood is found in the other appendages, 
so far as I am aware, has not been determined. To continue this account further, 
when a claw is shot, a short jet of blood is thrown from the stump, but the bleeding soon 
ceases, followed by a slight swelling of the tissues over the fresh surface; if the valves 
are pressed open the bleeding is resumed. 
