282 BULLETIN OF THE BUREAU OF FISHERIES. 



having been found to lie in the corresponding ganglion of the cord, but if the animal is 

 anesthetized it will "forget" to shoot its claw. 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 autotomy 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 {gj) 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. 235), but that a fusion has 

 taken place between the third and fourth segments, a statement which is not easUy 

 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 (see ch. 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 Umbs 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 

 reflexly 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 hmb; 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 sUght swelling of the tissues over the fresh surface; if the valves 

 are pressed open the bleeding is resumed. 



