46 



In order to demonstrate the method by which the lobster is 

 able to escape at the expense of the loss of a leg, it will be necessary 

 to examine the structure of the basal parts of the limb. 



The second segment consists roughty of a hollow triangular 

 pyramid hinged at two of its points to the first segment (figs. 

 3 and 4). Ventrally, a flexor muscle has its tendon attached to 

 the lower border, while dorsally, an extensor from the rounded 

 edge (which represents two sides of the pyramid base) passes 

 into the cavity of the first segment (fig. 4, e). 



The inner side of the dorsal wall of the second segment is 

 the origin of a muscle which is inserted by a small tendon into 

 the ventral edge of the third segment. This muscle (fig. 5, a), 

 by its insertion, may be regarded as a flexor, but since movement 

 between the second and third segments is very hmited, it probably 

 does not function as such. Its important role in the production 

 of autotomy gives reason for it being called the " autotomiser." 



The third segment articulates with the second by a hinge 

 joint. At the point of attachment of the muscle described above, 

 a furrow begins at its lower edge and passes round the greater 

 part of the limb at a distance of about 1 mm. from the 

 arthrodial membrane of the third joint (figs. 3, 4 and 5, /). 



After autotomy has occurred the appearance of the limb 

 stump is as shown in fig. 8. A ring of calcareous material (6), 

 corresponding to the part of the third segment central to the 

 groove, is found attached to the stump. The autotomiser (figs. 6, 

 7 and 8, a) is contracted, and thus the ring is pulled into the cavity 

 of the second segment, where it is often hidden among the soft 

 tissues. This probably accounts for the fact that previous workers 

 have not noticed it. Division of the hmb therefore takes place 

 along the groove at the base of the third segment, and where 

 that groove ends, along the arthrodial membrane joining segments 

 two and three. 



The factors controlling autotomy were revealed by the follow- 

 ing experiments : — 



(1) Division of the extensor muscle of the second segment 

 prevented it from occurring. 



(2) Division of the flexor muscle of the third (the autotomiser) 

 had the same effect. 



(3) Cutting of the nerve cord to the tail also prevented it. 



