AUTOTOMY AND REGENERATION 119 



tough membrane, is partly through brittle calcareous matter 

 as well. In the case of the great claws there is a breaking 

 joint at the base similar to that in the crab's leg, which we 

 are to study presently. Autotomy takes place only when the 

 limb is roughly held or crushed ; a clean cut with sharp 

 scissors across the limb does not result in the remainder 

 of the limb being thrown off, but produces attempts at 

 autophagy as in the prawn. The fact that the walking leg 

 of the lobster requires to be held to produce autotomy 

 suggests to Paul that autotomy is mainly of advantage as a 

 means of escape. 



It is, however, in the Brachyura, or true crabs, that the 

 autotomy reaches its highest development. In the walking 

 limbs and chelae of all Brachyura there is a definite breaking 

 plane situated in the middle of the second segment from the 

 base (basi-ischium), and marked externally by a ring-like 

 groove. On comparing the legs of the crab with those of 

 the lobster, it will be seen that the groove in the crab's leg 

 corresponds to the joint between the second and third 

 segments in the other form. When the leg of a crab is 

 injured it is invariably thrown off at the breaking plane. 

 The mechanism by which the autotomy is effected has 

 recently been reinvestigated by Paul {op. cit.), who, using 

 improved methods, reaches a somewhat different conclusion 

 from that formulated by Fredericq, whose researches in this 

 field have long been familiar. Paul's explanation is as 

 follows. The segment in which the breaking plane occurs 

 is connected by muscles to the basal limb segment and to 

 the body wall, a short extensor and a short flexor moving it 

 on the basal segment, and a long extensor and long flexor 

 joining it to the body. The tendons of the extensor muscles 

 are inserted on a ring-like thickening of cuticle which 

 surrounds the basal part of the second limb segment. In 

 the ventral part of this ring is a slanting joint formed of 

 material which is not calcified like the remainder (see right- 

 hand of Fig. 10). The action of short and long extensors 

 being in nearly opposite directions their simultaneous 

 contraction tends to pull the jointed parts of the ring 



