260 Proceedings of the Eoyal Society of Edinburgh. [Sess. 
segment be cut, the crab proceeds to bite the damaged limb down to the 
level of the breaking-plane in the second segment. Thus, by modifying 
conditions, we can produce three types of reaction to injury in the hermit 
crab — autotomy purely local, autotomy involving other levels of the 
nervous system, and autophagy. 
In Galatheids autotomy can be performed by muscles at the base of 
the damaged limb, alone. The reflex is unisegmental, but under changed 
conditions may be reinforced by arcs of other levels. 
The Brachyura are the most highly specialised group of decapods, and 
it is found that in them autotomy is a purely unisegmental reflex. After 
injury, the extensors of the second segment, acting in opposite directions 
on the ring of hard integunient central to the breaking-plane, cause 
weakening of the limb at this point, and division may take place at once, 
as may be the case in Portunus, or when the distal part of the limb meets 
an external point of resistance, usually the carapace. There is a definite 
time-relation between the contractions of the opposing extensors. In Hyas, 
when no external point of resistance can be found, the animal plucks off 
the damaged limb with its chelae. This is the only case in which the uni- 
segmental arc is reinforced from other levels. 
There is here an assemblage of types in which morphological complexity, 
as seen in the structure of the breaking-joint, goes hand in hand with 
physiological specialisation in the local or unisegmental arc. In the lower 
forms, where rupture occurs at a free joint and is only a tearing of soft 
tissues, many arcs are involved : in the Brachyura, where slight muscular 
contraction in certain directions causes profound weakening of the limb at 
the breaking-joint, only one arc is involved. 
It is generally held that reactions to nocuous stimuli have remained 
principally and most powerfully unisegmental in the higher vertebrates, 
because withdrawal from the source of injury is thus most rapidly 
effected. In other words, the unisegmental reflex is regarded as the 
primary one. 
In these decapods, however, we find that whereas the reaction to 
damage of a limb involves many levels of the nervous system in lower 
forms, the reaction is almost always carried by one arc in the more highly 
specialised forms. 
Much speculation is possible regarding the degeneration in reaction-type 
when conditions are altered, and especially on the changes in behaviour of 
the hermit crab. Are we to regard the plucking movements of the chelae 
and the occurrence of autophagy as referable back to habits useful to the 
ancestral stock ? If such is the case, what element in the change of environ- 
