AUTOTOMY AND REGENERATION 123 



autotomy and regeneration there tells the fact, which he 

 himself does not undertake to explain, that hermit crabs, 

 which in natural conditions are very rarely found to have 

 lost a limb, possess the power of autotomy and regene- 

 ration in a marked degree. Again, in those forms with a 

 breaking plane regeneration may take place not only from 

 the breaking plane, but from other parts of the limb as well. 

 It is possible that these two objections may be fairly met 

 by supposing hermit crabs to have inherited capacities of 

 autotomy and regeneration though they do not need them 

 particularly, and did not themselves evolve them. Never- 

 theless, even supposing this explanation to be correct, other 

 difficulties remain, such, for instance, as that of accounting 

 for the manner in which — if we call autotomy and regenera- 

 tion " adaptive " — these phenomena have been brought to 

 perfection. The argument based on the accumulation of 

 small variations in a particular direction is, in the case of 

 processes of this kind, by no means convincing. 



The forms which we have been considering are, of course, 

 not the only Crustacean species which exhibit autotomy. 

 Most shore collectors have had personal experience of the 

 autotomy of hermit lobsters (Galatheidae), and of porcelain 

 crabs. Similarly, crabs of the tropical genus Grapsus, when 

 seized by a limb, leave the limb in the hand of the collector, 

 and make off so readily as almost to force him to conclude 

 that the animal has a very precise appreciation of the 

 danger which threatens it ! Incidentally we may notice 

 that Pieron (1907), who has studied autotomy in Grapsus 

 and in other groups, distinguishes between " evasive " 

 autotomy, which seems to him to be " voluntary " in the 

 same sense as an endeavour to escape is voluntary, and reflex 

 autotomy, which is much more general. The muscular con- 

 tractions which cause autotomy in Grapsus are of the same 

 order as the normal locomotor contractions. Voluntary or 

 evasive autotomy is particularly well developed in Grapsus, 

 but it also occurs elsewhere, as in hermit crabs. Reflex 

 autotomy, according to Pieron, is not universally distributed, 

 even among the Brachyura. 



