AUTOTOMY AND REGENERATION 117 



as hydroids, anemones, flat-worms, ascidians, and so on, and 

 to confine our attention to this phenomenon only so far as 

 it occurs associated with the process known as autotomy. 

 The latter process is generally of very much more restricted 

 occurrence than regeneration and is yet of relatively frequent 

 occurrence on the sea-shore, an association of facts which 

 affords us considerable justification for regarding the two 

 processes together as of the nature of a response to environ- 

 ment. 



Autotomy is the term applied to the reflex act of throwing 

 off a limb or surrendering a portion of the body in relation 

 to some external stimulus. Probably the most familiar case 

 of autotomy is provided by the lizard, which, when seized 

 by the tail, at once breaks it off at a level corresponding to 

 that of the seventh caudal vertebra, or below, the act being 

 rendered easy of accomplishment by the presence of a 

 special weakened area in the middle of the vertebral " centra." 

 Another interesting example of a similar kind is recorded by 

 Semper and concerns snails of the genus Helicarion which 

 frequent trees in the Philippines. When grasped roughly 

 by the " tail " (posterior end of the " foot ") the snails 

 escape in the same way as does the lizard, by leaving this 

 part of the body behind. The most readily accepted 

 explanation of these and similar cases is that they are 

 protective adaptations. The tail of the lizard, for instance, 

 on account of its conspicuousness, is thought to be particu- 

 larly liable to attack, and since the autotomy provides a 

 valuable means of escape, the process would have been 

 fostered by natural selection. Autotomy does not necessarily 

 always require a breaking plane, but when one is present 

 we think we are right in saying that it is regarded as a 

 refinement of the process : affording the most favourable 

 conditions for subsequent regeneration or, it may be, reducing 

 haemorrhage.. 



This view of autotomy as a process which has been 

 fostered by natural secretion has also been criticised by 

 Morgan, who finds the same difficulty in picturing its evolu- 

 tion on these lines as he does that of regeneration. Morgan's 



