AUTOTOMY AND REGENERATION 129 



genuinely uninterrupted circular musculature in the body 

 wall. 



Miscellaneous Cases of Autotomy. — It remains to notice 

 one or two cases of autotomy which, though of minor impor- 

 tance, are not infrequently brought to the notice of shore 

 collectors. For instance, every shore naturalist will have 

 noticed the readiness with which Polynoid worms autotomise 

 segments, and, more especially, throw off their scales (elytra) 

 when captured or kept in captivity. We have heard of no 

 special provision by which this is achieved, but the observa- 

 tions of Michel (191 1) on Halosydna gelatinosa and Lagisca 

 extenuata prove that the casting of the scales is genuinely 

 autotomic, and not the result of mere fragility, since they 

 cease almost entirely to be shed when the animal is beheaded. 

 Further, after a number of elytra have been shed the 

 autotomy becomes more difficult, but after a period of rest 

 is continued as readily as before. An elytron takes no more 

 than five days to regenerate, and a remarkable feature is the 

 precision of the process. Not only is the number typical of 

 the species always reproduced, but the original arrange- 

 ment, in twos in certain segments, and in threes in others, 

 is also preserved. 



Another minor case of autotomy occurs in ^olid molluscs. 

 Eliot (19 10) remarks on his occasionally finding an Antiopella 

 with tiny regenerating cerata instead of the normal large 

 ones which have been lost, the animal presenting a totally 

 different appearance in consequence. Other British species 

 which commonly autotomise their cerata are Tergipes 

 despectus^ Galvina exigua, and Doto. In Proctonotus the 

 cerata are glandular, easily detached, and have a special 

 apparatus at the top which enables them to fasten to 

 extraneous objects. When seized by an enemy they are 

 said to adhere to it and disturb it with their secretion, while 

 the slug itself crawls away. The tropical Discodoris fragilis 

 and others are said to be capable of casting off the whole of 

 the mantle margin, and yet to remain in apparent good health. 



Retrospect. — While our survey of autotomy and regenera- 



K 



