504 COMPARATIVE ANATOMY chap. 



XX. Capacity for Regeneration and Asexual Reproduction by means of 

 Fission and Gemmation. 



The capacity for regeneration is, as a rule, highly developed in Echinoderms. 

 Defects in the body wall are in this way easily and quickly repaired in all Echino- 

 deniis. The Echinoidca, even, in which this capacity is in other ways slightly 

 developed, easily repair smaller or greater defects in the body epithelium which cover.s 

 the test. For example, in Doroddaris papillata, portions of the test over which 

 the epithelium has been damaged or destroyed are cast off, and as soon as a fresh 

 integument has formed a new test can undoubtedly be produced below it. 



The capacity for regeneration may increase to an extraordinary degree within the 

 different divisions, and the sensitiveness to external stimulus increases in proportion, 

 till a stage is reached when voluntary amputation by means of muscular contraction 

 takes place in response to external stimuli. 



Tlie Criihoids completely regenerate lost viscera, and it even appears as if such 

 loss is not altogether involuntary, in certain species and under certain conditions 

 voluntary amputation taking place. This, however, is not certain. 



Ci-lnoids easily regenerate broken-off portions of arms or whole arms ; several or 

 indeed all the arms may, under favourable circumstances, be regenerated. The arms 

 break off easily at their bases ; it even appears as \{ Antrdon, in response to injurious 

 stimuli, voluntarily throws off its arms. 



The regeneration of the portions of arms (bitten off, possibly by enemies) or of 

 whole arms takes place very easily in many Adcrcddca and Ophiuroidca. The 

 frequency with which Asteroids and OpMurkls with regenerated arms or arm tips are 

 met with demonstrates both the frequency of mutilation and the great utility of 

 regeneration. 



Species oi Asteroids in which the disc with the other arms are regenerated at the 

 base of broken-off arms are less common. Such regenerations give rise to the well- 

 known "comet" ioTTa ol Asteroids CFig. 397, B). Regeneration of the whole body 

 from one arm never occurs in Opihiuridti. It has been suggested that the difference 

 between Asteroids and Ophiurids in this respect is connected with the fact that, in 

 Asteroids, intestinal diverticula project into the arms, and that the genital products 

 are often developed in them, which is never the case in Ophiurids. 



Animals in which half the disc is retained can regenerate the rest of the body 

 both among the (Iphiuroidea and the Asteroidea. 



Defects both great and small in the disc are repaired. 



In Linckia vm/lifura, an Asteroid distinguished by au extraordinary capacity for 

 regeneration, cases have been known in which, after the animal has lost the greater 

 part of an arm, two new tips have been formed by the wounded surface, and in one 

 case regeneration led to the formation of a complete new Asteroid at such a 

 point. This latter case is illustrated in outline in Fig. 397 C. The new animal 

 consists of two discs with their arms, connected by the regenerating stump of the 

 arm. 



Holothurioidea. — Here also the capacity for generation seems to be very great. 

 Not only are tentacles and integumental defects repaired, but the ejected viscera 

 (intestine, respiratory trees, and even the calcareous ring, the water vascular ring, 

 and the gonads) can be regenerated. In Synapta, after the body has been completely 

 cut to pieces, its anterior portion can regenerate the whole. In a Ouctimaria, the 

 two separate halves can grow into complete animals. 



Increase in the capacity for regeneration is accompanied by increased irritability. 

 Jlany Holothurioidea, especially Aspidochirotce, when strongly stimulated, contract 



