REGENERATION AND LIABILITY TO INJURY 



105 



In all the main, groups of echinoderms, with one possible excep- 

 tion, regeneration has been found to take place. Probably all star- 

 fishes and brittle-stars regenerate their arms, and even if cut in two 

 or more pieces, new starfishes develop. The crinoids regenerate lost 

 arms, and even parts of the disk ; also the visceral mass. The holo- 

 thurians have very remarkable powers of regeneration. In some 

 forms regeneration takes place if the animals are cut in two, or even 

 in more than two pieces. The remarkable phenomenon of eviscera- 

 tion that take place in certain holothurians, if they are roughly 

 handled, or kept under unfavorable conditions, are well known and 

 have been described by a number of writers. It has even been sug- 

 gested that the holothurian may save itself by offering up its viscera 



B 



FlG. 39. A. Amphiuma means with left fore and hind leg regenerating. B. Necturvs maculatus 

 with right fore leg beginning to regenerate after eight months. C. Plethedon cinereus. A, 

 B, C. Drawn to same scale. 



to its assailant ! Unfortunately for this view, it has been found that 

 the viscera are unpalatable, at least to sea-anemones and to fishes. 

 Ludwig and Minchin suggest that the throwing off of the Cuvierian 

 organs, which are attached to the cloaca, is a defensive act, and if car- 

 ried too far, according to the latter writer, the viscera may also be 

 lost. The holothurians have remarkable recuperative powers and 

 may regenerate new viscera in a very short time. The sea-urchins 

 form, perhaps, an exception in this group, since there are no records 

 of their regenerative power, but no doubt this is because they have 

 not been as fully investigated as have other forms. 



In the vertebrates the lower forms, amphioxus, petromyzon, and 

 sharks, have not been studied in regard to their regenerative power. 



