Regeneration 579 



If an arm of a starfish is injured, it is usually cast off near its base, 

 and the arm with part of its central disk will regenerate a new starfish. 

 The lost arm on the original starfish will also be regenerated (Fig. 28). 

 Sea cucumbers (Fig. Ill) when irritated may cast out their respiratory 

 apparatus, and part, or all, of their intestine; in both cases the lost 

 parts may be efficiently regenerated. Autotomy is an advantage since 

 the wound heals more efficiently at the breaking point or area than if 

 the injury had occurred elsewhere. The idea seems to be that it is 

 more desirable to sacrifice an easily replaceable part than to jeopardize 

 the total organism. Autotomy without the subsequent regeneration 

 would not be very practical. 



REGENERATION (re jen e -ra' shun) (L. re, again; generare, to 

 beget) 



This phenomenon consists of the replacement or renewal of an organ 

 or structure which has been lost or injured, whether by autotomy or 

 otherwise (Fig. 28). Certain structures appear to be more easily re- 

 generated than others. In fact, certain ones are never regenerated if 

 once lost. Regeneration is common in such organisms as protozoa, 

 sponges. Hydra, earthworms, planarians, starfishes, sea cucumbers, cray- 

 fishes, lobsters, etc. Usually a renewed structure resembles the lost one, 

 but this is not always true. For example, the removal of a nonfunc- 

 tional, degenerate eye from a so-called "blind" crayfish may result in 

 the regeneration of a functional, antenna-like tactile organ. The rate 

 of regeneration is influenced by such factors as the age of the organism, 

 the extent of the injury, the specific tissue or organ involved, etc. 



In protozoa, during reproduction, two entirely complete individuals 

 may be formed from the two halves which have been divided by fission 

 (Figs. 162 and 169) . In many sponges, if the individual is cut into pieces, 

 each piece will regenerate a norma] animal. Bath sponges, if cut into 

 pieces of about two cubic inches, will regenerate a sponge about six 

 times this size in about two months. When certain species of sponges 

 are broken up and strained through fine cloth so as to dissociate the 

 cells, the latter will again fuse together and eventually form a sponge 

 with its typical skeleton, pores, canals, etc. 



Hydra (Fig. 28) may be cut into pieces and each part will regenerate 

 an entire animal. The part with the tentacles produces a new indi- 

 vidual. If split lengthwise into two or four parts, each part forms a 

 normal individual. A hydra with two "heads" with tentacles can be 

 produced by the splitting and separation of that region. Even pieces of 



