686 TEXTBOOK OF ZOOLOGY 



Mollusca 



Capacity for regeneration is extensive in this phylum. Foot or 

 head, mantle or shell, may be replaced under favorable environ- 

 mental conditions. 



Arthropoda 



Regenerative capacity of organisms in the phylum Arthropoda 

 is more limited than in the preceding groups. Regeneration takes 

 place in the walking legs of decapod Crustacea, such as shrimps, 

 crabs, lobsters, and crayfish. Spiders, which are members of the 

 order Arachnida, are also able to regenerate appendages, but the 

 insects generally lack this capacity. 



The hermit crab protrudes three pairs of thoracic legs from the 

 snail shell in which it lives. These three pairs of legs have breaking 

 joints, clearly demarked levels at which these legs break when 

 caught. The last two pairs of thoracic legs are not protruded from 

 the snail shell and have no breaking joint. When a leg of the first 

 three pairs is injured, it is thrown off at the breaking joint and the 

 leg regenerates from the breaking joint outward. Such a regen- 

 erated leg is small and of little use until the crab molts. The re- 

 generated leg grows to its proper proportions during the molt. If 

 a leg of the first three thoracic pairs is cut off between the body 

 and the breaking joint, a new leg forms with a new breaking joint. 

 Thus, the breaking joint is not essential to regeneration (Pig. 366). 



The last pairs of legs, without breaking joints, are not readily 

 subject to injury. If, however, they are cut off at any level, the 

 missing parts are regenerated. 



The eyes of Crustacea are also capable of regeneration. When 

 the eyestalk of Palaemon (shrimp) is cut off near the tip, a new 

 eye regenerates, but when the eyestalk is cut off near the base, an 

 antenna is formed. 



Echinodermata 



Regenerative capacity is very great among members of this phylum. 

 If an arm be torn from a starfish, not only will the major fragment 

 of the starfish regenerate a new arm but the detached arm will also 

 regenerate a new starfish provided even a small portion of the disc 

 remains with it. 



