296 REPRODIXTION IN ANIMALS 



ject to frequent mutilation, by the action of waves, for 

 example. In each case the damage is repaired and where 

 the body is torn in two the missing portions are often 

 replaced and two individuals result. A flatworm may be 

 broken into more than two hundred pieces and each will 

 form an entire new individual. 



Crabs, insects, and spiders, when seized by one of their 

 legs, will often separate the limb from the body, which 

 then scuttles off to a place of safety. Lizards and some 

 species of salamanders drop the tail when roughly treated, 

 and while the actively wriggling appendage occupies the 

 attention of the enemy the rest of the body generally 

 escapes. Many starfishes frequently undergo a similar 

 process of self -mutilation, while a number of species of 

 the related sea cucumbers will throw out the entire di- 

 gestive tract when molested. In most cases the missing 

 portions are replaced by the process of regeneration, 

 which is thus seen to be of prime importance to the exist- 

 ence of many species of animals. 



Regeneration in Higher Animals. — Among the 

 higher animals the process of regeneration is limited. In 

 the frog and related salamander, the limbs that have 

 been bitten or cut off will regenerate, but death results if 

 the body is cut in two. An amputated finger in the case 

 of man is never replaced, though regenerative changes 

 occur that are of high importance. The skin grows over 

 the damaged area, and beneath it the connective tissue, 

 which ordinarily binds the body-cells together, forms a 

 compact mass of scar tissue. This becomes penetrated by 

 regenerated blood-vessels and by nerves when the injury 

 is slight. Broken bones are repaired, but muscular and 

 nervous tissues are never regenerated to any large degree, 

 nor are internal organs replaced by regeneration. 



A workman in an automobile factory, who has always 

 made bolts, cannot successfully make tires. And appar- 

 ently in much the same way a nerve-cell that conducts 

 impulses, cannot assume the duties of digestion. In 



