426 COMPARATIVE ZOOLOGY 



plying the constant waste, in order to preserve the size 

 and shape of the organs. A child eats to grow and 

 repair; the adult eats only to repair. 172 Birds develop 

 rapidly, and so spend most of their life full-fledged; 

 while insects generally, fishes, amphibians, reptiles, and 

 mammals mature at a comparatively greater age. The 

 perfect insect rarely changes its size, and takes but 

 little food ; eating and growing are almost confined to 

 larval life. The crust of the sea urchin, which is never 

 shed, grows by the addition of matter to the margins of 

 the plates. The shell of the oyster is enlarged by the 

 deposition of new laminae, each extending beyond the 

 other. At every enlargement, the interior is lined with 

 a new nacreous layer ; so that the number of such layers 

 in the oldest part of the shell indicates the number of 

 enlargements. When the shell has reached its full size, 

 new layers are added to the inner surface only, which 

 increases the thickness. It is the margin of the mantle 

 which provides for the increase in length and breadth, 

 while the thickness is derived from the whole surface. 

 The edges of the concentric laminae are the " lines of 

 growth." The oyster is full-grown in about five years. 

 The bones of fishes and reptiles are continually grow- 

 ing ; the long bones of higher animals increase in length 

 so long as the ends (epiphyses) are separate from the 

 shaft. The limbs of man, after birth, grow more rapidly 

 than the trunk. 



The power of regenerating lost parts is greatest where 

 the organization is lowest, and while the animal is in the 

 young or larval state. It is really a process of budding. 

 The upper part of the hydra, if separated, will repro- 

 duce the rest of the body; if the lower part is cut off, 

 it will add the rest. Certain worms may be cut into 

 several pieces, and each part will regain what is needed 

 to complete the mangled organism. The starfish can 



