214 COMPARATIVE ZOOLOGY. 



Crustacea and Insects. The name is sometimes limited to 

 cases where the two kinds of generations differ in form. 



3. Growth and Repair. 



Growth is increase of bulk, as Development is increase 

 of structure. It occurs whenever the process of repair 

 exceeds that of waste, or when new material is added 

 faster than the tissues are destroyed. There is a specific 

 limit of growth for all animals, although many of the low 

 cold-blooded forms, as the Trout and Anaconda, seem to 

 grow as long as they live. After the body has attained 

 its maturity, i. e., has fully developed, the tissues cease to 

 grow ; and nutrition is concerned solely in supplying 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. 119 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 indi- 

 cates 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 



