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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 a lines of growth. 5 ’ The Oyster is full-grown in 
about five years. The bones of Fishes and Reptiles are 
