208 : COMPARATIVE ZOOLOGY. 
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, z.¢., 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.” Birds develop rapidly, and so spend 
most of their life full-fledged; while Insects generally, 
Fishes, 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 en- 
larged by the deposition of new laminze, 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 laminze are the 
“lines of growth.” The Oyster is full-grown in about 
five years. The bones of Fishes and Reptiles are contin- 
ually growing; 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 rap- 
idly 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 head of the Hydra, if separated, will reproduce a 
