Q54 PHILOSOPHY OF ZOOLOGY. 
while in its pupa state; while others, as many caterpillars, 
employ secreted threads in order to draw around them the 
leaves of the plants on which they feed. 
Other animals which likewise require only a temporary 
covering, fabricate their garment from the materials which 
they employ as food. ‘This is the case with the Tinea, or 
clothes-moths. They feed upon wool or hair; and, with 
the same materials, they construct a covering to fit their 
body, with which they move about, increasing its length by 
adding to its extremity, and its breadth, by slitting up the 
sides, and inserting new materials in the gap. The Phry- 
ganeze, or Caddis worms, on the other hand, while they pass 
the first portion of their existence in the water, clothe them- 
selves with bits of straw, sand, or shells. These they cut 
into proper shape, and form into a tube a little larger than 
the body, in which they dwell, and which they likewise carry 
about with them. However rough the outside of the cover- 
ing may appear, the inside is smooth, having a coating of 
slimy matter plastered round the cavity. ‘These animals 
do not, i obedience to this instinct, employ the materials 
within their reach indiscriminately ; for the covering might, 
in that case, become too heavy to be easily carried about 
with them at the bottom of the pool,—or too light, and, 
by rising buoyant to the surface, remove them from their 
sources of nourishment. They select and arrange the ma- 
terials, so as to avoid both these evils. 
But there are many animals which employ an artificial 
covering during every period of their lives. Some we ob- 
serve, although incapable of collecting and arranging the 
materials for a habitation, are still provided with a propen- 
sity for a covering, and avail themselves of the remains of 
other animals. The hermit-crab takes up his abode in a 
deserted univalve shell; and, when his body enlarges, he 
shifts to a larger shell, and thus preserves himself in a pro- 
