ROOM IV.] NATURAL HISTORY. 83 
Crustacea, nearly allied to, and indeed forming a part of 
the Entomostraca , and like them they change their skin 
and have jointed limbs. When first hatched these 
animals have only three pairs of legs ; they float about 
free in the sea, and have a pair of large eyes to direct 
their course. When they have found a fit place to 
which to attach themselves, the cartilaginous skin that 
covers their body thickens, and becomes hardened with 
calcareous plates, and as it becomes more opake, the 
eyes, which are no longer wanted, are absorbed. In 
the perfect state they live affixed -to marine bodies, by 
the part of the body near the head of the animal, and 
which is always inclosed in a case or hard skin. This 
skin has an opening at the end of the free part for 
the passage of the fringed feet, which, by their action, 
create a current to carry the small animals, in the sea, 
near the mouth placed at the base of the cavity. The 
edge of this opening in the case, is always furnished 
with four, more or less, distinct valves, and the base 
of the case is generally surrounded with other similar 
valves; the animals are divided into families and ge¬ 
nera according to the development of these valves. In 
the Sea Acorns, ( Balanus ,) the four valves, (usually to¬ 
gether called the operculum,) are nearly equal sized, and 
sunk into a flexible skin, which allows them to move in 
the cavity formed by the (four, six, or eight) valves which 
surround and inclose the base of the body. These latter 
valves are united together side by side by a dentated 
suture into a bell-shaped body, and thfey are increased in 
size (as the animals grow) by the addition of new matter 
to the base and outer side of each of the valves, deposited 
by the processes of the skin, which are placed for the 
purpose between their sutures. They are generally affixed 
to wood and stones, but a few attach themselves to the 
bodies of whales, and as they grow and their shells are 
enlarged by the addition of new matter to the base of the 
valves, they gradually raise themselves out of the sub¬ 
stance of the skin, in which they w T ere immersed in their 
young state. Some of the genera which live in this 
manner, as the Coronula , to enable them to hold more 
firmly to the skin, form a shell which is variously 
folded on its edge, the folds being refolded, and thus in- 
