NATURAL HISTORY. 
155 
ROOM IV.] 
which is merely covered with the general skin of the body. Both the 
head and the body are frequently enveloped in a free horny or coriaceous 
shield, formed of one, two, or more pieces. The feet are all partiaky 
divided and ciliated. 
This order has been formed into two divisions, according to the 
structure of the mouth and the form of the feet. The most prominent 
representative of it is the King Crab ( Limulus ). 
They were all referred to one group by the older entomologists under 
the name of Monoculus , because they were believed to have only a 
Sm fh ^Cirripedes (Cases 23 and 24) were formerly considered as 
molluscous animals, on account of their being inclosed in a hard shelly 
case, but now that their history and anatomy have been studied, they 
prove to be 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, to the mouth which is placed at the base 
of the cavitv. 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 gene¬ 
rally surrounded with other similar valves; the animals are divided 
into families and genera according to the developement of these 
valves. 
In the Sea Acorns, ( Balanus ,) the four valves, (usually together 
called the operculum,) are nearly equal si zed, 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 they 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 pur¬ 
pose between their sutures. They are generally affixed to wood and stones, 
and but a few attach themselves to the bodies of whales, and as they grow, 
their shells are enlarged by the addition of new matter to the base of the 
valves, they gradually raise themselves out of the substance of the skin, 
in which they were 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 increasing in number as tne 
shell enlarges in diameter. These shells are greatly altered in form ac¬ 
cording to their position; if they are separate they are broad ana ex¬ 
panded, if crowded they are narrow and high. In Balanus it is the 
( ^hse that is lengthened, and in Chirona it is the valves that are produced 
*wnd altered in shape. 
