156 
NATURAL HISTORY. [NEW BUILDING 
pair of feet, the following and last ones, (seven at most,) 
forming a sort of tail, ending in fins or filiform appendages. 
On the head are four antennae and a mouth composed of 
three pairs of jaws. The females carry their eggs between 
the scales in a kind of pouch under their thorax, and 
when the young are hatched, they remain attached to the 
feet or other part of the body of their mother, until they 
have acquired strength requisite to swim about and pro¬ 
vide for their own wants. They are all of a small size. 
A few are found in fresh water, and fewer are terrestrial; 
some are parasitic on other marine animals. 
This order has been divided into three sections; some, as 
the Amphipodes , have their jaws furnished with a palpus, 
while the Lcemodipodes and Isopodes are destitute of them. 
The order of Entomostraca, on the other hand, have a 
soft head, which is merely covered with the general skin of 
the body. Both the head and the body are frequently en¬ 
veloped in a free horny or coriaceous shield, formed of 
one, two, or more pieces. The feet are all partially di¬ 
vided 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 ento¬ 
mologists under the name of Monoculus, because^ they 
were believed to have only a single eye. 
The 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 
