Malacostraca.- 
286 
Malacostraca.- 
CRU STAGE A. Decapoda. 
these ganglions the nerves are distributed to the differ- 
ent organs and members of the body, and to the muscles 
which move them. 
The sexes are distinct. The males have not as yet 
been seen in all ; but there is good reason to believe 
that, with the exeeptidn of the cirripedes or barnacles, 
the two sexes exist separately in all cases, though in 
some the males differ very much in appearance from 
the females. They always produce their young by 
means of eggs, which, in most or all instances, are 
fecundated within the body of the female before she 
lays. In some genera the young are fully formed in 
the internal ovary of the mother, and are extruded 
alive. In many, the young at birth are like the parent. 
but in others they are very unlike, and undergo several 
changes, amounting almost to a kind of real transforma- 
tion, before they assume the perfect form. The Crus- 
tacea differ from each other very much in form and 
appearance ; they differ also very much in their habits. 
Most of them live constantly in the water, but there are 
some which during the greater part of their life inhabit 
the land, and are called Land-crabs. The larger num- 
ber are carnivorous, and many even devour their own 
species. 
For convenience sake, we divide the class Crustacea 
into two great sub-classes, the Malacostraca and the 
Et^TOMOSTRACA. 
Sub-class I. — MALACOSTRACA. 
Tins name was originally applied by Aristotle to the 
whole class in general, but by Latreille and others it 
is used to designate only a particular division of it. It 
contains the greater part of the large Crustaceans, and 
more especially those which are useful to man. 
It has been divided into two great sections, the Stalk- 
eyed Crustaceans or Podophthalma (those which have 
their eyes placed at the tip of a footstalk), and the 
Sessile-eyed or Edkiophthalma (those in which the 
eyes are not placed on footstalks). 
Section I. — STALK-EYED CRUSTACEA (Podophthalma.) 
The Stalk-eyed Crustacea are chiefly distinguished by 
the possession of organs exclusively, formed for the 
purpose of respiration (branchiae, or gills), and the 
existence of eyes placed at the extremity of a movable 
peduncle or footstalk — hence their name. A large 
portion of the front of the body is covered by a buckler- 
shaped shell or carapace, which extends more or less 
beyond the thorax, and this kind of covering generally 
tends to distinguish the animals of this division of Crus- 
tacea at first sight. 
They are divided into two orders, Decapoda, or 
Ten-footed Crustacea; and Stomapoda, or Sea- 
Mantes. 
Order I. — DECAPODA {Ten-fooled 
Crustaceans). 
This order contains the greatest number of species 
of all the orders of the Crustacea. The animals whicli 
belong to it are those which have the most complicated 
organization, and the most perfectly developed facul- 
ties. The branchiae, or gills by which they respire, 
are fixed on the internal surface of the thorax, and are 
lodged in a cavity formed by the prolongation of the 
carapace beyond the sides. The head and thorax are 
compactly united or soldered together, and are covered 
by a large shield-shaped carapace, which stretches be- 
yond the front of the body, descends on each side to 
the base of the feet, and extends backwards as far as 
to the origin of the abdomen. The feet, properly so 
called, are five pairs ; the first of which are transformed 
into arms, and are terminated by a pair of large claws 
or pincers, forming organs of prehension; the four other 
pairs being the true organs of motion. These animals 
are slow of growth, live long, and some of them become 
very large. 
They are divided into three large groups, which are 
distinguished from each other by the comparative length 
of the abdomen, or, as it is generally called, the tail — 
the Short-tailed, ovBrachyura; theLong-tailed, or Mac- 
roura; and the anomalous-tailed, or Anomoura. 
Brachyura. — The Short-tailed Crustaceans, orCrabs, 
have their short abdomen generally bent under the 
Fig. 183. 
The Common Crab. 
body and lodged in a cavity there, so that it is of little 
or no use to them in swimming. The opecies are very 
