THE MUSEUM 
NATURAL HISTORY. 
ZOOLOGY. 
Class CRUSTACEA. 
Forming part of the great division of the animal king- 
or shedding their carapace at certain periods. During 
dom, the Annulosa, and immediately following the 
the time the animal is growing in size, their external 
Insects, comes the important class of animals called 
covering does not increase in equal proportion. In 
Crustacea. 
order, therefore, to permit the growth of the body to go 
The length to which the article “Insects” has neces- 
on, this solid envelop is thrown off, and a new one ex- 
sarily extended, must be our apology for the summary 
actly fitted to the increased size of the animal is formed. 
manner in which this most interesting class must be 
When young these periodic moultings, as they are called, 
treated. 
are of frequent occurrence, and it is wonderful to see 
Crustaceous animals are true members of the Arti- 
the perfect manner in which the covering of the body. 
culata. having, like insects, no internal skeleton, but 
feet, antennse, &c., is thrown off, without any portion 
generally, on the contrary, an external tegumentary 
of it being destroyed or broken. 
skeleton, composed of flexible rings or joints, and the 
Crustacea respire by means of organs analogous to 
body provided with articulated or jointed organs of 
the gills of fishes, and called, as in them, branchice. 
locomotion. 
These organs are either exclusively formed as gills, or 
They differ, however, from insects in having their 
they are members modified and adapted for that pur- 
respiration carried on by means of gills or branchiae. 
pose. By their means the animals separate the oxygen 
and in having a double or complete circulation of blood. 
contained in solution in the water in which they live. 
Their body is in general covered with a hard external , 
They vary much in number and in shape in the differ- 
shell or crust (crusta), from which circumstance they 
ent species, but are generally placed near the base of 
derive their name. This shell has for its base a pecu- 
the legs, and are protected by the border of the cara- 
liar substance, which is found also in the covering of 
pace, though a few have them placed externally. 
insects, called chitine. It is commonly called the 
The blood in the Crustacea is colourless, and when 
carapax or carapace ; and in man}' of the species, as 
observed by the microscope is seen to contain a great 
the common crab and lobster, is extremely hard, almost 
quantity of albuminous globules suspended in it. There 
stony, being composed of a large portion of carbonate 
are no absorbent vessels, and it appears that the chyle 
of lime with a small quantity of chitine ; whilst in 
is carried into the blood by imbibition. Crustacea 
others, as in the Entomostracous species, it is of a 
possess a heart, which by its muscular action propels 
horny consistence, the composition partaking largely of 
the blood over every part of the body, through a well- 
chitine and albumen, and slightly of carbonate of lime. 
developed system of arterial vessels. After it has 
This carapace being divided into rings or joints, the 
served for the nourishment of the diffeient organs, it is 
animals, even in those which have it hardest, possess 
carried into the veins, which are rather reservoirs or 
a considerable, degree of freedom of motion. The nor- 
lacunce than vessels properly so called, and from them 
mal number of these rings or joints is twenty-one, but 
through the branchiae or gills. After traversing these 
two or three are often blended together or soldered into 
organs it returns directly to the heart, to run anew the 
one. To each of these joints except the last, there is 
circle just mentioned. The nervous system of the Crus- 
attached a pair of members, the forms and uses of which 
tacea consists of a chain of ganglions, more or less 
vary much in the different species, according to their 
numerous, united by nervous cords. The normal num- 
age, &c. These members are always divided into joints 
her of these ganglions is equal to that of the joints of 
or articulations, the number of which is very various. 
the body; but sometimes they are double, and at others 
and are covered with the same envelop or crust as the 
they appear to be much fewer in number, owing to 
body itself. Crustacea possess the power of casting off" 
several being united together into one mass. From 
