12 
Guide to Crustacea. 
Wall- 
cases 
Nos. 1- 
month and at least three pairs of jaw-like appendages behind the 
month, in being nearly always of aquatic habits, and in breathing 
by gills or by the general surface of the body. 
A Crustacean can usually be distinguished from any other 
Arthropod by the fact that its “ walking-legs ” do not correspond 
in number or arrangement with those found in the other groups 
Thus an Insect can usually be recognised at first sight by haying 
three pairs of legs, an Arachnid by having four pairs, and a 
Centipede or a Millipede by having a great number of legs, all 
nearly alike. The Crustacea, on the other hand, show a great 
variety in the arrangement of their walking or swimming legs, but 
they very seldom exhibit any special resemblance, in respect of 
these appendages, to the other large groups of Arthropods. 
THE LOBSTER AS A TYPE OF CRUSTACEA. 
The plan of structure common to the whole Class will be best 
3 understood by beginning with the study of a typical form. 
For this purpose the common Lobster has been selected as 
being easily accessible, of convenient size, and not too specialised 
to admit of ready comparison with other Crustacea. 
The Crayfish, which is the type more usually described in 
text-books, differs only in minor details from the Lobster. 
Like the other Arthropoda, the Crustacea have the body and 
limbs encased by a firm covering which gives support to the soft 
internal organs and in particular affords points of attachment for 
the muscles by means of which the animal moves. In other 
words, this covering plays the part of a skeleton ; but since, unlike 
the bony skeleton of Vertebrate animals, it is outside instead of 
inside the soft parts, it is distinguished as an “ exoskeleton.” In 
many Crustacea also, the exoskeleton is sufficiently strong to serve 
the purpose of defensive armour, and to enable the limbs to act as 
efficient and powerful weapons. 
Although the firm outer covering is really continuous over the 
whole of the surface of the body and limbs, it becomes thinned away 
in places to form joints permitting movement between the various 
parts. Thus, the body and limbs are divided into “ segments ” * 
* The word “ joint,” often applied to these divisions of the body and 
limbs, ought properly to be restricted to the hinge or connection between two 
segments. 
