12 Guide to Crustacea. 



mouth and at least three pairs of jaw-Hke appendages behind the 

 mouth, 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 having 

 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 LOBSTEE AS A TYPE OF CEUSTACEA. 



The plan of structure common to the whole Class will be best 

 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, wdiich is the type more usually described in 

 text-books, differs only in minor details from the Lobster. 



Like the other iVrthropoda, 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 

 pai-ts. Thus, the body and limbs are divided into " segments " * 



* The word " joint," often applied to these divisions of the hody and 

 limbs, ought properly to be restricted to the hinge or connection between two 

 segments. 



