sect, x PHYLUM ARTHROPODA 213 



tipede of the class Myriapoda, and the blue-bottle fly of the 

 class Insecta. 



1. THE CKUSTACEA 



The class Crustacea comprises a very large number of 

 Arthropods, the great majority of which are inhabitants 

 either of fresh water or of salt. Familiar examples of Crus- 

 tacea are the crayfishes, lobsters, shrimps, and prawns, the 

 crabs and hermit-crabs, the sand- hoppers, and woodlice, 

 the barnacles, and acorn-shells. As an example of the 

 Crustacea the Fresh-water Crayfish should be studied. 

 The following description applies more especially to the 

 common European crayfish {Potamobia pallipes), 1 but the 

 American" species of Astacus will be found to correspond in 

 all essential respects, while the lobster also presents but slight 

 differences. 



It is to be noticed, in the first place, that the crayfish, 

 like Nereis, is a bilaterally symmetrical animal, and that the 

 bilateral symmetry is complete, the right and left halves of 

 the body being exactly alike. The crayfish, it is to be 

 noticed, also resembles Nereis and the leech in being 

 metamerically segmented, the segmentation being most 

 clearly distinguishable in the posterior region of the body. 

 Here, however, the external resemblance ceases. Instead 

 of the soft integument of Nereis and the leech, the crayfish 

 has a hard enclosing crust or exoskeleton formed of the 

 thickened and calcified cuticle, and, in place of the un- 

 jointed, short parapodia of Nereis, there are a series of 

 variously modified appendages, feelers, jaws, legs, etc., 

 which, like the body itself, are enclosed in a hard exo- 

 skeleton, having a jointed character, the appendages thus 



1 More commonly named Astacus fluviatilis. 



