212 MANUAL OF ZOOLOGY SECT. 



poda, the Scorpion of the class Arachnida, the Centipede of 

 the class Myriapoda, and the Blue-bottle Fly of the class 

 Insecta. 



1. THE CRUSTACEA 



The class Crustacea comprises a very large number of 

 Arthropods, the great majority of which are inhabitants 

 either of fresh or of salt water. 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 may be studied. 

 The following description applies more especially to the 

 common European Crayfish (Potamobia pallipes)^ but the 

 American species of the same or allied genera, the Austra- 

 lian species of the genus Astacopsis, or the New Zealand 

 species of the genus Paranephrops, will be found to corres- 

 pond in all essential respects. 



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 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 integu- 

 ment of Nereis and the Leech, the Crayfish has a hard 

 enclosing crust or exoskeleton formed of the thickened 

 and chitinous 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., 

 1 More commonly named A status flnviatilis. 



