THE CRAYFISH. 253 



General Characteristics of Arthropods (to which primitive, 

 parasitic, and degenerate forms present exceptions). 



The body is bilaterally symmetrical, and consists of numerous 

 segments variously grouped. Several or all of the segments 

 bear paired jointed appendages variously modified. The 

 cuticle is chitinous. Ciliated epithelium is absent, except in 

 Peripatus. The dorsal brain is connected by a ring roufid the 

 gullet with a double chain of ventral ganglia. Above the food 

 canal lies the heart. The trtie or primitive ccelome is ahvays 

 small in the adult ; the apparent body cavity is of secondary 

 origin, and has in a great part a blood carrying or vascular 

 function. The sexes are almost always separate, the repro- 

 ductive organs and ducts are usually paired. There is often 

 some metamorphosis in the course of development. In habit 

 the Arthropods are predominatitlv active. 



Class Crustacea. 



General Characteristics of Crustaceans (to which primitive, 

 parasitic, and degenerate forms offer exceptions). 



With the exception of the land crabs, wood lice, and sand 

 hoppers, the Crustaceans live in ivater and breathe by gills 

 or through the skin. The head carries two pairs of antennce 

 atid other appendages ; the thorax or median part of the 

 body, sometimes distinct from, sometimes fused to the head, also 

 bears limbs ; the posterior region or abdomen is usually seg- 

 mented, atid often furnished with appendages. The typical 

 appetidage consists of tivo branches and a basal portion to 

 which gills may be attached. To the chitin of the cuticle, 

 carbonate of lime is added. 



A Type of Crustacea. The fresh water Crayfish 

 {Astacus fluvia tills) . 



(Most of the following description will apply also to the Lobsters 

 (Hoinarus and Palimiriis), and to the Norway Lobster (Neplirops 

 norvegicus), often called a crayfish). 



Mode of Life. 



The fresh water crayfish hves in streams, and burrows in 

 the banks. It is not found in Scotland, but occurs here 



