320 PHYLUM ARTHROPODA 



bear paired jointed appendages variously modified. The 

 cuticle is chitinous. Ciliated epithelium is almost always 

 absent. The dorsal brain is connected by a ring round 

 the gullet zvith a double chain of ventral ganglia. Above the 

 food canal lies the heart. The true or primitive coelom is 

 always 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 

 reproductive organs and ducts are usually paired. There is 

 often some metamorphosis in the course of development . In 

 habit the Arthropods are predominantly active. 



Class (Crustacea 



General Characteristics of Crustaceans (to which 

 primitive, parasitic, and degenerate forms offer 

 exceptions) 



With few exceptions, e.g. land-crabs, wood-lice, and sand- 

 hoppers, Crustaceans live in water. They breathe by gills or 

 cutaneously. The head carries two pairs of antenna in 

 addition to other appendages, e.g. at least three pairs of 

 jaws ; the thorax, sometimes distinct from, and sometimes 

 fused to the head, bears various kinds of limbs ; the abdomen 

 is usually segmented, and often has appendages. The 

 typical appendage consists of two 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 fluviatilis) 



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

 (Homarus), to the Rock Lobster {Paliniirus), and to the Norway 

 Lobster {Nephrops norvegicus), often called a crayfish.) 



Mode of life. — The fresh-water crayfish lives in streams, 

 and burrows in the banks. It is not found in Scotland, but 

 occurs here and there in England and Ireland, and is 

 common on the Continent. It is not found in districts 

 where the water contains little lime. The food is very 

 varied — from roots to water-rats ; cannibalism also occurs. 



