CRA YFISH. 281 



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 with a double chain of ventral ganglia. Above the food 

 canal lies the heart. The true or primitive c&lom 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 reproduc- 

 tive 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 (Palinurus\ 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. 

 The animals swim backwards by powerful tail strokes, or 



