CRAYFISH. 281 
bear paired jointed appendages variously modified. The 
cuticle is chitinous. Ciltated 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 celom is always 
small in the adult; the apparent body cavity is of secondary 
origin, and has ina great part a blood-carrying or vascular 
Junction. 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 antenne in 
addition to other appendages, e.g. at least three pairs of 
jaws; the thorax, sometimes distinct from, and sometimes 
Jused to the head, bears various kinds of limbs ; the abdomen 
zs 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 ts 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 (Paiimurus), and to the Norway 
Lobster (Wephrops 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 
