266 niYLUM ARTHROPODA. 



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 coclom 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 alwavs 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 predominantly 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 water and breathe by gills 

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

 in addition to other appendages ; the thorax or median part 

 of the body, sometimes distinct from, and sometimes fused to the 

 head, also bears limbs ; the posterior region or abdomen is 

 usually segmented, and often furnished with 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 Lobsters 

 Hoiiiants and IWiintrus. and to the Norway Lobster (Ncphrops 

 norvegiciis'), 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 absent from 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 creep 

 forwards on their "walking legs." Their life is tolerably 



