CRAYFISH. 231 



Type. The Fresh-water Crayfish (Astacus fluviabilis). 



(Most of the description will apply to the Lobsters (Homarus 

 and Faltnurus), 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 absent from districts with 

 little lime in the waters. Its food is very varied — from 

 roots to water-rats, and cannibalism occurs. The animals 

 swim actively backwards by powerful tail-strokes, or creep 

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

 secure, but the frequent moultings are expensive and 

 hazardous. The hatched young are like miniature adults, 

 and cling for a while beneath the tail of the mother. 



External Appearance. — The head and thorax are covered 

 by a combined (cephalothoracic) shield ; the abdomen con- 

 sists of six separate rings and a terminal piece (telson) on 

 which the food-canal ends. The stalked moveable eyes, the 

 two pairs of feelers, the mouth with many appendages crowded 

 around it, the gills under the side-flaps of the thoracic shield, 

 the varied appendages with sensory, masticatory, combative, 

 locomotor, and other functions, these and other features are 

 obvious. But the plan of the animal must be realised ; it 

 consists of three groups of segments — five in the head, 

 eight in the thorax, six in the abdomen, and the nineteen 

 rings bear nineteen pairs of appendages, which by division of 

 labour and other causes have come to be very heterogeneous. 

 Yet in development and plan of structure, they are all 

 similar, and illustrate clearly what is meant by " serial homo- 

 logy." 



r(T.) The external cuticle, including various strata of 



I chitin, plus or minus pigment and lime-salts ; 



The Body- Wall I (2) The ectoderm, epidermis, or hypodermis, which 



consists of — \ makes and remakes the cuticle ; 



I (3) Ah internal connective-tissue layer with pigment, 



I blood-vessels, and nerves. 



Between the rings and between the joints the cuticle is 

 not calcareous and is therefore pliable. As a sacrificed 



