XII. 



THE FRESH-WATER CRAYFISH (Astacus fluviatilis) 

 AND THE LOBSTER (Homarus vulgaris). 



THE Crayfish and the Lobster are inhabitants of the water, 

 the former occurring in many of our rivers and the latter 

 abounding on the rocky parts of the coasts of the European 

 seas. They are bilaterally symmetrical animals, provided 

 with many pairs of limbs, among which the large prehensile 

 'claws' are conspicuous. They are very active, walking 

 and swimming with equal ease and sometimes propelling 

 themselves backwards or forwards, with great swiftness, by 

 strokes of the broad fin which terminates the body. They 

 have conspicuous eyes, mounted upon moveable stalks, at 

 the anterior end of the head ; and two pairs of feelers, one 

 pair of which are as long as the body, while the other pair 

 are much shorter. 



The body and limbs are invested by a strong jointed 

 shell, or exoskekton, which is a product of the subjacent epi- 

 dermis, and consists of layers of membrane which remain 

 soft and flexible in the interspaces between the segments of 

 the body and limbs, but are rendered hard and dense else- 

 where by the deposit of calcareous salts; the exoskeleton 

 is deeply tinged with a colouring matter which turns red 

 when exposed to the action of boiling water. The body 

 presents an anterior division the cephalothorax covered 



