TEN-LEGGED CRUSTACEA 413 



GROUP L: THORACOSTRACA. 

 ORDER I. : TEN-LEGGED CRUSTACEA (DECAPODA). 



The Crayfish (Astacus fluviatilis). 



(Length up to 8 inches.) 



A. Its Occurrence and Habitat. 



The fresh-water crayfish is plentiful enough in some English rivers, e.g., 

 the Thames. It is to be found also in considerable numbers in the 

 artificial channel' known as the New Eiver, which supplies part of 

 London with water. 



The streams and rivers of Germany are now as poor in crayfish as 

 they once were rich in these animals ; indeed, in many waters these 

 creatures, so highly esteemed on account of their excellent flesh, have 

 been totally exterminated. This is chiefly owing to a disease, the 

 crayfish pest, which in 1876 made its appearance in France, and spread 

 thence further and further east. Man, however, is not without his 

 share in the blame of the depopulation of the waters. The capture of 

 the crayfish was in many cases carried on in a most reckless fashion, 

 neither young nor females with eggs (see Section C) being spared, with 

 the result that all the progeny was destroyed. Manufactories also 

 emptied their poison-laden waters into streams and rivers and killed 

 these sensitive animals, to whom pure water is a primary necessity. 

 (We shall see later what other conditions must be satisfied by the waters 

 inhabited by the crayfish.) 



During the day the crayfish rests in some safe hiding-place, under 

 stones, among the roots of plants growing on the banks, or in holes, 

 which it digs out for itself with its great claws (which see). Hence, in 

 the second place, the crayfish prefers streams where it can find recesses 

 of this description ready formed for it, or where the banks are of such 

 formation that it can easily dig them out for itself. Not until darkness 

 sets in does the animal leave its haunt in search of food. 



B. Structure of Crayfish, 

 i. Segmentation of the Body. 

 A mere superficial glance shows that the body of the crayfish consists 

 of two distinctly separate portions. The anterior of these divisions 

 corresponds to the head and thorax of the body of an insect (see p. 311), 

 both of which here (as in spiders, see p. 408) are intimately united to 

 form the cephalo-thorax. The original division of the two portions is, 

 however, still indicated by a curved transverse furrow (cervical groove), 



