THE CRAYFISH 113 



as they were. They are reared, however, in crayfish 

 "farms," in both Germany and France. In general appear- 

 ance, and indeed in almost every detail of its structure, the 

 crayfish resembles a small lobster, and dwellers by the 

 seaside who may find it more convenient to take the 

 latter animal as their type will find Httle in it that differs from 

 the following description : — 



The English crayfish is of a dull greenish-grey colour, and 

 seldom attains a greater length than three, or three and a 

 half inches, but a variety commonly found in France and 

 Germany is larger, attaining a length of five inches or even 

 more, and it is further distinguished by the red colour of its 

 pincers and walking legs. 



The crayfish's body, like that of Apus, is divisible into a 

 cephalic, a thoracic, and an abdominal region, each region 

 being composed of a number of segments bearing appendages. 

 The grouping and external aspect of these regions, however, 

 presents many differences. In the first place, the number of 

 segments in the crayfish's body is fewer than in that of Apus : 

 there are nineteen in all, excluding the posterior piece or 

 telson on which the anus opens, and each segment bears a 

 pair, and never more than one pair, of jointed appendages. 



In the second place, the cuticular armour or exoskeleton of 

 the crayfish is far more developed than in Apus, and attains 

 considerable thickness and strength by the deposition of salts 

 of lime in its chitinous organic basis. The salts of lime may 

 be dissolved out by placing the animal in dilute acid, and the 

 organic residue which is left, though preserving faithfully the 

 outlines of the exoskeleton, is soft and yielding. 



In the third place, the fusion of segments, found only in the 

 cephalic region of Apus, is carried to a much greater extent in 

 the crayfish, for in the latter animal the thoracic segments are 

 firmly fused to one another and to the head, and thus con- 

 stitute a cephalothorax. The dorsal integument forms a con- 

 tinuous shield known as the carapace. The abdomen, oh the 

 contrary, is composed of six distinct rings or segments in 

 addition to the terminal telson, the last named structure corre- 

 sponding to the caudal piece of Apus, but differing from it in 

 being devoid of caudal styles. 



The abdomen of the crayfish is flexible ; it can be extended 

 in a straight hne with the cephalothorax or can be bent down 



II. H 



Digitized by Microsoft® 



