ARTHROPOD A. 83 



CHAPTER VI ARTHROPODA. 

 10. ASTACUS (The Crayfish). 



THE Crayfish is a small greenish lobster-like animal, living in 

 streams and canals, under the banks of which it burrows. An 

 average mature specimen is about five inches long. There is only 

 one British species, Astacus fluviatilis, a large variety of which, 

 A. fluviatilis var. nobilis, is common on the Continent, where it is 

 largely used as food. As the points of difference are but small, 

 the following description will apply to either : 



Crayfishes are mainly carnivorous, devouring water-snails, tad- 

 poles, insect larvae, worms or other animals, dead or alive, but 

 they also feed on vegetable matter. They either walk by means 

 of their four pair of jointed legs, or else swim swiftly by alter- 

 nately bending and straightening their powerful tails, which 

 propel them backwards; they can also employ their large nippers 

 for climbing. 



The sexes are distinct, but there is not much difference exter- 

 nally between the male and female. The most obvious distinction 

 is that the tail of the latter is somewhat broader, and in specimens 

 examined during winter or spring a considerable number of rather 

 large eggs will be found attached to its under side. These are 

 hatched in May or June. 



MORPHOLOGY AND PHYSIOLOGY. 



1. External Characters (Figs. 25, 26, and 27). The bilater- 

 ally symmetrical body is segmented, as in the Earthworm, but the 

 segments are definite in number, and, instead of being nearly all 

 alike, vary much in character, and many of them are fused 

 together. Owing to these differences the body is marked out 

 into regions from before backwards, head, thorax, and abdomen or 

 tail. The two first are united to form the cephalothorax, which 

 is covered by a firm continuous shell, the carapace, dorsally and 

 laterally. The segments of all the regions bear paired, jointed 

 appendages of various kinds, which differ greatly in form and 



