CHAPTER VI 



THE CRAYFISH. — CHARACTERS OF THE PHYLUM 

 ARTHROPODA. 



\\'e have now to study an animal formed on a very similar 

 plan of structure to the Earthworm as regards segmentation 

 and arrangement of many of the organs, but which reaches 

 in every respect a far higher grade of organisation. 



The common British Fresh-water Crayfish is usually known 

 as Astaiiis Jliroiatilis, and is found in many of the streams and 

 rivers of England and Ireland, hiding under stones, or in 

 holes, into which it darts very suddenly on the approach of 

 danger. Its ordinary creeping movements are slow, and are 

 effected by means of a number of jointed limbs, for which 

 reason it is included, together with insects, spiders, scorpions, 

 tKrc, in the phj-lum Arthropoda. 



In colour, the Crayfish is greenish grey, and in form it is 

 very similar to the marine Eobster {Hotnarus vulgaris), to 

 which the following description will apply almost equally 

 well. 



In addition to the presence of paired limbs or appendages, 

 one of the most striking points of difference between an 

 Earthworm and a Crayfish is the smaller and constant 

 number of segments or metameres in the latter, as well 

 us the fact that certain of these are more or less 



