CHAPTER VII 



THE CRAYFISH CHARACTERS OF THE PHYLUM 

 ARTHROPODA 



WE 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 Astacusfluviatilis, 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, &c., 

 in the phylum Arthropoda (p. 220). 



In colour, the crayfish is greenish-grey, and in form 

 it is very similar to the marine Lobster, 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 as the fact that certain of these are more or less 



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