CHAPTER XXI. 



THE ARTHROPODA 

 THE CRAYFISH 



As an example of a gill-breathing arthropod, the crayfish has become 

 the classic laboratory type, and this because, like the frog, it is already 

 known to the student to some extent. 



The phylum to which man and the frog belong the Vertebrate 

 is in point of numbers much smaller than the phylum Arthropoda, to 

 which the crayfish belongs a group embracing more than three- fourths 

 of all living animals. 



The Arthropoda are usually divided into branchiata* ( ) 



commonly called Crustacea ( ) those animals 



possessing a hard chitinous ( ) exoskeleton and 



breathing with gills, practically all of which live in water ; and tracheata 

 ( ), consisting of those animals breathing through 



little tubules called tracheae. The tracheata include grasshoppers, bees, 

 wasps, ants, spiders, and insects of all kinds. While 400,000 of the 

 600,000 known species of animals belong to the Arthropoda, the greatest 

 sub-group of these is, in turn, the insects. 



The crayfish is large enough to be studied profitably in the labora- 

 tory. All who have lived or spent any of their youth near ponds and 

 rivers, know at least one or two species of crayfish. These they have 

 found lying quietly under the stones in running streams, and when such 

 stones were lifted, the animal's pincers were threateningly brought for- 

 ward to clasp the fingers of the supposed attacker. Then followed a 

 darting backward until the animal again pushed itself under some shel- 

 tering object or was able to find some close corner in which its body 

 could be pressed. 



The exterior skeleton so prominent in the Arthropoda, is in thor- 

 ough contrast to that of the frog, whose supporting tissues are placed 

 on the innermost portion of its body ; yet it is not from this character- 

 istic that the phylum is named, but from the fact that the animals be- 

 longing to this group have jointed legs. The word arthropoda means 

 jointed feet. 



The crayfish will be used in this book more as a type to introduce 

 nomenclature and general arrangement of the phylum Arthropoda than 

 as a study of detail. 



The entry into a more minute investigation of the phylum will come 

 with a study of the grasshopper. The larger and more convenient size 



*This classification into Branchiata and Tracheata lacks scientific foundation, but is convenient 

 for the beginner and for the student of medicine. As an example of why this classification is not 

 scientific, we may mention the true spiders which have no tracheae and yet are called Tracheates. 



