78 THE STORY OF ANIMAL LIFE 



"chelae," or claws, such as the large claws of the 

 lobster ; " chelicerae," or " claw-horns," tearing 

 appendages attached to the head ; " mandibles," 

 mouth appendages used for biting, etc., etc. The 

 reader who wishes to attain a clear idea of the 

 structure of a segmented animal, and of the ways 

 in which its parts are modified, should consult 

 Huxley's classical study of " The Crayfish " (In- 

 ternational Science Series). 



The Arthropoda include two main groups 

 the Crustacea, or Jointed Animals of the water, 

 which breathe by gills; the Insects, or Jointed 

 Animals of the land, which breathe through tubes 

 in their sides, called tracheae. 



The Crustacea include the familiar Crabs and 

 Lobsters. These are among their highest forms 

 as well as their largest, and if we begin at the 

 beginning we must seek much smaller forms. 

 The group called Entomostraca include the so- 

 called Freshwater Flea, a very active little thing 

 found in English ditches, and a great many other 

 freshwater forms : also the little Cypris, which 

 has a shield forming a sort of bivalve-shell, and 

 is interesting from its wide occurrence as a fossil 

 form. Most of the Entomostraca have a larval 

 form called a Nauplius ; but this larva refuses to 

 tell us anything about the past history of the 

 Arthropods. It is itself already a jointed animal 

 with legs. So we see that the Arthropods, unlike 

 the worms and the Chordata, have obliterated 

 all record of their poor relations. The parasitic 

 " fish-lice," so-called, are entomostracous Crus- 

 tacea, often greatly degenerated in consequence 

 of their habit of life. Some live in the gill- 

 chambers of a fish, some on, or even embedded in 

 the skin. 



