KEY TO THE CLASSES OF ARTHROPODA 



1. Animal enclosed in a clam-like shell 



Crustacea 

 Not so 2. 



2. With no apparent antennae 3. 

 With one or two pairs of antennae 6. 



3. Body almost cylindrical and divided into ring-like segments; with three 



pairs of segmented legs anteriorly or without legs 4. 



Body more spherical, or with four or more pairs of segmented legs 5. 



4. With three pairs of segmented legs anteriorly and with three pairs of 



undeveloped legs on the abdomen in the adult; rare (grouped indc 

 pendently or with the Insecta or with the Arachyiida by various writers) 



M.yrientomata, (Order Protura) 

 Larval forms; with or without legs 



Insecta or Hexapoda Insects 



5. With many pairs of body appendages 



Crustacea 

 With four pairs of legs 



Arachnoxdea, Subclass Arachnida Arachnids 



6. With two pairs of antennae 



Crustacea 

 With one pair of antennae 7. 



7. With three pairs of segmented legs; usually, but not always, with wings 



in the adult 



Insecta or Hexapoda Insects 

 With more than three pairs of segmented legs; no wings 8. 



8. Body long and narrow, with regular segments most of which bear one 



or two pairs of legs 



Is/lyriapoda Myriapods 

 Body divided into thoracic and abdominal regions; appendages often ir' 

 regular in size, shape or distribution 



Crustacea 



CRUSTACEA 



The Crustacea constitute one large class of the great phylum Arthropoda. 

 They arc usually distinguished from the other arthropods by the presence of 

 two pairs of antennae. Most of them breathe by means of gills and almost all 

 of them are aquatic. The class is divided by many writers into two sections, 

 the Malacostraca and the Entomostraca, the former usually being relatively 

 large animals with appendages on the abdominal segments, while the latter are 

 usually small and have no abdominal appendages. 



The term Entomostraca means creatures with insect-like shells or bodies 

 and was applied by O. F. Miiller in 1785. Before that time they had been 



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