ARTHROPODA. 



With segmented bodies that are provided with segmented 

 appendages. 



CLASS 1. Crustacea. 



Usually aquatic. With a more or less hardened 

 outer covering and many thoracic appendages. 

 Subclass 1. Entomostraca. 



Usually small. Appendages little differentiated. 

 The number of post-cephalic segments variable. 



Order 1. Phyllopoda. 



Appendages with leaf-like expansions. (Bran- 

 chipus, Daphnia.) 



Order 2. Ostracoda. 



Free-swimming, with the body inclosed in a bi- 

 ^ valve shell. Seven pairs of appendages. (Cy- 



pris.) 



Order 3. Copepoda. 



Body elongated and distinctly segmented (ex- 

 cept in parasitic forms). Four or five pairs of 

 biramous appendages. (Cyclops, Argulus.) 



Order 4. Cirripedia. 



Comparatively large and usually attached. Usu- 

 ally with six pairs of biramous appendages. 

 Forms that are not parasitic are covered by cal- 

 careous plates. (Lepas, Balanus.) 

 Subclass 2. Malacostraca. 



Usually of considerable size and generally highly 

 organized. Except in one order, thorax of eight 

 and abdomen of seven segments. 



Order 1. Phyllocarida. 



Body inclosed in a large, bivalve cephalic cara- 

 pace. Abdomen of eight segments. Thoracic 

 segments free from the head. (Nebalia.) 



Order 2. Schizopoda. 



Thoracic appendages all biramous. Shrimp-like 

 in shape. (Mysis.) 

 133 



