196 



Elementary Zoology. 



thoracic segments ; the Stomatopoda, also with a number of free 

 thoracic segments, and with movable eye and antennule seg- 

 ments ; Sc'iizopoda, with biramous thoracic limbs ; and Decapoda, 

 with all the thoracic segments fused with the head. The thoracic 

 appendages are uniramous in the adults, e.g. crayfish, crab, 

 lobsters. 



(3) Arthrostraoa. — In this group the eyes are sessile, and 

 there is no cephalothoracic shield. It is divisible into the 

 Amphipoda (Fig. 90), with thoracic gills, of which examples 

 are the shore-hopper and the fresh-water shrimp, and the 



Fig. %(^.~As<!Uus aquaiicus. 

 Magnified. 



Fig. go. — The Fresh-water Shrimp. 

 Magnified. 



Isopoda (Fig. 89), with abdominal gills, of which the best- 

 known example is the terrestrial woodlouse. 



2. TRACHEATA. 



These Arthropods breathe either by tracheae or lung-books, 

 which may be external or invaginated. There is but one pair 

 of antennas. They are prevalently terrestrial, while the 



