266 ELEMENTARY ZOOLOGY. 



Two pairs of feet to the segment; body cylindrical, antennae 



short Order 2, Diplopoda. 



Thirty or more segments; coils up when disturbed Julus 



(the galley- worm). 



279. Key to the Orders and to Some Common Forms of Crustacea. 

 Higher, larger forms; 19 pairs of appendages, 



Sub-class Malacostraca . 

 Segments fused into cephalo-thorax ; ten thoracic legs; eyes 



on stalks Order Decapoda, 



Abdomen long and usually well extended. 



Swimming forms Penaeus (prawns) . 



Crangon (shrimps). 



Creeping forms Cambarus (crayfish). 



Homarus (lobster). 

 Abdomen soft and degenerate; inserted in shell of 



mollusk Eupagurus (hermit-crab) . 



Abdomen poorly developed; turned under thorax. 



Body triangular in form; legs spider-like . .Libinia; etc., 



(spider-crabs) . 

 Body rounded ; broad in front and arched ; legs short, 



Cancer (burrowing crab) . 



Calinectes (soft-shelled crab). 



Body roughly four-sided ; one chela much larger than the 



other Gelasimus (fiddler-crab) . 



Thoracic segments free; eyes not stalked. Order Arthrostraca. 

 Body broad and flattened dorso-ventrally ; gills on legs 



(Isopoda) Asellus (water-bug) . 



Oniscus (sow-bug). 



Porcellio (pill-bug). 



Body flattened sidewise; legs partly adapted for swimming, 



partly for leaping Gammarus (sand-hoppers). 



Smaller forms with variable number (usually less than 19) 



appendages Sub-class Entomostraca. 



Attached in the adult; with limy shell, 



Order Cirripedia (barnacles) . 



Lepas, goose-neck barnacle. 



Balanus, acorn-shell barnacle. 



Free-swimming forms (fresh-water forms only included here) , 



Water-fleas. 

 Carapace grows ventrally and forms protective shell. 



Shell bivalved; body unsegmented ; 7 pairs of appendages. 



Cypris (Ostracoda). 



