ARTHROPODA — CRUSTACEA 307 



under boards and stones and when disturbed rolls itself up into 

 a ball. (/) Amphipoda (Silurian to present), e.g. beach- 

 fleas, (g) Euphausiacea (not known fossil). 



(h) Decapoda. Triassic to present. 



The posterior five pairs of thoracic legs are long and are used 

 in walking, whence the name from Greek deka, ten, + pous 

 {pod), foot. The abdomen may be long and straight as in the 

 lobster and crayfish or permanently flexed beneath the long 

 cephalothorax, as in the crab group (examples 4-6). The latter 

 group include the most advanced of all Crustacea. No un- 

 doubted decapods have been found in the Paleozoic, but their 

 ancestors are to be sought for in the generalized schizopods 

 (orders h, c, and g) of that era, and were probably not difl^eren- 

 tiated from the latter until the Triassic. 



Examples: (i) Cambarus (Fig. 123). Crayfish of the 

 family Astacidae, to which Cambarus belongs, have been found 

 from the Cretaceous to the present day. The distinction of 

 Cambarus from Astacus, the other genus of this family, lies in 

 the character of the gills, features not preserved in the fossil 

 forms known. The crayfish is descended from marine lobster- 

 like ancestors. (2) Homarus (Cretaceous to present). H. 

 americanus (Fig. 125), the single Atlantic coast species of the 

 American lobster, ranges from North Carolina to Labrador and 

 from near shore to a depth of 100 fathoms. It is omnivorous 

 and is attracted to its food mostly by the sense of smell. A 

 female seventeen inches long will lay at one time about 60,000 

 eggs. The young pass through a stage in which they resemble 

 the Mysidacea. The lobster molts about eight times the first 

 year, five the second, and three the third. (3) Eryon (Jurassic 

 to Cretaceous). (4) The Japanese spider crab (Macrocheira 

 kdmpferi) is the largest living crustacean, with at times a spread 

 of legs of 18 to 20 feet. (5) The blue or edible crab {Callinectes 

 sapidus or hastatus) ranges from Florida to Cape Cod ; it prefers 

 muddy shores and like the lobster lives largely upon decomposing 



