FOOD AND FEEDING 



the three anterior pairs of limbs as foot-claws and the five pairs of posterior 



limbs as walking legs. The eyes are stalked and the gills thoracic. 



Sub-Order Amphipoda. This sub-order includes the Water-fleas 



and the marine Sand-hoppers, of which one genus is present in both 



running and standing fresh water. 



Gammarus. Fig.71. This freshwater Crustacean, known as the Water- 



fleaand FairyShrimp,Ga;»Wi3r«j'^a/<?;c,hasaflattenedform,theanteriorportion 



consisting of the head and thorax covered with a carapace, and the posterior 



portion with six segments and a terminal flap ending 



in a short bristle. The anterior three pairs of legs 



serve for swimming and the posterior legs for swim- 

 ming and the hopping motion by which it moves 



more rapidly through the water. The color is a 



translucent dusky grey. It is found in muddy 



streams among half-rotten brush wood and other 



litter, usually hidden under stones or aquatic plants, 



generally feeding at dusk or at night, when it either 



swims leisurely through the water or hops by rapid 



strokes of the posterior legs. Its food is decaying fig. 71. Gammarus puiex. 



vegetation, small animals and spawn; and will serve reatyenarge . 



as an eff^ective scavenger in tanks for larger fishes and amphibia but should 



not be introduced with young fishes. 



Sub-Order Isopoda. This sub-order includes the Water-Asel, 



Wood Louse and a number of marine forms. 



AsELLOPUs.'Fig.ya. This freshwater Crustacean is known astheWater- 



Kse\,Asellopus ienax, and has a compressed or flattened form and a segmental 



body covered with an olivate armor marked with lighter spots. It reaches 

 a length of i to 1.5 centimeters, and inhabits still and 

 slow-flowing water containing considerable vegetation. 

 Its movements are slow and it is usually secreted in 

 the mud or on the under side of the leaves of aquatic 

 plants; feeding on decaying vegetal substances, smaller 

 animals and spawn, and is an active enemy of very 

 young fishes. Another more slender and longer-limbed 

 form is Asellus communis, a New England States species. 

 Sub-Order Decapoda. This sub-order includes 

 the freshwater Crayfishes, marine Prawns, Shrimps, 

 the true Crabs and Lobsters. These will be treated 

 of elsewhere. 



The minute Crustaceans are of the greatest bene- 

 fit to the growth and survival of the young brood of 



FIG. 72. Asellopus tenax. 

 Greatly enlarged. 



