78 CRABS AND INSECTS. 
and the other a cutter; the other four pairs (Fig. 81, B) 
are long and slender, the first two ending in nippers, the 
hinder ones being provided with points or claws; these 
are the true organs of locomotion on the bottom. Each 
segment of the abdomen except the last bears a pair of flat 
appendages or swimmerets (Fig. 81, 2), by which the cray- 
fish can swim ahead, and to these the eggs are attached. 
Digestion.—The digestive organs, seen in Fig. 83, con- 
sist of the mouth, surrounded by the mandibles, that leads 
Fic. 82.—Tail of a cray-fish, Fic. 83.—Ideal section of prawn, 
showing flattened append- showing : s, stomach, below this 
ages for swimming. the mouth ; /, liver; 7, intestine; 
4, heart ; g, chain of ganglia or 
nerve-masses ; 4g, head-ganglia. 
by the cesophagus into the large stomach s ; the latter is 
provided with crushing teeth, by which food is still further 
masticated, then passing through a strainer at the posterior 
end, and so into the intestine 7, that leads into the telson. 
The liver / is very large and of a dark-green hue. 
Respiration.—The higher crustaceans breathe by gills, 
the plume-like object in Fig. 81. They are attached to 
the base of the legs, and are protected by the carapace or 
shelly covering of the cephalo-thorax. Water containing 
air reaches the gills by flowing under the edge of the car- 
apace back of the great claws. In the oyster (Fig. 55), we 
saw that cilia kept up a current over the gills, but here 
there is a curious appendage attached to the base of the 
second pair of maxillipeds (Fig. 81, /), called the “ gill- 
