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, a), 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 



V 

 r^v ; 



FIG. 82. Tail of a cray-fish, 

 showing flattened append- 

 ages for swimming. (After 

 Morse.) 



FIG. 83. Ideal section of prawn, 

 showing : s, stomach, below this 

 the mouth ; /, liver ; z', intestine ; 

 //, heart ; g, chain of ganglia or 

 nerve-masses ; hg, head-ganglia. 



by the oesophagus 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 /, 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. 8i,/), called the "gill- 



