296 ^^^ ANIMAL KINGDOM 



Corneal le.ns Tactile bristles 

 Epidermis 



Chemoreceptor^ 



B 



Opening 



Statocyst 



Pigment 



Retinula cell 

 Nerve fiber 



Nerve 



^ Statocyst 

 bristle 



Figure 16.7. Sense organs of the crayfish. A, Two ommatidia from the compound 

 eye. In each, light passing through the two lenses is focused on the outer end of the 

 rhabdome, which is made of seven fused rods or rhabdomeres, striated thickenings along 

 the inner edges of seven retinula cells. Pigment screens out stray light. B, Sensory bristles 

 on the antenna. A chemoreceptor is enlarged at the right, viewed from two directions. 

 C, The statocyst (above) and a still greater enlargement of one of the sensory bristles 

 inside the statocyst (below). (B and C from Huxley, 1880.) 



cephalic maxillae have much flattened protopodites expanded medially 

 to serve as plates for holding food against the jaws. Endopodites are 

 similarly flattened. The first maxillae lack exopodites, but those of the 

 second are expanded laterally with part of the protopodites to form 

 large flaps, the gill bailers. The mandibles (segment 4) are deeply 

 seated under the mouth. Each is a stout protopodite expanded medi- 

 ally to form teeth, which bears a small tactile endopodite, the man- 

 dibular palp. These jaws chew the food which is brought by chelate 

 pereiopods, shredded by maxillipeds, and held against the mouth by 

 maxillae. The simultaneous activity of all these pieces is bewildering 

 to the observer! 



Anterior to the jaws are two pairs of antennae. The second pair 

 each have a very long, many jointed endopodite, the flagellum, and a 

 flat exopodite, the scale. \VhiIe the crayfish is scooting backward the 

 scales are held outward to serve as rudders. Each first antenna has a 

 base with two flagella, producing an apparent biramous condition in 

 what is embryologically a uniramous limb. The flagella of both pairs 



