PHYLUM ARTHROPODA 297 



of antennae are used for exploration of the environment. Those of the 

 second antennae are primarily tactile, while the others have many small 

 chemoreceptors. 



In addition to appendages the crayfish has several sense organs. 

 Compound eyes are borne on stalks at the front of the cephalothorax. 

 Each is a cluster of 10,000 or more ommatidia (Fig. 16.7 A) arranged 

 radially, with the outer facets forming the eye surface. Each omma- 

 tidium functions as a complete eye looking out at a restricted part of 

 the world. The visual fields of adjacent ommatidia overlap consider- 

 ably, but all together provide a kind of mosaic view of the world. 



Chemoreceptors are small blunt bristles (Fig. 16.7 B) usually 

 found in groups of three or four. They are especially abundant on the 

 first antennae and on the mouth parts. Tactile bristles (Fig. 16.7 B) are 

 small bristles jointed to the body surface and supplied with nerve cells 

 at the base. These are scattered all over the body and are especially 

 abundant on the second antennae. 



The basal segment of each first antenna contains an ingenious 

 statocyst (Fig. 16.7 C). During development the dorsal surface in- 

 vaginates to form a sac lined with numerous tactile bristles. The open- 

 ing remains as a slit concealed by a tuft of surface hairs. The crayfish 

 pushes its head into the sand until each sac contains a gioup of sand 

 grains, which then provide stimuli for the sense of balance by the way 

 they lie against the sensory bristles. 



The sides of the carapace, arching over the body, enclose a pair of 

 gill chambers. Numerous gills (20 in Astacus) lie in each chamber, pro- 

 jecting ujiward from their origins on the limbs and body wall (Fig. 

 16.9). Each gill resembles a bottle brush, having a central axis and 

 numerous radiating filaments. On each side six gills (podobranchiae) 

 arise from the basal segments of the second and third maxillipeds and 

 first four pairs of pereiopods. Eleven more (arthrobranchiae) emerge 

 from the joint membrane between these legs and the body. Three addi- 

 tional pairs of gills (pleurobranchiae) originate on the sides of the 

 body above the last three pairs of pereiopods. Gills of adjacent body 

 segments are separated by flattened plates, the epipodites, attached to 

 the bases of the legs (Fig. 16.9). 



The carapace fits snugly against the bases of the legs, leaving sizable 

 openings only at the posterolateral edge and anteriorly beside the 

 mouth parts. The gill bailers of the second maxillae (Fig. 16.5) extend 

 back over the gills and undulate to produce a water current. Most of 

 the time water is drawn in posteriorly and expelled anteriorly, but 

 occasionally the direction is reversed to flush out debris that may have 

 collected on the gills. 



136. Internal Anatomy of the Crayfish 



Muscles extend between various parts of the body, but are prominent 

 only in the abdomen and legs. The abdomen is nearly filled with mus- 

 cle, including straplike dorsal extensors (Fig. 16.8 C) and very stout 

 complex ventral flexors. Obviously flexion is a much more powerful 

 movement than extension. In the floor of the thorax muscles to the 



