PHYLUM ARTHROPODA 293 



Figure 16.3. Nauplius larva. (After Dietrich.) 



limbs are usually biiamous when they first appear, the basic limb plan 

 in crustaceans is similar to that oi tiie trilobites: one pair oi uniranious 

 antennae followed by a series of biramous limbs. The nauplius larva 

 is found in all of the major groups of crustaceans, which suggests that 

 an animal resembling it may have been the common ancestor of the 

 class. 



Crustaceans are traditionally divided into the large and the small. 

 Large members form a natural subclass, the Malacostraca. In this group 

 the order Decapoda (ten walking legs) includes the familiar shrimps, 

 crayfish, lobsters and crabs. The crayfish will be described as an ex- 

 ample of the subclass. Similarities among malacostracans are close 

 enough so that knowledge of one form is a key to the understanding of 

 others. Small crustaceans are grouped in several orders that form 

 several subclasses. Of these the water-llea will be described as an ex- 

 ample. Unfortunately the orders of small crustaceans are so diverse 

 that one example is not an adequate introduction to the others. 



134. Astacus, a Crayfish 



Crayfish of the genus Astacus are common in this country west of the 

 Rockies. To the east the slightly dilferent genus, Cambarus, is abundant. 

 Crayfish are found in or near ponds, lakes and streams. Those in the 

 water excavate holes beneath logs and stones to serve as temporary 

 shelters, while those on the banks may dig deep burrows. They are 

 most active at dusk and after dark, scavenging the neighborhood for 

 plant or animal debris and occasionally capturing unwary insects, tad- 

 poles and fish. 



1 35. External Morphology of the Crayfish 



The crayfish body (Fig. 16.4) is divided into a solid cephalothorax 

 and a jointed abdomen. If we include the embryonic first segment, the 

 cephalothorax represents the fusion of six cephalic and eight thoracic 

 seginents. All except the first have appendages. The back extends 

 laterally as a pair of skeletal folds that bend down over the sides of 



