168 ARTHROPODA 



eyes on stalks? What appendages are used in walking? Are 

 all of these appendages used in just the same way? Does 

 the animal move equally well in all directions? Perhaps 

 you can make the animal swim; if so, observe the method. 

 Feed a specimen with portions of a clam or fish, and see how 

 food is torn to pieces and transferred to the mouth, and de- 

 termine, if possible, how the mouth appendages are used. 



Appendages may be missing. If any are, notice at what 

 point they are broken. Possibly small appendages may be 

 growing from the old stubs. Autotomy may be studied by 

 crushing a claw or a leg of the fiddler crab, Uca. Other 

 forms will respond, but sometimes not promptly. What is 

 the importance of this reaction? 



External Anatomy. — As in Nereis, the body is segmented. 

 The five segments of the head and the eight segments of the 

 thorax, however, are immovably fused to form a cephalo- 

 thorax. This is covered dorsally by a single piece, the cara- 

 pace. 



1. Note, on the carapace, the cervical groove between the 

 head and thorax, and the beak or rostrum forming an an- 

 terior spine. The ventrolateral edge of the carapace is not 

 attached. A flat object thrust between it and the body 

 passes into the gill chamber. This free plate of the carapace 

 is called the gill cover. Notice the hairlike spines along its 

 free border. What purpose do these serve? 



2. The abdomen is composed of seven movable segments, 

 each bearing a pair of jointed appendages except the last, 

 which is sometimes not considered a true segment and is 

 called the telson. Each abdominal segment consists of a 

 dorsal piece, the tergum, which is continued as a free plate 

 laterally (the pleuron) , and of a ventral piece, the sternum. 

 Move the abdominal segments and see where they are hinged. 

 How are the terga and sterna arranged to allow free move- 

 ment? In the thorax the sterna, though fused, can be dis- 

 tinguished. 



3. Appendages. — Aside from the stalked eyes, whose ho- 



