AETHROPODA. 43 



3. What is the advantage of such a shell to the cray- 



fish? 



4. What disadvantages has it ? 



5. The Carapace. 



The shell over the dorsal and lateral surfaces of the 



cephalo-thorax is called the carapace. 

 a. Is it continuous with the shell on the ventral 



surface ? 

 i. Observe the extension of the shell forward into a 



pen-like projection (rostrum). 

 c. Observe a transverse groove (cervical groove) 



between the head and the thorax. 



6. The Abdominal Shell. 



a. How many ring-like parts (somite shells) are 



there in the shell of the abdomen ? 



b. How are the somite shells arranged to permit the 



bending of the abdomen and still give protec- 

 tion? 



c. Exactly where are the hinges between the movable 



somite shells ? 



D. Appendages of the Thorax. 



The appendages of the thorax are called "walking- 

 legs." The large pair are often called chelipeds. 



1 . How many pairs of walking-legs are there ? 



2. Which are provided with pinchers ? (In numbering 



serial structures always begin with the anterior.) 



3. What special uses have those which are provided 



with pinchers ? 



4. In walking, are the walking-legs used in regular 



order, i.e. in coordination ? 



5. Why do the chelipeds often vary in size ? (Examine 



several specimens.) 



