AETHKOPODA. 47 



I. A Study of Muscles. (Dem.) 



The abdomen of the crayfish in swimming is moved 

 by two sets of muscles, the extensors and the flex- 

 ors. The extensor muscles straighten the abdo- 

 men, while the flexors bend it. 



1. Are the flexors located on the dorsal or the ventral 



side of the abdomen ? 



2. Where are the extensors located ? 



3. Where must the muscles be attached anteriorly ? 



4. Which are the stronger, the extensors or the flexors ? 



Why? 

 In the same manner locate the position of the mus- 

 cles which operate the pinchers of the chelipeds. 

 They are called abductor and adductor muscles. 



5. Why are these mixscles so named ? 



6. Do these muscles differ in size ? Why ? 



J. Drawings. 



1. Draw the crayfish from the dorsal view, with the 



appendages and caudal fin outspread. 



2. Draw one of each of the following appendages, — 

 (a) the third maxilliped with its gill ; 



(6) the second maxilla with the paddle ; 



(c) the mandible ; 



(c?) the cheliped with its gill ; 



(e) the fifth walking-leg ; 



(/) the third swimmeret. 



K. Experimental Studies. 



Note. — These experiments require careful and patient study. Do not 

 be satisfied with one trial. Great care must be taken to avoid frightening 

 or tiring out the animals. Try to secure conditions as nearly natural as 

 possible. If the crayfish fails to respond to your tests, make a record of 

 it. These may be called negative results and are often as important as 

 the positive results which you usually expect. 



