THE CRAYFISH 149 



piece. Float out in water, and cut off the left digestive gland close to 

 the tract. Note the region between the gastric mill and the intestine. 

 Open the gastric mill along the ventral mid-line, find the teeth, work 

 them together, and see how they grind. 



(e) Draw a side view from the left (X 2 or 3), showing the diges- 

 tive tract and the right digestive gland in position and the place where 

 the left one opens into the tract. 



Exercise 5. — The Nervous System and the Excretory Organs. 



(f) Carefully remove all the muscles and viscera from the abdo- 

 men. The nerve cord will be seen lying in the mid-ventral line. 

 Notice the ganglia. How many do you count? In what somites do 

 they lie? Notice the lateral nerves. How are these arranged with 

 reference to the ganglia? In the cephalothorax, the nerve cord is con- 

 cealed beneath transverse ridges of the ventral wall of the skeleton. 

 Cut these with heavy scissors and expose the nerve cord, beginning at 

 the posterior end of the cephalothorax and working forward. How 

 many thoracic ganglia do you find? Just back of the esophagus is the 

 large subesophageal ganglion from which two circumesophageal con- 

 nectives pass to the dorsal ganglion, or brain, behind the eyes. Find 

 the nerves passing from the brain to the eyes and to the antennae and 

 antennules. Draw a figure (X 2 or 3) of the nervous system thus 

 exposed, showing accurately the ganglia, the somites in which they lie, 

 and the lateral nerves. Compare the relation of ganglia to somites 

 with that found in the earthworm. 



(g) At the anterior end of the body, near the external openings 

 already noted, find the excretory organs, or. green glands. The thin 

 bladder and underlying glandular portion of the organ can be readily 

 distinguished. Refer to your textbook for further details. 



IV. APPENDAGES 



Exercise 6.^ — Serial Homology and Functional Modification. 



(a) Examination of the appendages shows that they are obviously 

 different in each region of the body. Anteriorly one finds that they 

 have sensory functions. In the region of the mouth they are modified 

 in relation to the seizing and mastication of food. In the middle re- 

 gion they serve for walking legs. In the abdomen the smaller appen- 

 dages are modified with reference to respiration and reproduction, 

 while the last pair and the telson constitute the powerful tail-fin. 

 Comparison of their structure reveals a certain fundamental plan 

 which will be appreciated as study progresses. Remove the appen- 

 dages one at a time from the right side of the animal, placing each one 



