350 ZOOLOGY 



4. Draw right side of thorax (carapace previously removed) to 

 show the number and position of gills, x 2. 



5. Draw dorsal aspect of eye-stalk, x 4. 



QUESTIONS ON EXTERNAL ANATOMY 



1. How many segments in the last thoracic appendage? Xumber 

 the segments upon your drawing, beginning at the proximal (attached) 

 end. Number as far as you can the homologous segments upon the 

 other thoracic appendages drawn. 



2. How can you homologize 1 with one another the swimmerets, the 

 legs, and the mouth parts ? Give evidence for each step and indicate 

 results on the drawings already made. 



3. Which is the least differentiated of the series of appendages? 2 



4. What is the gill formula of the animal in hand? 



OBSERVATIONS ON THE LIVING ANIMAL 



Each student should be provided with the following: (1) soup- 

 plate; (2) needle; (3) vial of dilute ammonia; (4) vial of 5% acetic 

 acid ; (5) vial of carmine. 



1. Hold a living crayfish in a soup-plate of water so that the free 

 edge of its carapace is near the surface. Put a few drops of the carmine 

 into the water in front of the thorax ; behind the thorax ; between 

 the legs. Draw a diagram showing the currents. What is the use of 

 these currents, and by what mechanism are they produced? 



2. Hold the living crayfish so that its head and anterior half only 

 of thorax are submerged ; observe what takes place at hinder edge of 

 carapace. Hold the crayfish so that abdomen and posterior half only 

 of carapace are submerged ; what takes place at anterior edge of cara- 

 pace ? Interpretation ? 



3. Determine the function of each pair of legs used in locomotion 

 and the correlation in their movements. 



1 The idea of homology should be made clear. The pupil should indicate the 

 protopodite (/>), exopodite (ex), and endopodite (en) upon all the drawings of 

 appendages. 



2 The teacher should make use of the crayfish to explain the principle that 

 different organs have a different degree of specialization or " differentiation." 

 This will lead to the topic of Division of Labor, as indicated below. 



