266 ZOOLOGY 



sound. Are all parts of the body equally sensitive to touch? 

 To chemical stimuli? Make use of a 5 per cent, solution of 

 acetic acid; strong salt solution; strong beef extract. What 

 inferences may be drawn from your experiments? Place a 

 crayfish on his back. Describe his actions. What is the nature 

 of the stimulus that arouses this reaction? Evidences? 



II. Symmetry. (This group is especially favorable for this 

 study.) 



Notice what is implied in bilateral or tri-axial symmetry. 



Antero-posterior axis : are the poles alike or different ? 



Make a memorandum of all the chief differences. 



Dorso- ventral axis (as above). 



Right-left axis. Record the points of agreement. 

 Contrast the axes in length. Can you think of any causes 



for the differences and likenesses discovered above? 



Any advantages arising therefrom ? 



III. General Form. Distinguish two regions; Cephalo- 

 thorax and abdomen. 



Cephalo-thorax ; carapace. 

 Head; rostrum, eyes, mouth. 

 Cervical groove. 

 Thorax. 



Abdomen; how many segments do you find? What 

 seems to determine a segment ? 



Applying these criteria can you find any indications of 

 segmentation in the cephalo- thorax ? (Make a tem- 

 porary estimate of the number of segments in the 

 animal.) 



Make two sketches showing a dorsal and a ventral view 

 of the crayfish, preserving proportions. 



Examine one of the abdominal segments (the third or 

 fourth from the front). How is it joined to those next 

 it ? Follow the line of union. Note tergum, or dorsal 

 piece; sternum, or ventral piece; pleura, the lateral pro- 

 jections from the tergum. 



Make a sketch of an imaginary cross section showing the 

 relation of these parts to each other, together with the 

 attachment of the appendages. 



