ARTHROPOD A 267 



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

 To chemical stimuli? Make use of a s 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 a,bove? 



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 proportibns. 



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. 



