262 ZOOLOGY. 



5. Evidences of sensitiveness : Devise experiments of your 

 own to prove whether the cray-fish is stimulated by light; 

 contacts; the presence of food in any other way than by sight; 

 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? 



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 seg- 

 mentation in the cephalo-thorax ? (Make a temporary 

 estimate of the number of segments in the animal.) 



Make two sketches showing a dorsal and a ventral view of 

 the cray-fish, 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. 



