THE LOCUST OR THE CRAYFISH. 71 



Make a diagrammatic view of an imaginary cross section of the 

 thorax in the region of the heart, and one of the abdomen show- 

 ing the relative position of the internal organs. Also a diagram 

 of a mid-longitudinal (sagittal) section. 



76. Comparative Exercise. Sum up the points of like- 

 ness in the the crayfish and the locust. The points of 

 difference. Keep in mind the degree of union of head 

 and thorax ; the number of segments represented in each 

 of the three regions; the degree of differentiation among 

 the appendages ; the mode of respiration ; food ; and general 

 activities and habits. 



7 7 . Supplementary Library Work. After the laboratory 

 and field work has been done the student should be en- 

 couraged to follow up some good description of the cray- 

 fish or lobster or locust in the reference texts. Let him 

 see to what extent his own observations are supported 

 by those of other people. In case of difference, what 

 should you do, in the spirit of the scientific method? 



