THE LOCUST OR THE CRAYFISH. 6l 



The condition which you have studied out in 

 the grasshopper is known as "bilateral sym- 

 metry." Undertake to define this kind of 

 symmetry. What advantages are there in this 

 kind of symmetry to the animal possessing it? 

 Why would we expect the anterior end to be 

 different from the posterior end; that is, what 

 causes would tend to make it so? What would 

 make the dorsal surface different from the 

 ventral? What facts would tend, in the long 

 run, to make the right and left sides duplicates? 

 Be sure to distinguish carefully between the 

 evolutionary causes of bilateral symmetry and 

 the advantages of it after it is once produced. 



b. Segmentation. By segmentation in animals 

 we mean the tendency to repeat similar struc- 

 tures in line, along the principal axis as verte- 

 brae, ribs, legs, etc. Parts are repeated on 

 opposite sides of the body, as we have seen, but 

 this is not segmentation. 



What external signs of segmentation can you 

 find in the locust? How many such segments 

 do you think the animal has? Enumerate all 

 the kinds of organs or marks that show this 

 tendency to repeat structures. Which show 

 it most strikingly in the thorax (or body) region ? 

 Which in the abdomen (the softer posterior 

 region)? Are there any rings showing the 

 actual division of these segments? Where do 

 they show to best advantage? Do the append- 

 ages assist in any way in indicating the number 

 of segments? 



