VERTEBRATES 113 



ferentiation into regions, its covering, its fins, mouth, gill 

 slits, and sense organs. 



Perch: If you have not already studied the bony fish, 

 the points suggested for the shark will be sufficient for 

 this exercise. 



In each case, find out the condition of the skeleton. 



Questions. 



1. Which of these animals seem most simple in form, 

 and which most complex ? Give a reason for your answer. 



2. Give the stages which show how the fold of skin 

 develops into separate fins. 



3. How does the number of gills and gill slits change 

 in the series .' (Give definite numbers.) How may the 

 reduction in the number of gills be compensated for in the 

 amount of surface exposed for the exchange of gases in 

 breathing } 



4. How is protection afforded the delicate structure of 

 the gills in the final form } 



5. Give the stages in the formation of a definite, sym- 

 metrical mouth with jaws of equal size. 



6. The presence of sense organs may be taken to indi- 

 cate that there is an organ of control, or brain. How is 

 the development of this organ like or unlike that of the 

 other structures in the series .' 



7. For the developing brain and nervous system what 

 protection and support is afforded in each case } 



The foregoing questions may be answered in tabular 

 form by arranging the names of the animals in a line and 

 the questions in a column. 



Suggested drawings. 



a. Acorn-tongued worm, x I. 



b, Lancelot, x i. 



