60 ELEMENTARY ZOOLOGY. 



B. The form and structure. 



i. The general plan of the body, 

 a. Symmetry. Is there any plane which would 

 divide the body into symmetrical halves? If 

 so define its position. Is there more than one 

 such plane? 



If we imagine an axis drawn lengthwise along 

 the middle of the animal we would call this 

 axis the anterior-posterior axis. Are the two 

 poles of this axis alike or unlike? Enumerate 

 all the differences you can find between the 

 anterior part and the posterior part as shape 

 of body, and the like. What organs are found 

 at the anterior end? The posterior end? 

 What is the prime meaning of anterior! What 

 advantage is there to the animal in having 

 these organs at the anterior end? Why do 

 you think these particular organs have come 

 to lie at the anterior end rather than elsewhere? 

 Similarly, imagine an axis perpendicular to 

 this principal axis, and extending from the 

 back to the belly of the animal. This is called 

 the dor so-ventral axis. Are its ends alike? 

 Enumerate all the differences you can find be- 

 tween the dorsal and ventral surfaces of the 

 body. What are the advantages in having the 

 wings, etc., dorsal? the legs and mouth-parts 

 ventral? 



In the same way think of a line perpendicular 

 to both of these and extending from left to 

 right. Are its poles alike or unlike? What 

 value is there in this condition? What organs 

 are thus duplicated, or paired? 



