26 APPLIED BIOLOGY 



small holes are punched in the cover. By looking through the glass, 

 it will be possible to learn many things about the frog's external 

 structure and habits. 



Notice that the frog's body consists of head, trunk, and limbs. 

 How many limbs ? Is there a neck, or a tail ? 



Just as in geography we use the terms north, south, east, 

 and west to indicate directions, so in zoology we must have 

 terms for directions or positions on bodies of animals. The 

 head-end of an animal is called anterior, the opposite end 

 of the trunk is posterior, the lower surface of the body is 

 ventral, and the back or upper surface is dorsal. 



Imagine your own body supported on all four limbs (i.e., walking 

 on hands and feet) and locate anterior, posterior, dorsal, and ventral. 

 Make an outline sketch of the frog as seen in profile and another one 

 of a boy as you imagine him walking on hands and knees, and then 

 write the four terms given above on your sketches so as to indicate the 

 parts of the body to which they are applied. Compare the right 

 and left sides of the frog. Remember that in the study of animal 

 biology right and left refer to the frog's body, not to your own. Hold 

 the tumbler so that the frog will sit with its head pointed away from 

 you, and your right will be the frog's right. If the animal were 

 lying on its back with head pointed away from you, as you look down 

 upon its ventral surface, would your right be right or left of the frog ? 

 Are the two sides of the frog's body alike ? Are the right and left 

 sides of your own body similar externally ? 



Any animal having differentiated (meaning made different) 

 dorsal and ventral surfaces, and anterior and posterior 

 ends, might be divided into similar halves (right and left) 

 only by cutting in the median plane from anterior to posterior 

 and from dorsal to ventral. Can you think of any other 

 plane in which a frog can be equally divided? Such an 

 animal is bilaterally symmetrical. Do you know any animal 

 which is not so? Look at a jellyfish in the school-museum 

 or at a picture of one in a book of zoology. In how many 

 places could a wheel with eight spokes be divided? Make 

 a rough sketch to prove your answer. Animals, like jelly- 

 fishes, which have the wheel-like plan of structure are said 



