FROM SINGLE CELLS TO MANY CELLS 



11 



planes, and straight lines. In describing ani- extending from the tip of the nose to the 



mals a reference to planes describes most of tip of the tail, through the midline, and this 



them. A plane has length and breadth, but cut becomes a plane of symmetry (Fig. 



no depth, that is, it is two-dimensional. 4-8 ). Such a bisected pig now consists of two 



Therefore, dividing an animal by a plane mirror halves. A bilaterally symmetrical ani- 



results in two halves, each of which is a mal may also be cut transversely; such cuts 



Universal 



R a d i a 



Bilatera 



Planaria 



Asymmetry 



Fig. 4-9. Various kinds 



mirror image of the other. Note that each 

 half is not a duplicate of the other because 

 all of the parts are reversed. When only one 

 plane can be drawn, the animal is said to 

 be bilaterally symmetrical. This is the case 

 with most animals. A pig, for example, can 

 be divided into two halves by a single cut 



of symmetry in animals. 



are called transverse sections. Additional 

 terms useful in orientation are the follow- 

 ing: The back side is the dorsal side, while 

 the opposite or belly side is the ventral 

 side, the head is the anterior end, and the 

 opposite end is the posterior end. The pig 

 also has a left and right side. These terms 



