EXTERNAL CHARACTERISTICS 7 



passing through the body longitudinally. The median 

 plane is that sagittal plane which passes through the mid- 

 line of the body and divides it into two mirrored or iden- 

 tical halves. 



b. The horizontal or frontal plane or section is any hori- 

 zontal plane passing longitudinally through the body. It 

 is at right angles to the median plane and parallel to the 

 dorsal and ventral surfaces. 



c. The transverse or cross section or plane is that plane 

 which passes through the body at right angles to the sagittal 

 and the horizontal planes. 



d. The longitudinal or anteroposterior axis is any axis 

 which is in the median plane extending from the head to 

 the end of the tail. 



e. A sagittal or dorsoventral axis is any axis in a sagit- 

 tal plane and passes from the dorsal to the ventral surface. 



Drawings 2, 3, 4. Diagram the planes and axes of a vertebrate 

 and demonstrate them using plastic clay. 



5. Symmetry. 



An animal which has symmetry may be divided by a line 

 or plane in such a way that one of the halves will be a 

 mirrored image of the other. Asymmetrical animals cannot 

 be so divided. The opposite like parts of a symmetrical 

 animal are called antimeres. There are three well marked 

 types of symmetry in animals, universal, radial and 

 bilateral. The embryo of many vertebrates passes through 

 all stages of symmetry; universal symmetry in which no 

 opposite sides are different (blastula) ; radial symmetry in 

 which opposite ends are different (gastrula) ; and bilateral 

 in which opposite ends and two opposite sides are differ- 

 ent, which is the symmetry of adult vertebrates. 



There is but one plane of symmetry in a bilateral animal. 

 The median sagittal plane is the one and only plane that 



