20 THE ANIMALS AND MAN 



As you look at the toad note the similarity of the parts 

 on one side to those of the other, as right leg correspond- 

 ing to left leg, right eye to left eye, etc. This arrangement 

 of the body in similar halves among animals is known as 

 bilateral symmetry. As a rule animals which show bilateral 

 symmetry move in a definite direction. The part that moves 

 forward is the anterior end, while the opposite extremity is 

 the posterior end. In most animals we note two other views 

 or aspects; that which is called the "back" and with most 

 animals is, under ordinary conditions, uppermost is called 

 the dorsum or dorsal aspect, while that which lies, below is the 

 venter or ventral aspect. When referring to a view from one 

 side we speak of it as a right or left lateral aspect. These 

 terms hold good for most of the animals that we shall study. 



Note on the head of the toad the wide, transverse mouth. 

 What other openings are on the head? Note the two large 

 eyes, the organs of sight. Just back of each eye find an 

 elliptical, smooth membrane. This is the tympanum of 

 the outer ear, and through this membrane the vibrations 

 produced by sound-waves are transferred to the inner 

 ear, which receives sensations and transmits them to the 

 brain. Open the mouth by drawing down the lower jaw. 

 Note just within the angle of the lower jaw the tongue. 

 How is it attached to the wall of the mouth ? On the tongue 

 are a great many fine papilla in which is located the sense 

 of taste. It has now been seen that most of the special senses 

 of the toad have their seat in the head. Pass a straw or bris- 

 tle into one of the nostrils. Where does it come out ? These 

 internal openings to the nose are the inner nares. Note in 

 the roof of the mouth just posterior to each of the eyeballs 

 an opening. These are the internal openings to the wide 

 Eustachian tubes, which lead to the mouth from the chamber 

 of the ear behind the tympanum. 



Note far back in the mouth an opening through which 

 food passes. This is the oesophagus or gullet. Note just 



