8 LABORATORY STUDIES IN ZOOLOGY 



contact, changes in temperature and moisture, the chemical 

 nature of surroundings, and even hght. 



A substance which stinmlates the sense of taste must be in 

 solution. It penetrates the surface membrane of the mouth 

 and stimulates special taste organs. In the frog these organs 

 occur apparently on the floor and roof of the mouth and particu- 

 larly on the flattened surfaces of special mushroom-shaped 

 elevations, fungiform papilloB, on the tongue. The organs of 

 taste, like the organs of touch, cannot be made out in a macro- 

 scopic study but they belong to the surface of the body, since 

 the mouth lining and the skin form a continuous sheet of material 

 identical in origin. 



Locate the two nostrils or external nares on the upper surface 

 of the snout. Pass a beaded bristle through one and observe 

 that it comes out of a round opening in the roof of the mouth 

 cavity. This is the internal nostril or naris. The canal between 

 the external and internal nares serves for the passage of air on 

 its way to and from the mouth cavity. This short canal is 

 enlarged during its course to form the olfactory sac the lining 

 of which is thickened and otherwise modified to form the recep- 

 tive organ of smell, olfactory sense organ. Gases given off from 

 odorous bodies are detected by these organs. The olfactory 

 sense organs constitute the doorway through which the animal 

 communicates with the outside world as regards the nature of 

 gases (odors) which surround it. The advantage of an organ 

 of smell located in the walls of the breathing passages is obvious. 



Posterior to the external nares are the large prominent eyes, 

 doorways for communication by means of light waves. In each 

 may be distinguished the iris, surrounding a central opening, 

 the pupil. By the proper manipulation with forceps eyelids 

 may be discovered. Determine their locations and relative 

 size and importance in covering and protecting the eye. The 

 nictitating membrane is a thin, transparent "third eyelid" which 

 is really an outgrowth of the lower lid. This may be observed 

 in a living animal. In most animals, the nictitating membrane 

 when present is distinct from the true lower lid. It reaches a 

 high degree of development in birds, where it may be stretched 

 over the whole front of the eye. In man it is represented by a 

 vestigial fold at the inner angle of the eye. What advantage 

 may reside in the presence and transparency of the nictitating 

 membrane? Compare the position of the eyes in frog and man. 



