370 



METAZOAN PHYLA 



the removal of the anterior part of the brain in front of the medulla has 

 so little effect upon its activities indicates plainly the low grade of 

 intelligence possessed by the animal. The roof of the cerebrum in all 

 forms up to Amphibia has been epithelial and without nerve cells. In 



Olfactory lobe 



Cerebrum 



Medulla 



Fig. 252. — Brain of European frog, Rana esculenta Linnaeus, viewed from above. 

 {From a Ziegler model, after Wiedersheim.) The roots of the cranial nerves are marked by 

 roman numerals. 



Amphibia it contains nerve cells, but these are inside and are covered by 

 fibers and are not organized into a cortex. 



The frog responds directly to many external stimuli. It is sensitive 

 to light, the whole skin being affected. The animal is said to exhibit a 

 negative phototropism since it avoids bright light and, when exposed 

 to it, faces it, that being the position in which the smallest amount of 

 light will be received by the skin of the body as a w^hole. Frogs are 

 also stimulated by contact and tend to crawl under objects and into 

 crevices. Both of these responses are modified by temperature. Natu- 



