THE FROG 4? 



by nerves. To know the internal structure of an animal one must know 

 all that can be known in regard to the following systems : 



1. Digestive 



2. Circulatory 



3. Respiratory 



4. Excretory 



5. Nervous 



6. Skeletal 



7. Muscular 



8. Reproductive. 



After an incision is made along the mid-line (Fig. 6) of the ventral 

 ( ) surface of the animal from the lower angle of 



the jaw to its most posterior end, the internal organs are seen. These 

 are called the viscera ( ). The cavity in which they 



are found is known as the coelom ( ) or body cavity. 



If the animal has just been chloroformed, the heart will still be beat- 

 ing. The heart is contained in a sac-like structure called the pericardium 



( ). 



Surrounding at least a portion of the pericardium, are three promi- 

 nent lobes of the reddish-brown liver, while the' lungs, looking like small 

 strawberries, lie, one on each side, near the anterior end of the abdomi- 

 nal cavity. 



The stomach is easily recognizable, together with the coiled intes- 

 tine attached to it. 



The kidneys are flattened reddish bodies attached to the dorsal body 

 wall. 



If it is the breeding season, and the frog is a female, almost the 

 entire body-cavity may be filled with thousands of eggs. The eggs in 

 turn are contained in a film-like covering known as the ovary ( ) 



and oviducts ( ), the latter organs serving as tubes 



through which the eggs leave the body. If the specimen should be a 

 male, the two testes ( ) will be suspended by little 



membranes at the side of the digestive canal (t, Fig. 6). The entire 

 lining of the abdominal cavity in all the higher forms of animals is called 

 the peritoneum ( ). When one or two layers of 



this peritoneum suspend, or hold up an organ, such as the digestive canal 

 and the reproductive organs, such suspending peritoneum is called a 

 mesentery ( ). 



THE DIGESTIVE SYSTEM 



It will be noticed that the tongue is what is called extensile 

 ( ), that is, it can be thrown forward and outward. 



There is a sticky substance secreted on the tongue which causes objects 

 with which it comes in contact to adhere. It is also interesting to ob- 

 serve that unless an object is moving, the frog pays no attention to it. 



