AN INTRODUCTION TO ANIMAL BIOLOGY 33 



The largest organ in the frog's body is the liver, a large bi-lobed 

 (two lobes or divisions) organ behind and at the sides of the heart. 

 Its color is reddish brown in frogs not preserved in chemicals. It 

 will be studied more carefully later. 



Ovaries. In a full-grown female frog, masses of spherical black 

 and white eggs (ova) lie among the other organs (Fig. 5). These 

 ova are attached to, or are really united to form, two organs (the 

 ovaries or egg-organs) which later will be found to be fastened to the 

 dorsal wall of the body-cavity, one on the right and one on the left 

 of the median line; i.e., bilaterally symmetrical. With forceps, 

 pull out the masses of ova, taking care not to injure other structures. 



Alimentary Organs. These are the organs concerned with 

 receiving food and preparing it for the use of the frog's body. The 

 alimentary canal, or food-tube, extends through the body from the 

 anterior to the posterior extremity (Fig. 8). The mouth is the an- 

 terior opening; the posterior opening is called anus. Turning 

 the organs in the body-cavity, but not cutting, examine the 

 various parts stomach, intestine, etc. of the food- tube. The 

 stomach lies dorsal to the left lobe of the liver. The short tube 

 from stomach to throat is the esophagus (gullet). The throat or 

 pharynx connects the esophagus with the mouth-cavity. Care- 

 fully push a probe (such as a small stick) down the throat into 

 the stomach. The tube extending from the stomach backwards or 

 posteriorly is the intestine. Note that the first part lies parallel 

 to the stomach. The constriction between the stomach and this 

 first part of the intestine is the pylorus. The small intestine is a 

 slender and much convoluted tube. The large intestine (or rectum) 

 is a short, straight tube, of greater diameter than the small intestine. 

 The expanded end of the large intestine is called cloaca. Tubes from 

 the kidneys* and reproductive organs open into the cloaca ; but this 

 is not so in the highest animals. The opening (anus) of the large 

 intestine to the exterior is on the dorsal surface of the body near the 

 end of the backbone. 



Carefully cut the membrane (mesentery) which holds the alimen- 

 tary canal in place, cutting close along the canal, and pin the intes- 

 tine to one side and out of the body-cavity. Cut open the stomach 

 longitudinally. It may contain food, such as softened and disin- 

 tegrated pieces of worms, etc. ; the condition of the food suggests 

 that solid foods are dissolved in the stomach. Wash out the con- 

 tents, and notice the longitudinal folds which line the stomach and 

 increase the surface with which food comes into contact. Cut open 

 the intestine in several places. Do you find folds arranged as in the 



