210 VERTEBRATA 



be easily killed by immersing a quarter of an hour in a 

 half pint jar of water containing an ounce of one per 

 cent chloretone or a quarter of an ounce of chloroform. 

 A frog is preferable for this work. Before killing, its 

 method of respiration and locomotion should be noted. 

 Among the external features to be observed are the moist, 

 smooth skin, the teeth in the upper jaw, the peculiar tongue, 

 the two minute apertures, called external nares, at the tip 

 of the nose, the two eyes, each with two eyelids of which 

 the lower is the more movable, the circular tympanic mem- 

 brane just back of the eye, and the webbed condition of 

 the feet. The three parts of the fore limb are the arm, 

 forearm, and hand, with the four digits corresponding to 

 the four fingers of man. The thumb is scarcely large 

 enough to be noted. The parts of the hind limb are the 

 thigh, leg, and foot, with the five toes webbed together. 

 To study the internal features, cut off with the scissors 

 the entire ventral wall of the abdomen, lay the frog on its 

 back in a waxed-bottom dissecting pan containing enough 

 water to cover the animal. Pin down the sides of the 

 abdominal wall and identify the various organs by the use 

 of the illustration and comparison with the same struc- 

 tures in a fish. If the animal is a male, the sexual glands 

 will be two oval solid yellowish bodies near the kidneys. 

 The delicate membrane supporting the intestine is the 

 mesentery, which is a part of the peritoneum, the thin 

 glistening membrane lining the body cavity and closely 

 surrounding most of the organs. The heart of the amphibia 

 is composed of a ventricle and two auricles in addition to 

 a venous sac and an arterial trunk. The circulation of the 

 blood may be observed under the microscope in the tail 



