28 



GENERAL ZOOLOGY 



cutaneous connective tissue, divided into inner and outer layers by- 

 large lymph spaces. These spaces are readily seen when the skin 

 is removed from the frog. A thickened ridge in the skin extends 

 back from the eye on either side of the body to form a lateral 

 dermal fold or plica. In addition to mucus-secreting glands, 

 which are very numerous, the skin also contains smaller numbers 

 of poison glands, located principally in the skin of the dorsal body 



surface, lateral dermal plicae and in 

 the skin of the hindlegs. The poison 

 is in the form of a whitish fluid. It 

 is not so toxic as the secretions of 

 the poison glands of toads. 



In a sitting posture the body rests 

 on the folded hindlegs and the ex- 

 tended forelegs. From this position 

 the sudden extension of the hindlegs 

 sends the animal through the air in a 

 long leap. This is its usual form of 

 locomotion on land. In water the 

 swimming strokes are also provided 

 by the hindlegs. The hindfoot has 

 five elongated toes connected by 

 interdigital membranes which increase 

 the surface of the foot and add power 

 to the swimming stroke. Each 



Fig. 8.— Palmar surfaces of feet f ore f oot has four toes and a very 

 of Rana pipiens. A, right fore- n-i 



foot of female; B, right forefoot rudimentary thumb. A padlike 

 of male; C, hindfoot. enlargement on the proximal segment 



of the first digit is present in the male and is practically the only 

 external anatomical distinction between the sexes (Fig. 8). 



The eyes are prominent structures at the sides of the head. 

 Each eye has an immovable upper eyelid in the form of a thick 

 fold of the integument. The lower lid is well developed and can 

 be moved upward, covering the eye. When this happens the 

 eyeball is withdrawn into the socket by a special muscle, the 

 retractor bulbi. The upper part of the lower lid is thin and 

 transparent and is called the nictitating membrane. When the 

 lower lid is withdrawn, the nictitating membrane folds inside 

 on the lower thicker part of the lid. On the forehead between 

 the eyes an unpigmented spot, the brow spot, can usually be seen. 



