CHAPTER IX 



MULTICELLULAR ANIMALS: THE FROG (RANA) 

 GENERAL DESCRIPTION 



THE body of the frog is composed of a head and a trunk, 

 but there is neither neck nor tail. The wide mouth extends 

 far back to the end of the head. On the upper side of the head 

 in front are two nostrils (nares) that open directly through 

 the bones of the skull into the mouth. Farther back on either 

 side of the head are the eyes, provided with two loose folds 

 of skin which serve as eyelids. The upper lid is immovable, 

 but the lower can be brought up over the eye for protection. 

 It is called the nictitating membrane (Lat. nictare = to wink), 

 is semi-transparent, and does not prevent sight wholly when 

 closed. Behind the eyes are two round flat surfaces, which 

 are membranes stretched over a shallow cavity in the skull. 

 They are the tympanic membranes (Lat. tympanum = drum) 

 and serve to collect sound waves and transfer them to the 

 ears which lie within the head. The part of the body behind 

 the anterior appendages or arms is called the abdomen, and 

 the cavity within, which holds the stomach and intestines, 

 is the abdominal cavity. The organs of the abdomen are 

 sometimes called viscera. 



Of the two pairs of appendages, the fore legs are provided 

 with only four toes, while the hind legs have five toes con- 

 nected by a web. The hind legs are much longer than the 

 fore legs and are the chief organs used in locomotion. The 

 rest of the body is smooth, gradually tapering behind and end- 

 ing abruptly just above the attachment of the hind legs. Near 

 the posterior end of the body on the dorsal side is a good-sized 

 opening, the cloacal aperture (Lat. cloaca sewer), which 

 serves as the common outlet of the intestine, the kidneys, 

 and the reproductive organs. 



175 



