THE FROG AND VERTEBRATES IN GENERAL 165 



THE FROG 



Having considered the characteristic features of the Vertebrates 

 and their classification in so far as is necessary for our present 

 discussion, we are now in a position to proceed with a detailed 

 description of the structure and life processes of a representative 

 Vertebrate. Because of its availability, convenient size, and gen- 

 eral adaptability to laboratory work, we shall select the Frog as 

 the type animal, but an endeavor will be made to consider the 

 various features from the comparative standpoint, particularly 

 with reference to the condition found in the Mammals. 



A. External Structure 



The body of the Frog is divided into two regions, the head and 

 trunk. The neck region, which is typically present in the higher 

 Vertebrates and permits a wide variety of head movements inde- 

 pendently of the rest of the body, is entirely lacking. (W. f. 78.) 



A study of the external features of the Frog's head reveals at 

 once the very large mouth with upper and lower jaws ; the latter 

 also present on all of the Vertebrates except the Cyclostomes. 

 The upper jaw has a projecting, fleshy upper lip with a ventral 

 groove into which the lower jaw fits very perfectly so as to form 

 an airtight joint which, as will be shown later, is a necessary 

 feature of breathing in the Frog. A pair of dorsally placed eyes 

 with upper and lower lids is another prominent feature. Nor- 

 mally they project a considerable distance above the surface. They 

 can, however, be drawn back into the eye sockets, or orbits, of the 

 skull and entirely covered over by the lower lids ; the upper lids 

 being almost immovable in this animal. A slight excrescence in 

 the skin, just anterior to a line drawn between the eyes, marks 

 the brow spot which is connected with the brain at an early 

 stage of development. It is believed to represent the remains of a 

 so-called pineal eye, which at present is known to be functional 

 only in one species (Sphenodon punctatum) which belongs to the 

 Reptiles. Also on the dorsal surface and near the anterior end 

 of the head is a pair of small openings, the external nares, which 

 open into the mouth cavity and function in connection with the 

 olfactory and respiratory structures, in essentially the same manner 

 as the nostrils in the nose of Man or other higher forms. Posterior 



