GENERAL BIOLOGY 



RANA, The Frog. 

 (Chordata, Vertebrata, Amphibia.) 



Read'. Parker and Parker, Practical Zoology, pp. 1-228; or 

 Holmes, The Biology of the Frog, pp. 1-358. 



A. GENERAL STRUCTURE. 



I. EXTERNAL CHARACTERS. 



Note the smooth moist skin over the entire animal ; the absence 

 of exoskeleton; the head, trunk, two pairs of limbs; the absence 

 of a tail and of a neck. 



a. The head. Observe: 



1. The eyes are prominent and have lids ; the ears are marked 

 out by a modified part of the skin, membrana tympani posterior to 

 the eyes ; the two anterior nares, or nostrils ; the position of the 

 mouth opening; the soft flexible throat and hard parts of the endo- 

 skeleton felt on the dorsal side of the head. Observe the move- 

 ments of the throat in respiration. 



2. After the frog has been killed with chloroform (see below 



II, i, a.) pass a bristle far into the anterior nares and one into 

 the ear through a hole cut in the membrana tympani; on opening 

 the mouth the bristles will indicate its communications with the 

 nostrils and tympanic cavity. The second bristle appears in the 

 Eustachian recess at the side of the posterior part of the mouth. 

 In the male a small opening anterior to this recess leads into the 

 buccal sac which can be distended, by means of a small blow- 

 pipe. Turn the fleshy tongue forward and notice its mode of attach- 

 ment. Note the slit of the glottis and the posterior opening of the 

 mouth into the oesophagus ; pass a bristle into the former and a 

 large probe into the latter. There are thus two median openings 

 from the mouth cavity and six paired openings in the male frog; 

 four in the female. Note the small teeth. 



b. The Trunk. 



This tapers towards the posterior end where the cloacal aperture 

 is seen near the dorsal surface. Beneath the skin the hard endo- 

 skeleton can be felt on the dorsal side and on the anterior part of 

 the ventral side. 



c. The limbs. 



i. The anterior pair divided each into three regions, brachium 



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