96 GENERAL BIOLOGY. 



aldehyde in water, but is often used indiscrimi- 

 nately for solutions of various strengths. 



I. External Characters. 



1. The division of the frog into head, trunk, and 



limbs. 



2. The skin: moist and smooth. Note its 



looseness. 



Make short slits in several places and inflate the 

 lymph-sacs. 



3. The head : its shape. 



(a) The eyes : each with two eyelids (are 



they movable ?). 



(b) The tympanic membranes; modified parts of 



the skin covering the ears. 



(c) The nostrils or anterior nares. 



4. The limbs. 



(a) The anterior limb is divided into three 



regions : brachium, antibrachium, and 

 maims. The manus has four digits. In 

 the breeding season the first digit of the 

 male bears a swollen cushion. 



(b) The posterior limb is divided into three 



regions : femur, cms, and pes. The pes 

 has five digits connected by a web. 



5. Identify all openings into the body. 



6. Look at a living frog in an aquarium-jar with 



a little water to see its position, its eyes, 

 etc., and its mode of breathing. 



