Chapter 23 



THE FROG— AN AMPHIBIOUS 

 VERTEBRATE ANIMAL 



The common leopard frog is known as Rana pipiens (ra' na pip' i enz) 

 (L, rana, frog; pipiens, piping). Its body is smooth and covered with 

 mucus secreted by glands in the skin. Like many lower vertebrates, the 

 frog has the ability to change color due to changes in the black and yel- 

 low pigment cells in the skin. Because of its coloration the frog is af- 

 forded a certain degree of protection from enemies. This is known as 

 protective coloration. When in water, the frog need keep only the tip 

 of the nose above the surface because of the location of the nostrils (ex- 

 ternal nares) . Two large eyes are located on the top of the head. The 

 tympanum (eardrum) is external and just posterior to each eye. The 

 body may be divided into head and trunk. The latter bears two pairs of 

 appendages, but there are no claws upon the toes. 



Integument and Skeleton. — The skin does not fit tightly and is com- 

 posed of (1) a rather thin outer layer called the epidermis and (2) a 

 thicker, inner layer the dermis (corium) (Fig. 208). The epidermis con- 

 sists of several layers of cells: (1) the outer ones, composing the stratum 

 corneum (stra' tum kor'neum) (L. stratum, layer; corneus, horny), 

 are flat, compact, and horny (shed several times during the active season 

 when the frog moults) and (2) the inner ones, next to the dermis, com- 

 posing the Malpighian layer, are columnar and by mitosis give origin to 

 the outer layer. The dermis consists of connective tissues in which are 

 glands, blood vessels, pigments (Fig. 209), nerves, muscle fibers, and 

 lymph spaces. The dermis is made of (1) an outer layer, called the 

 stratum spongiosum, consisting of loose connective tissue and containing 

 (a) pigment bodies which give the frog its spotted pattern (pigments may 

 also be present in the epidermis), (b) small spherical mucous glands 

 which pour a slimy secretion out upon the surface of the skin, (c) larger 

 spherical poison glands which secrete a whitish, acrid fluid for protection, 

 and (d) numerous sensory and tactile papillae (just below the epidermis) 



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