46 



General Biology 



are divided into three portions. The upper portion is known as the 

 thigh; the middle, the crus or shank; and the distal ( ) 



portion, the foot or pes. The foot, which is well developed, has five 

 toes and the rudiment of a sixth, called the prehallux ( ), 



situated on the inner side of the foot. The toes themselves are con- 

 nected with a web, making the foot quite efficient as a swimming organ. 

 There are also small cushions, called subarticular pads, ( ) 



between the bones of the toes. 



Fig. 5. Pineal eye of a Lizard; diagrammatic. A brain and upper wall of the 

 skull, the latter cut through; B, pineal eye alone, in section. V, Z, M, H, cerebrum, 

 thalamencephalon, optic lobes, cerebellum; h skin, ^ roof of skull, o unpigmented 

 portion of skin below which the pineal eye lies in a hole in the roof of the skull; 

 p epiphysis, i hypophysis, 2 optic nerve. L lens, R retina, N nerve of pineal eye. 

 (After Boas.) 



The skin is smooth and loose, containing large black pigment spots 

 ( ) and some green and golden pigments as well. 



As with other vertebrates, the skin has two layers, an outer called the 

 epidermis ( ) and an inner, the dermis ( ). 



Nothing similar to hair or scales can be found on the frog. In the skin 

 there are large mucous glands ( ) which keep the 



surface slimy, and there are also some poison glands secreting 

 ( ) a whitish fluid, supposedly for defensive pur- 



poses. Behind the eyes there are usually two light colored ridges, 

 formed by a thickening of the skin and called the dorso-lateral dermal 

 plicae ( ) or folds. There may be some smaller, 



irregular, longitudinal folds ( ) of skin between 



these. It will also be observed that the color of the skin is much darker 

 on the upper, or dorsal, surface than below, where it is almost white. 



INTERNAL STRUCTURE 



As with man, the various organs ( ) and tissues 



( ) of the frog's body are supported by an internal 



skeleton of bones. This is called an endoskeleton ( ) 



to distinguish it from the skeletons in such types of animals, as the 

 crayfish, which have their entire skeletal structure on the outside of the 

 body, forcing that animal to grow an entirely new skeleton whenever 

 the animal itself grows larger than its skeleton jacket will stretch. The 

 higher forms of animals all have endoskeletons. The different parts 



