CLASS AMPHIBIA 517 



Another function is in maintaining the concentration of body 

 fluids. Frogs absorb water through their skin at a rather constant 

 rate, varying with the temperature. The kidney in turn expels 

 water at the same rate and thus maintains the proper balance. In 

 addition to its usual function the urinary bladder may be used as a 

 storage reservoir for water during temporary drought. The water 

 may be absorbed from it by other tissues until the proper osmotic 

 equilibrium of the tissues with the blood is produced. The excretory 

 function is further developed in the chapter on The Vertebrate An- 

 imal. 



Skeletal System 



The bullfrog has no exoskeleton, its body being covered by 

 smooth skin. The endoskeleton may be considered in two main divi- 

 sions, the axial and appendicular portions. The axial part includes 

 the skull and vertebral column; the appendicular portion consists of 

 the bones of the limbs and their supports, the pectoral and pelvic 

 girdles. 



Bones are joined to one another by structures made up of connec- 

 tive tissue which allow varying degrees of movement between them. 

 These structures are called joints or articulations. In some cases, as 

 in the skull, the joints are immovable and the bones are separated only 

 by a thin sutural ligament of connective tissue. In other cases, the 

 joints are slightly movable, as in the vertebral column where a plate 

 of dense tissue and cartilage connect the vertebrae. In still other 

 cases the bones are freely movable, as in the limbs, and here the bones 

 are entirely separated, but are held in place by ligaments. 



The Axial Skeleton. — The skull, which is composed of cartilage, 

 cartilage bones, and membrane bones, forms a case for the brain and 

 capsules for the sense organs. The frog's cranium has considerably 

 more cartilage than do the skulls of higher vertebrates and less than 

 those of lower vertebrates. The cartilage bones are so called because 

 of their origin in cartilage which has subsequently been partly re- 

 placed by ossified tissue, forming bones separated by sutures. These 

 cartilage bones are found at various points on the cartilage box that 

 composes the foundation of the cranium. Cartilage bones are the 

 sphenethmoids, pro-otics, exoccipitals, pterygoids, palatines, and car- 

 tilaginous quadrates. The membrane bones develop from ossifications 

 of membranes which cover the cartilage and cartilage bones. They 

 are thin and may be separated from the others. The membrane bones 



