The Frog. 75 



that in reality the various organs, which are exposed by opening 

 it up, do not lie within it. The alimentary tract, which is the 

 most conspicuous of these, is tied down to the dorsal side of 

 the cavity by thin sheets of tissue, the mesenteries. These 

 mesenteries are double, and embrace the alimentary canal, 

 thus shutting it off from the coelom. The same applies to the 

 kidneys, heart, and other organs. 



The only organs that can be said to really lie actually in 

 the body cavity are the male and female gonads, the testes and 

 ovaries. They are formed by a proliferation of the membrane 

 which lines the coelom. The mouths of the oviducts in the 

 female frog, moreover, open actually into the coelom, the lining 

 membrane of which is continuous with their mouths. A special 

 part of the coelom is separated off from the rest, and forms the 

 pericardium, enclosing the heart. Mention has been made of the 

 subintegumental lymph spaces ; these spaces are in communi- 

 cation with a system of vessels which ramify through the body, 

 often accompanying the blood-vessels, and which open into the 

 coelom. This lymph system contains a colourless fluid, in 

 which float colourless nucleated corpuscles, similar to those of 

 the blood. Lying beneath the skin of the back, just in front 

 of the cloacal aperture, and between the third and fourth 

 vertebra, are certain muscular contractile sacs, the lymph 

 hearts, which pump the lymph into the veins in their neigh- 

 bourhood ; the coelom thus, through the lymph system, is in 

 communication with the vascular system. 



The alimentary canal of the frog commences with the mouth 

 cavity and ends posteriorly in the cloacal cavity. The mouth 

 and teeth have been already referred to, and the latter will be 

 more particularly described in the section dealing with the 

 skeleton. The mouth cavity is followed by the (esophagus, 

 which leads into the stomach. The stomach is a bent tube, 

 wider at first, and gradually diminishing in calibre as it passes 

 into the small intestine ; the junction of the two is marked by 

 a fold of the lining membrane. The small intestine is narrow 

 and coiled.i The large intestine is wide and short, ending in 



' The coiling of the intestine allows a large secretory and absorjj- 

 tive surface to be stowed away in a small space. This surface is 



