312 



THE RISE OF ANIMAL LIFE 



nerves, but strangely enough, the frog seems 

 to have lost them somewhere along the way. 

 The spinal nerves that pass to the legs are 

 grouped together in two plexi, the anterior 

 brachial plexus and the posterior sciatic 

 plexus (Fig. 13-32). From these regions the 

 nerves spread out again and pass to all parts 

 of the appendages. The larger nerves are in 

 the regions of the legs, as one might expect, 

 since it is there that more of the messages 

 must travel. 



Digestive system. This system starts with 

 a disproportionately large mouth which 

 when fully open can enclose a body one- 

 fourth the size of the frog itself (Fig. 13- 

 33). The jaws are feebly armed with a top 

 row of teeth and a few on the front edge of 

 the palate. The teeth function only in hold- 

 ing prey and are incapable of crushing or 

 chewing. The large protrusible tongue lies 

 on the Hoor of the mouth with the two 

 pointed tips directed down the throat. It 

 is attached in a peculiar manner, the ante- 

 rior end being fastened just inside the lower 

 jaw (Fig. 13-34). When in use, the mouth 

 is opened wide and the tongue is flipped 

 out with lightning-like speed, so fast, in 

 fact, that it has no difficulty in capturing 

 agile insects, since the sticky mucus se- 

 creted by mouth glands makes a good adhe- 

 sive agent. There are no digestive enzymes 

 in the saliva and hence no digestion occurs 

 in the mouth. 



Openings into the mouth cavity are 

 those of the eustachian tubes which con- 

 nect with the cavity under the ear drums, 

 the internal nares, which connect with the 

 nostrils, and tlie esophagus which leads ab- 

 ruptly into the large U-shaped stomach. 

 The back part of the mouth, the pharynx, 

 is lined with cilia which beat continuously 

 and help carry food down to the stomach. 

 The stomach is merely a portion of the di- 

 gestive tract which is enlarged for storage 

 of food, and the frog has occasion, indeed, 

 to use a sac of such ample proportions. 

 Some digestion takes place in the stomach, 

 much as in the stomach of man. The lower 



extremity is marked by a constriction, the 

 pyloric sphincter (a band of circular mus- 

 cles which, when contracted, closes the 

 opening). The stomach is followed in turn 

 by the small intestine which receives the 

 pancreatic juice and bile from a single duct 

 (Fig. 13-34). The pancreas is a long, light- 

 colored, ribbon-like organ that lies between 

 the stomach and the first part of the intes- 

 tine. The liver is composed of three lobes, 

 two large lateral lobes and one smaller 

 median lobe. The gall bladder usually lies 

 dorsal to the smaller lobe, and the bile 

 duct passes from it through the substance 

 of the pancreas on its way to the intestine, 

 picking up the pancreatic duct along the 

 way. The gall bladder is green in color be- 

 cause of the bile which it contains. 



The small intestine of the herbivorous 

 tadpole is very much longer than that of the 

 carnivorous adult frog, a distinction that 

 generally separates animals that feed on 

 vegetation from those that feed on meat. 

 Digestion takes place much faster where 

 meat is the principal diet and therefore a 

 shorter gut is sufficient. On the other hand, 

 the cellulose found in plant tissue requires 

 a longer period to digest; hence a longer 

 gut is necessary in animals that are vegetar- 

 ians. The small intestine of tlie frog opens 

 directly into the short expanded large intes- 

 tine which soon constricts down to the 

 rectum and then opens into the cloaca 

 (sewer). Here the genital and urinary 

 ducts also empty. Undigested food de- 

 posited in this region is soon voided to the 

 outside through the anus. The cloaca is 

 found among reptiles, birds, and low mam- 

 mals, but among all higher mammals the 

 urogenital and digestive tubes have sepa- 

 rate openings to the outside of the body. 



Circulatory system. Just as in the animals 

 already studied, the circulatory system in 

 the frog must transport food and oxygen 

 to the cells of the body and waste products 

 away from them. In fish, the heart is a 

 simple pump in which all of the blood is 

 carried through a single circuit; the blood 



