314 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



The asymmetry of the Snail affects the internal organs, includ- 

 ing those of digestion, especially as regards the position of the 

 anus. The odontophore is a very special modification for rasping 

 food, and not only is a large digestive gland present, but also 

 salivary glands, the function of which is, however, doubtful. 



Amphioxus feeds on small organisms carried into the gut by 

 ciliary currents. The nature of the so-called liver is doubtful. 



The Dogfish is carnivorous and possessed of rows of sharp teeth 

 fitted for securing prey. As in all the vertebrate forms there are 

 two digestive glands liver and pancreas. The gut is short, as 

 in all cases where the food is of easily digestible animal nature, 

 but the spiral valve offers a considerable absorptive surface. 



In the Frog, insects are secured as food by means of the 

 tongue, aided by the sticky secretion of numerous small glands. 

 Their escape from the mouth-cavity is prevented by the minute, 

 pointed teeth. The absorptive surface is much increased, not 

 only by the length but by the ridged internal surface of the small 

 intestine. The gut is relatively much longer in the herbivorous 

 tadpole. 



The Pigeon, by means of its horny beak, picks up food, which 

 is temporarily stored up in the crop, and acted upon mechanically 

 in the gizzard. The finger-shaped absorptive projections from 

 the lining of the intestine are here, and in the Rabbit, called 

 villi. 



The Eabbit possesses teeth adapted for cutting and grinding 

 vegetable food. The flexible, fleshy lips largely aid in seizing 

 food. The alimentary canal is here immensely long, as a result 

 of the vegetable food, and its absorptive surface is, in consequence, 

 extremely large. Not only are liver and pancreas present, but 

 also several pairs of salivary glands. 



Circulation. In Amoeba and Vorticella this is aided by the 

 presence of a contractile vacuole, and in the Tapeworm and Fluke 

 one of the functions of a circulatory system i.e., the conveyance 

 of waste-products to organs which get rid of them is rendered 

 superfluous by the large extent of those (excretory) organs them- 

 selves. The general movements of the body effect circulation in 

 Ascaris. 



The Earthworm possesses a well-defined and closed blood 

 system as distinct from a coelomic system. The propulsion of 

 blood is effected by the paired hearts. The Leech exhibits a 

 well-developed blood system in communication with a reduced 



