44 COMPARATIVE ANATOMY. 



dentine. The newly erupted tooth, therefore, presents a 

 cemental surface for mastication. This may be a simple 

 membrane (membrane of ISTasmyth) as in man, when it will 

 soon disappear, and the enamel at once become the opposing 

 surface; or it may contain cementum in abundance. In the 

 latter case its gradual disappearance from the convexities of 

 the grinding surface leaves exposed the enamel cusps. These, 

 after a longer time, will in like manner be worn through, 

 and expose circumscribed areas of dentine. So that in the 

 molar tooth of an adult herbivore, as in Castor (beaver), or 

 Equus (horse), the islands of dentine are encircled by en- 

 amel, while cementum fills up the depressions. In the her- 

 bivorous type, the canines are often absent. The incisors are 

 absent as in upper jaw of Ruminantia, or very largely devel- 

 oped, as in Rodentia. In the latter order they are pecu- 

 liar in having no closed fang, but in remaining open and 

 continuous with the pulp. The tooth is therefore always 

 growing; a definite length being secured by the constant loss 

 of substance by attrition at the crown. A rodentic incisor 

 is covered with a thick layer of enamel anteriorly, a thin 

 layer posteriorly; the latter wearing away more rapidly than 

 the former, a scalpriform surface is always presented. 



VII. 



DIGESTIVE SYSTEM, 



THE solids and fluids taken into the body as food are sub- 

 jected to a process called digestion, by which the solid por- 

 tions are reduced to a fluid state, the nutritive particles 

 separated from the excrementitious, and the whole prepared 

 to become blood, skeleton, muscle, etc. (Agassiz and Gould.) 



The digestive apparatus is composed of an alimentary canal 

 and appendages. 



PROTOZOA. With the simplest forms of life, as in the Rhizo- 



