593 



ALIMENTARY SYSTEM 



It was suggested on p. 569 that the mouth of vertebrates 

 was formed from an anterior pair of gill slits which coalesced. 

 This accords with the primary function of the otlier gill slits, 

 and allows for the loss of the more primitive mouth presumable- 

 possessed by the invertebrate ancestors. But speculations on 

 such thin grounds are idle, and all that matters is that in all 

 vertebrates there is a mouth which is either terminal or a little 

 way down the ventral surface. The opening leads into a space, 

 also known as the mouth but less ambiguously as the buccal 

 cavity. In most vertebrates this bears teeth of some sort, but 

 they are absent from birds and some smaller groups, such as 

 the turtles. In general, food passes straight through the buccal 

 cavity, and the teeth are mere backwardly directed points which 

 prevent food from escaping, but in most mammals there is a 

 greater or less degree of chewing and the teeth are complex 

 and of different sorts. Some birds also, such as the parrots, chew 

 their food, using the beak for this purpose. A tongue is only 

 developed in the tetrapods, its musculature and skeleton coming 

 from the parts which helped to form the branchial arches. In 

 lower forms, as in the frog and lizards, it is chiefly used for food 

 gathering, but in chewing species it helps in the manipulation 

 of food. It usually bears taste organs, and in the snakes it is 

 an accessory olfactory organ ; in its rapid darting in and out of 

 the mouth it picks up particles of volatile matter, and when it 

 is withdrawn the forked tip fits into two sensory pockets on the 

 roof of the mouth. Mucous-secreting glands open into the buccal 

 cavity of most land-Hving vertebrates, but it is only in a few 

 mammals and birds, such as man and the fowl, that it contains 

 a starch-digesting enzyme. 



The pharynx, the passage from which the gill slits open, is 

 hardly distinguishable in form from the buccal cavity, especially 

 in tetrapods. 



The rest of the alimentary canal, or gut, was probabl\- little 

 speciahsed in the earliest microphagous vertebrates, and that is 

 still the condition of the lamprey. More often, however, there is 

 some specialisation of parts for different functions, and since 

 the names of these parts are taken from those of mammals we 

 will first list these in order; they are, gullet or a?sophagus, 

 stomach, duodenum, ileum, colon, rectum. Duodenum and ileum 



