DIGESTIVE SYSTEM 121 



to its length, and the oral and anal apertures are diametrically 

 situated, approaching each other only in the cephalopoda and 

 pteropoda. 



The brachiopoda are organised on the simplest scale, 

 having a mouth situated between the two arms, or arm-like 

 processes. The lamellibranchiata have a short oesophagus 

 attached to the mouth, hepatic ducts leading into the stomach 

 and the terminal intestine coiled. Together with the develop- 

 ment of a head in the gastropoda and cephalopoda, a 

 pharynx with a deposit of chitine is formed, whereupon follow 

 an oesophagus, a stomach, commonly divided into cardia and 

 pylorus, a large intestine and a terminal intestine, generally 

 expanded. 



To arrive at a clear understanding of the nature of the 

 digestive system of the vertebrates we must trace their em- 

 bryonic development. Here we find that the entoderm and 

 an inner section of the mesoderm form the nucleus of devel- 

 opment, that in the higher vertebrates the intestinal sac is 

 primarily closed and only secondarily furnished with anterior 

 and posterior apertures. After a further differentiation into an 

 anterior and posterior section the latter, which passes through 

 the visceral cavity, forms the intestinal tube proper. 



A further peculiarity common to all vertebrates, including 

 man, is the fact that the anterior section of the primitive in- 

 testinal tube acts primarily (as in the tunicata) as respiratory 

 cavity, and that the alimentary canal originates in the fundus 

 thereof. In spite of the broad breathing cavity of the acrania 

 being reduced in the craniata, it persists, nevertheless, up to 

 the highest vertebrates as a cavity serving both for respiration 

 and for the reception of food. From the reptiles upwards the 

 cavity is reduced by the insertion of the palate. The median 

 elongation of the velum palatinum forming the uvula (see Fig. 

 66) is no distinctive character of man, since it is present in the 

 apes and in the hare, and is found in a rudimentary state in the 

 camel and giraffe. 



Except in size, the tonsils of the apes differ but little from 

 those of man. The tonsils most closely resembling human ton- 

 sils are those of the carnivora ; they are oblong and tuber- 

 culated, consisting of follicular glands invested simply in a 



