THE ALIMENTARY SYSTEM 347 



Placentals), in which the alimentary and urino-genital systems 

 open separately to the exterior. 



The first special digestive gland to appear is the liver, 

 which is present in Amphioxus. In the higher forms it 

 connects with the anterior part of the intestine by the bile- 

 duct, and usually possesses a gall-bladder. A rudimentary 

 pancreas appears in the Cyclostomes, and in higher forms it is 

 well developed, connecting with the intestine by one or more 

 pancreatic ducts. It may be mentioned that in mammals at 

 least, the secretion of pancreatic juice is started by a substance 

 " secretin," which is released from the lining of the intestine 

 into the blood-stream and carried therein to the pancreas 

 which it stimulates. This is of interest, for secretin was the 

 first of the hormones (chemical stimulants with specific effects 

 and which are carried about in the blood) to be properly 

 recognised. 



Salivary glands are lacking from the lower water-living 

 chordata, as is readily understood when it is remembered 

 that a current of water is constantly sweeping through the 

 mouth to the gill-slits. Salivary glands make their appearance 

 in the Amphibia. In the snakes, some of the salivary glands 

 may be modified into poison-glands. 



Literature 



Oppel, A. Der Magen, Schlund und Darm, Mundhohle, Bauchspeichel- 

 driise und Leber. Lehrbuch der Vergleichenden mikroskopischen 

 Anatomie der Wirbeltiere, Parts I, 2, and 3. Fischer, Jena, 1896. 



