XIII 



PHYLUM CHORDATA 



87 



In some Fishes the scales or elements of the dermal exo- 

 skeleton pass insensibly into the teeth over the ridges of the 

 jaws, and agree with them in structure, so that there can be no 

 doubt as to the homology of the two. Teeth are, in fact, to be 

 looked upon as portions of the exoskeleton which have migrated 

 from the skin into the buccal cavity, and even into the pharynx, 

 and have there increased in size and assumed special functions. 



The tongue is a muscular elevation of the floor of the mouth, 

 supported by the basi-hyal, and usually more or less protrusible. 

 The roof of the buccal cavity in the embryo sends off a pouch, the 

 pituitary diverticulum (Fig. 760, A, pty. s.), which grows upwards, 

 and, losing its connection with the mouth, becomes attached to 

 the ventral surface 

 of the brain as 

 the pituitary body 

 (pty. ft.). It may 

 correspond with the 

 neural gland of 

 Urochorda. 



In terrestrial 

 Craniata buccal 

 glands are present, 

 opening by ducts 

 into the mouth : 

 the most important 

 of these are the 

 racemose salivary 

 glands which se- 

 crete a digestive 

 fluid saliva, cap* 



able of converting starch into sugar. There are also two large and 

 highly characteristic digestive glands in the abdominal cavity, 

 both developed as outpushings of the intestine, but differing 

 greatly from one another in their fully developed state, both in 

 outward appearance and in histological structure : these are the 

 liver and the pancreas. 



The liver (Fig. 7 60, A, Ir.) is a dark-red organ of relatively immense 

 size : it not only secretes a digestive juice, the bile, which has the 

 function of emulsifying fats, but also forms an amyloid substance 

 called glycogen or animal starch, which, after being stored up in 

 the liver-cells, is restored to the blood in the form of sugar. The 

 liver is formed of a mass of polyhedral cells (Fig. 770, /.) with 

 minute intercellular spaces which receive the bile secreted from 

 the cells and from which it passes to the ducts (b). The pancreas 

 (Fig. 760, A, pn.) is a racemose gland, and secretes pancreatic juice, 

 which acts upon proteids, starch, and fats. The ducts of both 

 glands usually open into the anterior end of the intestine : that of 



G 2 



FIG. 770. Diagram of structure of liver. J>, a small branch of 

 hepatic duct ; b', its ultimate termination in the intercellular 

 spaces ; c, blood-capillaries ; ?, liver-cells. (From Huxley's 



Physiology.) 



