428 INTRODUCTION TO CRANIATA [CH. 



intimately associated with a downgrowth of the brain, called the 

 ibfundibulum, to form an organ having an influence on the 

 well-being of the animal (see p. 416). Since in the case of the 

 Urochorda the subneural gland is fashioned out of the persistent 

 communication of the sense vesicle with the exterior, one is tempted 

 to regard the close connection of the infundibulum and the pituitary 

 body as remnants of the former connection of the brain and stomo- 

 daeuin in the ancestors of the Craniata. Some authors maintain 

 that a rudiment of the infundibulum is to be seen even in the 

 cerebral vesicle of Amphioxus (see Fig. 193). 



Except in the lowest forms the alimentary canal is differentiated 

 into several well-marked divisions. There is to begin with a 

 stomodaeum called the buccal cavity lined by an epithelium con- 

 sisting of many layers similar to that forming the epidermis. The 

 first division of the endodermal tube is called the pharynx, and 

 into this the gill-slits open. Following on the pharynx is a tube of 

 narrow diameter, termed the oesophagus or gullet, which leads 

 into the stomach. The line of demarcation between ectoderm and 

 endoderm is entirely obliterated in the adult, since the epithelium 

 lining both pharynx and oesophagus is many layered. The 

 stomach consists of the first of the loops into which the alimentary 

 canal is bent in consequence of its being longer than the body; it is a 

 greatly dilated portion of the canal and in it the food is stored until 

 a large amount of digestion is accomplished. As in other animals, 

 the food is moved from place to place by peristaltic contractions of 

 the visceral muscles derived from the inner wall of the coelom. 

 There is a particularly powerful girdle of these called the pyloric 

 sphincter, which by remaining contracted keep the distal end of 

 the stomach, the so-called pylorus, closed until the work of 

 digestion is accomplished, when they relax and allow the food to 

 pass on into the next division of the canal, the intestine. The 

 avails of the proximal part of the stomach are produced into small 

 pouches, termed the gastric glands, the cells lining which secrete 

 a substance called pepsin, which has the power of turning the 

 proteid of the food into soluble peptone. 



Pepsin is an example of the class of substances known as 

 digestive ferments or enzymes : these are complex substances of 

 unknown constitution which have the power of effecting a large 

 amount of chemical change without themselves undergoing a 

 permanent alteration. The object of their action on food-stuffs is 

 to render them soluble, and therefore fitted for absorption by the 



