Xtll 



PH'STLUM CHORDATA 



given off a blind pouch, 

 the liver (Ir.) or hepatic 

 ccecum, which extends 

 forwards to the right of 

 the pharynx : it is lined 

 with glandular epithelium 

 and secretes a digestive 

 fluid. 



The gill-slits (br. d.) 

 are long narrow clefts, 

 nearly vertical in the 

 expanded condition, but 

 very oblique in preserved 

 and contracted specimens 

 hence the fact that a 

 large number of clefts 

 always appear in a single 

 transverse section (Fig. 

 751, /4, M). The clefts 

 are more numerous than 

 the myomeres in the 

 adult, but correspond in 

 number with them in the 

 larva : hence they are 

 fundamentally m e t a - 

 meric, but undergo an 

 increase in number as 

 growth proceeds. 



The branchial lamella} 

 (Fig. 752, br. sep., Fig. 

 751, A, kb.), or portions 

 of the pharyngeal wall 

 separating the clefts from 

 one another, are covered 

 by an epithelium which 

 is for the most part en- 

 dodermal in origin, and 

 is composed of greatly 

 elongated and ciliated 

 cells. On the outer face 

 of each lamella, however, 

 the cells are shorter and 

 not ciliated, and are, as a 

 matter of fact, portions of 

 the epithelial lining of 

 the atrium, and of ecto- 

 dermal origin. Each 



