254 Elementary Biology. 



but is also the organ of respiration. It will be necessary for 

 us, therefore, to study the alimentary system in connection 

 with the respiratory system. 



The pharynx is a wide tube or sac, and extends fully 

 half the length of the body. Its lateral walls are pierced by 

 a series of oblique slits, putting the cavity of the pharynx 

 in communication with a space which lies round the ali- 

 mentary canal known as the atrial cavity. These slits are 

 lined by columnar ciliated cells, the cilia of which during 

 life are constantly in motion. The slits are known as bran- 

 chial slits, and the columns between them as branchial 

 arches. Each arch is strengthened by having developed in 

 its interior a chitinous rod. The long oblique bars are 

 connected to each other by short crossbars, so that the slits 

 rather resemble an open meshwork than anything else. 

 Dorsally and ventrally the pharynx has developed on its 

 wall a couple of ridges, the hollow between which is known 

 as the hyperbranchial groove and hypobranchial groove 

 respectively. 



Posteriorly the pharynx rapidly narrows into a short 

 straight intestine. The intestine is not of the same calibre 

 throughout. Immediately after leaving the pharynx it is 

 very narrow ; it then widens considerably, giving off for- 

 wards a diverticulum or sac known as the liver, again 

 narrowing gradually unul it ends at the anus at the base of 

 the caudal fin. 



It will not be possible to understand the mechanism of 

 alimentation and respiration until we have considered the 

 chamber the so-called atrial cavity into which the bran- 

 chial slits open. The relation of this cavity to the cavity 

 of the pharynx on the one hand, and to the exterior on the 

 other, may be best understood by studying their mode of 

 origin. In the very young Amphioxus there is no atrial 

 cavity, and the branchial slits open directly to the exterior 

 as do the branchial slits of an ordinary fish. Late in the 

 course of development two ridges appear along the sides of 



