111 PHARYNX 153 



where they an- tin- enormously enlarged 



glands. 



Similar Idcali/.cd de\ vlnpmriii s of tlie buccal glands occur in 

 r.inls and some of them may reach a ^n-at si/c as, for example, the 

 enormous subliminal ^hmds of the Woodpeckers. 



PHAUYNX. The part of the alimentary canal which follow- 

 immediatelv behind the huccal cavity is highly characteristic from 

 the fact that in Vertebrates it is concerned with the function of 

 breathing. The special organs which are developed to carry out this 

 respiratory function tall into two groups one represented by the 

 Lung adapted for respiratory exchange with the atmosphere, the 

 other by the Gills adapted for respiratory exchange with gases 

 in solution in the water. As the balance of probability is in 

 favour of the latter being the more archaic they will here be 

 considered first. 



The gills are seen in their most typical and familiar form in the 

 various groups of Fishes where there is present upon each side of the 

 pharyngeal region a series of visceral clefts slit-like openings lead- 

 ing from the pharyngeal cavity to the exterior separated from one 

 another by masses of solid tissue known as the visceral arches or 

 gill septa. The walls of the clefts are highly vascular and their 

 surface is commonly raised into conspicuous plate-like projections 

 the respiratory lamellae which serve to increase the area of 

 respiratory tissue. 



In the most archaic arrangement, seen in Elasmobrauch fishes, 

 the front lip of each cleft, except the first, is prolonged backwards to 

 form a small valvular flap overlapping the external opening. In the 

 Holocephali, Teleostouii, and Dipnoi the anterior one of these flaps, 

 that projecting back from the hyoid arch, becomes greatly enlarged 

 to form the operculum which overlaps not merely one but the whole 

 series of clefts lying behind it. Correlated with this the outer 

 portion of each succeeding septum, which in the Elasmobranch gave 

 origin to its valvular flap, has disappeared, leaving only the portion 

 lying next the pharyngeal cavity. 



The cleft lying in front of the hyoid arch the spiracle is usually 

 modi tied, its respiratory tissue having been reduced and even its 

 opening being diminished in size or completely absent, but its general 

 relations in the adult are such as to permit of no doubt as to its 

 serial hoinology with the clefts behind it. 



Most usually there are on each side six clefts a spiracle and five 

 branchial clefts -but there is reason to believe that there was a 

 greater number present in primitive Vertebrates seeing that the 

 number of persistent clefts becomes on the whole less as one ascends 

 the vertebrate scale and that here and there among the more archaic 

 forms a greater number than the usual is found (BdeUostoma up to 14, 

 Notidanus cinereus 7, N. griseus 6). 



In a few of the more archaic Vertebrates there develop durin- 

 .larval life, in addition to the visceral clefts with their respiratory 



