554 STRUCTURE OF VERTEBRATA 



Associated with the mouth cavity or stomodaeum are — {a) teeth 

 (ectodermic rudiments of enamel combined with a mesodermic papilla 

 which forms dentine or ivory) ; (b) from Amphibians onwards special 

 saUvary glands ; (c) a tongue — a glandular and sensitive outgrowth 

 from the floor. The tongue develops as a fold of mucous membrane 

 in front of the hyoid, and afterwards becomes increased by growth of 

 connective tissue, etc. In larval Amphibians muscle strands find their 

 way into it, and Gegenbaur suggested that their original function was 

 to compress the glands. As they gained strength they became able 

 for a new function, that of moving the tongue. In all higher animals 

 (above Fishes), the nasal sac opens posteriorly into the mouth ; in 

 some Reptiles and Birds, and in all Mammals, the cavity of the mouth 

 is divided by a palate into an upper nasal and lower buccal portion. 



The origin of the oral aperture is uncertain. In Tunicates it is 

 formed by an ectodermic insinking which meets the archenteron ; in 

 Amphioxus it seems to arise as a pore in an ectodermic disc ; in other 

 cases it is a simple ectodermic invagination ; or it may owe its origin to 

 the coalescence of an anterior pair of gill-clefts innervated by the fifth 

 nerve. If the last interpretation be true, its origin illustrates that 

 change of function which has been a frequent occurrence in evolution. 

 But if the mouth arose from a pair of gill -clefts, and in some cases it 

 actually has a paired origin, then there must have been an older mouth 

 to start with. Thus Beard in his brilliant morphological studies dis- 

 tinguished between " the old mouth and the new." The new mouth 

 is supposed to have resulted, as Dohrn suggested, from a pair of gill- 

 clefts ; the old mouth was an antecedent stomodaeum, of which the 

 so-called nose of Myxine and the oral hypophysis of higher forms may 

 be vestiges. This theory harmonises with the observations of Kleinen- 

 berg on the development of the mouth in some AnneUds {Lopado- 

 rhynchus), in which the larval stomodaeum is replaced by a paired 

 ectodermic invagination. 



The mouth cavity leads into the pharynx, on whose walls 

 there are the gill-clefts. Of these the maximum number is 

 eight, except in Amphioxus. If we exclude the hypo- 

 thetical clefts, such as those possibly represented by the 

 mouth, the first pair form the spiracles — well seen in skates. 

 In the position of the spiracles the Eustachian tubes of 

 higher Vertebrates develop. In front of the spiracle there 

 is sometimes a spiracular cartilage, which Dohrn dignifies 

 as a distinct arch. The other gill-clefts are associated with 

 gills in Fishes and Amphibians, while in Sauropsida and 

 Mammals, in which there are no gills, four " visceral " 

 clefts persist as practically functionless vestigial structures. 

 In some cases their openings are very evanescent. The 

 clefts are bordered by the branchial arches, and supplied by 

 blood vessels and nerves. 



