452 STRUCTURE OF VERTEBRATA. 



forming the anus. This is the hind-gut or procto- 

 deum. 



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

 (ectodermic rudiments of enamel combined with a mesodermic papilla 

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

 salivary 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 it seems likely, as Gegenbaur has recently in- 

 dicated, 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 Myxine, Dipnoi, and higher animals, 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 fiequent 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- 

 tinguishes 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 stomodseum, 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 Annelids (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 



