390 STRUCTURE OF VERTEBRATES. 



which is convenient, especially in regard to Invertebrates, 

 where the fore-gut or stomatodseum and the hind-gut or 

 proctodseum are often large. 



Associated with the mouth-cavity or stomatodseum are (a) 

 teeth (ectodermic rudiments of enamel combined with a 

 mesodermic papilla which forms dentine or ivory) ; (b) from 

 Amphibians onwards special salivary glands ; (c) a tongue or 

 muscular and sensitive outgrowth from the floor. In 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 not quite certain. In Tunicates it 

 is formed by an ectodermic insinking which meets the archenteron ; in 

 Amphioxus it seems to be formed as a pore in an ectodermic disc ; in 

 other cases it is either a simple ectodermic invagination or stomatodseum, 

 or it may have resulted from the coalescence of an anterior pair of gill- 

 clefts innervated by the fifth nerve. If so, its origin well illustrates that 

 change of function which seems to have 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, 

 distinguishes 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 stomatodasum, of which the 

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

 vestiges. This theory harmonises with the observations of Kleinenberg, 

 on the development of the mouth in some Annelids {Lopadorhynchus), 

 in which the larval stomatodseum is replaced by a paired ectodermic 

 invagination. 



The mouth-cavity leads into the pharynx, on whose walls 

 are the persistent gill-clefts. Of these the maximum number 

 is eight, for the hundred slits in Amphioxus cannot be 

 directly compared with the ordinary clefts. If we exclude 

 the hypothetical clefts, such as those which are possibly 

 represented in the mouth, the first pair form the spiracles — 

 well seen in skates. In the position of the spiracles the Eusta- 

 chian tubes of higher Vertebrates develop. In front of the 

 spiracle there is sometimes a spiracular cartilage, which we did 

 not mention in discussing the skull. This 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 



