THE ALIMENTARY SYSTEM. 449 



mid gut. This region is the fore gut or stomatodffium. 

 Finally, there is usually a shght posterior invagination of 

 ectoderm, forming the anus. This is the hind gut or proc- 

 todeum. 



Associated with the mouth cavity or stomatoda;iim are (a) teeth 

 (ectodermic rudiments of enamel combined with a mesodermic papilla 

 which forms dentine or ivory) ; (1^) from Amphibians onwards special 

 salivary glands ; (c) a tongue or muscular 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, &c. In larval Amphibians muscle strands find their way into it, 

 and it seems likely, as Gegenbaur has recently indicated, 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 not quite certain. In Tunicates it 

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

 Aniphioxus it seems to be formed 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, 

 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 stomatodoeum, of which the 

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

 vestiges. This theory harmonises with the observations of Kleinenberg, 

 on the development of the mouth in some Annelids {Lopadorhyjichits)^ 

 in which the larval stomatodfeum 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, for the hundred slits in Aniphioxus cannot be 

 directly compared with the ordinary clefts. If we exclude 

 the hypothetical clefts, such as those possibly represented 

 in 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 



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