1 8 MOUTH 



crush the segments behind. At the same time, the correspondence 

 between my ornery and branehiomery is to a great extent lost, 

 though evident in the nerve-supply. As the row of slits bars the 

 way Lo the downward growth of the myotomes, the latter have to 

 pass round the hinder edge of the series to form the ventral hypo- 

 glossal musculature (Fig. 3). A cartilaginous visceral arch develops 

 in front of the first or spiracular cleft, and behind this and each 

 succeeding cleft. Related to each arch and cleft is a segmental 

 branchial nerve and a blood-vascular arch. 



The first or mandibular and the second or hyoid arch become 

 closely connected with the skull in Gnathostomes ; the mandibular 

 arch bends over the mouth, and becomes subdivided into the 

 primitive upper and lower jaws (Fig. 5). In the lips, in front and 

 at the sides of the mouth, labial cartilages are often present in fish, 

 which Gegenbaur considered to be remnants of preoral gill -arches. 

 Huxley suggested that the trabeculae cranii represent gill-arches. 

 But there is really no definite evidence that preoral gill-slits have 

 ever existed, and it is difficult to see how they could have been 

 functional. Gegenbaur subsequently abandoned this view [163], 

 and inclined towards that of Pollard [333]. This author 

 considered the labial cartilages to be remnants of a primitive 

 system of cirrhi, such as are found in the Myxinoids, and which he 

 compared with those of Amphioxus. 



The nature of the mouth of Vertebrates is by no means easy 

 to determine. Dohrn (1882) believed it to be a new mouth 

 derived from a pair of anterior coalesced gill-slits [114]. Traces 

 of the original mouth, the palaeostome, homologous with that of 

 invertebrates, passing through the brain to open dorsally, were 

 supposed by Kolliker to be represented by the hypophysis and the 

 epiphysis. A less phantastic theory (Beard [33a] and von Kupfier 

 [275]) is that the hypophysis represents the original mouth or. 

 palaeostome, which opened into the alimentary canal as it still does 

 in Myxinoids (p. 46). This connection, however, appears to be 

 secondary, and at present the most reasonable view seems to be 

 that the ancestral vertebrate mouth has been retained, although it 

 may have shifted its position backwards. It is possible that during 

 this process of shifting some anterior gill-slits may have been 

 obliterated, or combined with the mouth ; but convincing evidence 

 of this is missing. 



The upper lip is formed in fish by the junction in the middle 

 line below the snout of two upper-jaw processes (Fig. 117, p. 154) ; 

 in the Tetrapoda these lateral processes combine with the median 

 fron to-nasal process to complete the upper margin of the mouth. 



We have briefly discussed above the subject of the segmentation 

 of the head in the Craniates ; but there is yet to be mentioned 



