26o THE OLD MOUTH AND THE NEW. 



In the early larval stages, all traces of the paired oral arches have disappeared 

 and we have instead the characteristic larval mouth of the amphibia, with the 

 V-shaped mandibles and the maxillae sheathed in horn, surrounded by prominent 

 lips. The latter form a shallow antechamber fringed with sensory papillae, that 

 resembles the pre-oral in chamber Amphidxus, Bothriolepis and the cyclostomes. 

 (Figs. 164, 165, 166, 177-178-) 



The mouth itself, owing to the gradual union of the paired arches on the 

 haemal surface, undergoes a remarkable transformation. It first appears as a 

 very long median furrow. As the anterior end is obliterated by the union of the 

 premaxillae and the posterior end by the union of the mandibles, the remaining 

 median portion widens, taking on first an hexagonal contour, and then the form 

 of a transverse slit, with a continuous maxillary arch in front and a mandibular 

 arch behind. (Fig. 161.) 



The median groove that initiates the opening into the enteron, may be 

 regarded as the remnant of the primitive cephalic navel of arthropods, and its 

 subsequent changes in form, as the result of the way in which the oral arches 

 unite around it. 



The anterior end of this groove, or the part lying, during the earliest stages, 

 between the premaxillary lobes, becomes deeper than the rest, and marks the 

 beginning of the mouth, C. A little later, this depression is most pronounced 

 between the maxillary lobes, E and F. 



The hypophysis may be regarded as the remnant of a pair of excretory 

 glands similar to those on the outer margin of the anterior cephalic appendages in 

 arachnids. In ancestral vertebrates they were probably situated on the margins 

 of the premaxillary lobes, their unpaired condition being due to the median fusion 

 of the appendages to which they belonged. 



The "tear duct" may be regarded as a specialization of the groove that 

 originally separated the maxillary and premaxillary arches. 



The development of the mouth and oral arches of the frog may be regarded 

 as the typical mode of development in vertebrates. A condition like that just 

 described is clearly present in other amphibia, as in Amblystoma (Fig. 168), and 

 in the sturgeon (Fig. 174). Even in the mammals we may see indications of the 

 same structure, the fronto-nasal process probably representing in part the fused 

 premaxillary lobes of the frog. 



In Bdellostoma, three pairs of oral lobes, comparable with those in the frog 

 are preserved even in adult stages. (Fig. 175.) In petromyzon there has been a 

 greater median fusion, for the remnants of the premaxillary lobes, the olfactory 

 pits, and the hypophysis have apparently been absorbed in a single median infold- 

 ing lying in front of the maxillary arch. 



In the most primitive vertebrate-like animals of all, the ostracoderms, the 



mouth parts of the adult were in a condition similar to those of Bdellostoma, or to 



those in the embryonic stages of the frog and sturgeon. (See Chapter XX, p. 373.) 



Conclusion. — We may therefore conclude that the mouth of vertebrates is 



