586 



MANDIBULAR AND HYOID BARS. 



view is the striking similarity which Huxley has pointed out between 

 the oral skeleton of the Lamprey and that of the Tadpole ; and certain 

 peculiarities of the mandibular arch of Chimaera and the Dipnoi can perhaps 

 best be explained on the supposition that the oral skeleton of these forms 

 has arisen in a manner somewhat similar to that in the Frog ; though with 

 reference to this point further developmental data are much required. 



On the other hand the above suppositions would necessitate our 

 admitting that a great abbreviation has occurred in the development of 

 the mandibular arch of the otherwise more primitive Urodela ; and that 

 the simple mode of growth of the jaws in Elasmobranchii, from the 

 primitive mandibular arch, is phylogenetically a much abbreviated and 

 modified process, instead of being, as usually supposed, a true record of 

 ancestral history. 



If the view is accepted that the characters of the mandibular arch of 

 the Tadpole are secondary, it will be necessary to admit that the adaptation 

 of the mandibular arch to the suctorial mouth took place after the suctorial 

 mouth had come to be merely a larval organ. 



In view of our imperfect knowledge of the development of most Piscine 

 skulls I would refrain from expressing a decided opinion in favour of 

 either of these alternatives. 



or.p 



eth 



FIG. 339. TADPOLE WITH TAIL BEGINNING TO SHRINK; SIDE VIEW OF SKULL 



WITHOUT THE BRANCHIAL ARCHES. (From Parker.) 



n.c. notochord; au. auditory capsule; between it and eth. the low cranial side wall 

 is seen; eth. ethmoidal region; st. stapes; 5. trigeminal foramen; 2. optic foramen; 

 ol. olfactory capsules, both seen owing to slight tilting of the skull; c.tr. cornu 

 trabeculae; ./. upper labial, in outline; su. suspensorium (quadrate); pd. its pedicle; 

 ot.fr. its otic process; or.p. its orbitar process; t.m. temporal muscle, indicated by 

 dotted lines passing beneath the orbitar process; pa.pg. palatopterygoid bar; ;;//. 

 Meckelian cartilage; /./. lower labial, in outline; c.h. ceratohyal; b.h. basihyal. The 

 upper outline of the head is shewn by dotted lines. 



As the tail of the Tadpole gradually disappears, and the 

 metamorphosis into the Frog becomes accomplished, the 

 mandibular arch undergoes important changes (fig. 339): the 



