A treatise on zoology . ally comparedto the labials, and the suboculararch to the branches of the trigeminalnerve pass below the suboculararch, not above as they shouldif this arch were homologous withthe pterygo-quadrate bar (, 21, 22). It i^ust be confessedthat the exact homology of thesecartilages is at present impossibleto determine, and that many ofthem may be new formations intlie Cj^clostome head. To understand the characterof the head skeleton other systemsmust be^takeir-itito has already been shown thatin Petromyzon the first meta-otic som
A treatise on zoology . ally comparedto the labials, and the suboculararch to the branches of the trigeminalnerve pass below the suboculararch, not above as they shouldif this arch were homologous withthe pterygo-quadrate bar (, 21, 22). It i^ust be confessedthat the exact homology of thesecartilages is at present impossibleto determine, and that many ofthem may be new formations intlie Cj^clostome head. To understand the characterof the head skeleton other systemsmust be^takeir-itito has already been shown thatin Petromyzon the first meta-otic somite, corresponding to theglossopharyngeal nerve, developsthe first permanent myotome ofthe adult (p. 5). The musclesof the head are therefore lessspecialised than in other Crani-ates. Moreover, not only doesthe notochord extend fully intothe base of the cranium, butthere is no sort of articulationbetween the hind part of theskull and the anterior region ofthe vertebral column. More im-portant still: whereas in all other. 38 CYCLOSTOMATA Craniata apertures for the 9th and 10th cranial nerves areincluded in the occipital region of the skull, in the Cyclostomataboth these nerves pass out freely behind the auditory capsule,beyond which the cranium does not extend backwards. Thereis a considerable gap, in Pdromyzon, between the auditory capsuleand the large corapovind first neural arch through which alsopass the combined ventral roots of the first three spinal nerves(Fig. 25). Considerable controversy has taken place concerning thecharacter of the branchial basket. It is often alleged that it isnot homologous with the branchial arches of the Gnathostomes,since it lies in too superficial a position with regard to the muscles,nerves, and blood-vessels, and pharyngeal Avail. For this reasonalso it is sometimes compared to the extrabranchials of Elasmo-branchs. This objection is, however, unconvincing, for, althoughit is true that the ventral aorta lies internal to the basket, yet there
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Keywords: ., bookcentury1900, bookdecade1900, bookpublishe, booksubjectzoology
