. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates. SKULL 85 present in all other Craniata; for this reason these forms are spoken of as Cydostomata to distinguish them from the other craniate Vertebrates or Gnathostomata. In addition to the peculiar histological differences in the struc- ture of the cartilage, many other special characters of the skull are seen not only in the Class as a whole, but also in the two Orders, which must have become specialised very early along different lines, so that a comparison between them is rendered difficult. The low character of the s
. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates. SKULL 85 present in all other Craniata; for this reason these forms are spoken of as Cydostomata to distinguish them from the other craniate Vertebrates or Gnathostomata. In addition to the peculiar histological differences in the struc- ture of the cartilage, many other special characters of the skull are seen not only in the Class as a whole, but also in the two Orders, which must have become specialised very early along different lines, so that a comparison between them is rendered difficult. The low character of the skull is marked by the imperfect develop- ment of the cartilaginous brain-box, and thus most of the cerebral nerves on making their exit from the skull are but slightly or not at all surrounded by cartilage: moreover, the anterior region of the spinal axis, which in Gnathostomes becomes assimilated to the cranium, remains undifferentiated, so that an occipital region is wanting and the vagus makes its exit behind the skull ("palceo- cranium ") and not through its walls. The jaw-apparatus has doubtless become degenerated, and indications of its former presence may possibly lie recognised. Fit;. 04. — SKULL WITH BRANCHIAL BASKET OF Petromyzon marinus. (After W. K. Parker.) The cartilaginous parts are dotted. , anterior dorsal cartilage; , anterior lateral cartilage; , annular cartilage; , auditory capsule; , 1—7, vertical bars of branchial basket; , 1—7, external branchial clefts; , cornual cartilage; , cranial roof; , 1—4, longitudinal bars of branchial basket ; fy. c, lingual cartilage ; , median ventral cartilage; , nasal aperture ; nch, notochord ; Nv. 2, foramen for optic nerve; , olfactory capsule; , pericardial cartilage ; pdc, posterior dorsal cartilage ; , posterior lateral, cartilage ; , subocular arch ; , styloid process; , styliform cartilage; t, teeth. T
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