. The elasmobranch fishes . Fig. 58. Development of cranium, Acanthias (Modified from Sewert- zoff.) , auditory capsule; asp., alisphe- noidal cartilage; pr., parachordal plate; tr., trabecular cartilage. The cartilaginous cranium or brain case in the Elasmobranchs is unlike the bony cranium of higher forms in that its sides, roof, and floor are welded into a single piece of cartilage. Joined to it also are the auditory and olfactory cap- sules for organs of special sense. The cranium is perforated by nerves and blood vessels which enter or leave the brain. Below the orbit the cranium may p
. The elasmobranch fishes . Fig. 58. Development of cranium, Acanthias (Modified from Sewert- zoff.) , auditory capsule; asp., alisphe- noidal cartilage; pr., parachordal plate; tr., trabecular cartilage. The cartilaginous cranium or brain case in the Elasmobranchs is unlike the bony cranium of higher forms in that its sides, roof, and floor are welded into a single piece of cartilage. Joined to it also are the auditory and olfactory cap- sules for organs of special sense. The cranium is perforated by nerves and blood vessels which enter or leave the brain. Below the orbit the cranium may project outward as an in£raorl)ital plate (most sharks) or such a plate may be absent (rays). The rudiments for the brain case are laid down in the embryo of Acanthias according to Sewertzoff (1897) as three pairs of cartilages (see figs. 58 and 70). These are (1) the parachordal plates (j^r.) which lie at the sides of the notochord and posterior to the internal carotid foramina; (2) a pair of ali- sphenoidal cartilages (asp.); and (3) a pair of trabecular cartilages (tr.) anterior to the internal carotid foramina. The parachordal and trabecular
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