Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 SKULL. 133 the conical cavities of which contain a part of the remains of the notochord. The notochord as a rule persists also in the centre of the vertebral body as a thin cord (connecting the dilated inter- vertebral portions, fig. 570 «). In the bony Ganoids and the Teleosteans the biconcave:;: vertebral bodies are completely ossified and fuse with the corresponding upper and lower bony arches, so as to form a complete vertebra. In some parts of the trunk ribs are attached to the piec
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 SKULL. 133 the conical cavities of which contain a part of the remains of the notochord. The notochord as a rule persists also in the centre of the vertebral body as a thin cord (connecting the dilated inter- vertebral portions, fig. 570 «). In the bony Ganoids and the Teleosteans the biconcave:;: vertebral bodies are completely ossified and fuse with the corresponding upper and lower bony arches, so as to form a complete vertebra. In some parts of the trunk ribs are attached to the pieces of the ventral arches (hsemapophyses) which here diverge from one another; and there are often in addition ossifications of the inter-muscular ligaments. The structure of the skull in Fishes presents a series of grades of development culminating in the complicated skull of the Teleostei. The primordial skull of the Cydostomes is the simplest. It consists of a cartilagino-membranous cranial capsule, in the hard basilar part Q FIG. 585.—Cephalic skeleton of the Sturgeon (after Wiedersheim). So, rostrum ; C«, nasa\ pit; O, orbit; Hrn, hyomandibular ; S, symplectic ; Pq, palatoquadrate ; Mil, lower jaw; Hy, hyoid bone ; V, foramen for the vagus ; R, ribs. of which the notochord ends. Two bony capsules—lateral appendages of the bony basilar region—enclose the auditory organ, while two anterior pieces are connected with the complicated apparatus of the facial and palatal cartilages. The primordial skull of the Selachians (fig. 571) shows a further advance in development. It has the form of a simple cartilaginous capsule which is not further divided into separate pieces. The notochord ends in its base. In the sturgeon (tig. 585), there are bony pieces as well as the cartilaginous cranial capsule. These consist of a flat basilar bone—the parasphenoid—and a system of dermal membrane bones. A true bony cranial invest- ment is first developed round the primordial skull o
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