| OCR Text |
Show 972 PROF. OWEN ON THE [Nov. 19, mollusks suggests to the physiological mind the probability that some corresponding step in advance of the ordinary condition of a testaceous defence would be likely ; and the retention of the parts of such defence vacated in the course of growth, with the superaddi-tion of a vascular by-way running through the whole, verifies such anticipation of the means whereby the camerated and siphoniferous shell is thus brought into closer harmony with the rest of the organic structure. WThen the first simple, single-chambered, nuclear dwelling is added to and, in part, abandoned, a closer connexion is therewith preserved than in Vermetus, e. g. ; and, as the artery penetrating the membranous siphon continued from the last-formed dwelling-chamber was demonstrated by mercurial injection1, there is no reason to doubt that such organic connexion was maintained between the fabricator of the second chamber after it had advanced from and vacated the first or nuclear one. The constancy of this siphuncular connexion running through all the chambers of the largest and most complex of the polythalamous sbells, with the great size and singular complexity of the siphuncle in several extinct species, form the grounds on which I still hold to my original belief in the function of the siphuncle as related to a maintenance of the vitality of the shell. But this relation may be connected, also, with a greater share assigned to the siphon in the protection of the soft parts of the Cephalopod at the earlier stages of its existence. The chief character of the tetrabranchiate chambered and siphonated shell is its affording, besides a sheath or ease to the whole animal, a special protection to a part of the animal. Such twofold office is performed by the shell in certain Gastropods, conspicuously in the genus Calyptrcea, in which an accessory "cup," springing from the concavity of the larger " saucer," lodges part of the muscular system2. Every cbambered and siphonated shell begins in this simple fashion. The protoconch is cup-shaped or flask-shaped, and includes a similar but smaller blind beginning of the siphon. The proportion of the inner partially protecting shell to the outer wholly encasing shell is greatest in the Silurian Orthoceratites; and with the large proportional siphon are associated complexities characteristic of the genera Ormoceras, Huronia, &c.3 Modifications of the contents of such siphons in the Silurian Vagi- 1 "The lesser aorta" sends off a small branch (14, pl. 5 & 6) "which, winding round to the ventral aspect of the ventricle (to which it is connected by a process of membrame), passes through a foramen in the septum dividing the pericardium from the cavity at the bottom of the pallial sac, is then continued through that cavity, passing between the ovary and gizzard, and lastly enters, without diminution of size, the membranous tube that traverses the partitions of the shell."-Memoir on the Pearly Nautilus, p. 36. 2 " Its cavity is filled by what maybe termed the apex of the foot, which here loses its muscular character, and assumes a gelatinous texture."-" Anatomy of the Calyptrceidcc," Trans. Zool. Soc. vol. i. p. 208, pl. 30. figs. 2, 6/, 1834. 3 Charles Stokes, in Geol. Trans. 2nd ser. vol. v. p. 706, 1837. |
