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Show 968 PROF. OWEN ON THE [Nov. 19, basis of support of the mollusk. Thus the septa, as they are successively formed, adhere, not only to the circumference of the growing valve, but to a central part of the preceding septum, and for an extent more or less corresponding to the circumference of the correspondingly advancing adductor muscle. If the adductor were a tube instead of a solid mass, the central confluent part of the septa would be perforated, and a siphon would result, the calcareous walls of which would be continuous, as in Spirula, Nautilus striatus, the Orthoceratites, &c. The contents of the deserted chambers in Spondylus varius are sea-water with an increased proportion of the saline ingredients. The efficient cause of the forward movement of the Spondylus varius appears to be the need of a shell of a size suitable to the growing bulk of the animal, coupled with the frequeut fixation of the lower valves of the young shell to an overcrusting mass of coral, in advance of which the growing shell must increase. Such increase and the testaceous provision for it are not, therefore, attributable to special expansion of one organ, but to the concomitant growth of the whole of the soft parts of the Palliobranch. It has been suggested that the periodical increase of the ovarium or testis might initiate and constrain the forward movement of the soft parts in the Cephalopods with chambered shells, and that a polythaiamous structure is related conditionally to the generative function1. But it will be observed, in both Spirula and Nautilus, that the formation of the chambers commences from the embryonal cup (proto-conch, fig. 5 «, p. 973), and continues through an early period of growth antecedent to the acquisition of the procreative function, or the adult stage of existence-and, moreover, that those early chambers are relatively deeper2 than the succeeding ones, indicative of a more extensive forward movement of the soft parts, in accordance with the more rapid growth of the animal which characterizes the period of nonage, when all the assimilative functions are concentrated on general increase and no degree of that power is diverted to the development of special organs, such as the testis or ovarium. The last or open chamber of Nautilus, and, by analogy of size and certain known contents, in Ammonites, was occupied by the entire soft parts. In Spirula it contains only the hind end of the liver and portions of the origins of the retractor muscles of the head and funnel. It has been stated to contain the ink-bladder in Spirula 3; but in m y dissections of that Cephalopod made subsequently to that detailed in the ' Zoology of the Voyage of the Samarang,' I find the same positions and relations of the ink-bag as are described and figured in that monograph4. 1 Prof. Seeley, Proceedings of the British Association, Bath, 1864, Section Zoology, "Nautiloid shells," 8vo, p. 229. 2 By depth is meant the diameter from septum to septum; by breadth that between wall and wall. ' Woodward, ' Manual of Mollusca,' p. 77. 4 " A very minute pyriform ink-bag, z, is situated close to tbe rectum ; and its duct opens within the verge of the anus'' (p. 10, pl. iv. fig. 11). |
