9G8 PROF. OWEN ON THE [Nov. 19, 



basis of support of the mollusk. Thus the septa, as they are succes- 

 sively 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 corre- 

 spondingly 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 

 function 1 . 



But it will be observed, in both Spirula and Nautilus, that the 

 formation of the chambers commences from the embryonal cup (proto- 

 conch,fig.5rt, 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 re- 

 latively deeper 2 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 my 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 monograph 4 . 



1 Prof. Seeley, Proceedings of the British Association, Bath, 1864, Section 

 Zx>logy, "Nautiloid shells," 8vo, p. 229. 



2 By depth is meant the diameter from septum to septum ; by breadth that 

 between wall and wall. 



J Woodward, ' Manual of Mollusca,' p. 77. 



4 " A very minute pyriform ink-bag, s, is situated close to the rertum ; and 

 its duct opens within the verge of the anus" (p. 10, pi. iv. fig. 11). 



