THE PEARLY NAUTILUS. 187 



but as it is only a film, and presents no structure under the microscope, I conclude it to be 

 deposited or left behind by the secreting surface of the mantle when the nacreous septum was 

 formed. And this opinion is strengthened by Dr. Carpenter's statement, " that in every distinct 

 formation of shell-substance there is a single layer of membrane," and " that this membrane was at 

 one time a constituent part of the mantle of the mollusc." 



The nacreous covering of the siphuncle was entire, and on removal it was found to enclose a 

 simple membranous tube, composed of an extension of the periostracum, and exhibiting no structure, 

 even under a one-tenth objective. 



As I could not detect any artery or vein, I conclude that they probably do not extend beyond the 

 first chamber. This view coincides with Professor Owen's statement that " neither the contents nor 

 the vital properties of the siphon are, however, yet known; an artery and vein are assigned for its life 

 and nutrition, and to extend a low degree of the same influence to the surrounding shell." But the 

 structure of the membranous siphon, in the specimen from which I had the opportunity of examining 

 it in a recent state presents, beyond the first chamber, an inextensible and almost friable texture, 

 unsusceptible of dilatation ; it is also coated beyond the extremity of the short testaceous siphon with 

 a thin mother-of-pearl deposit. 



We know that the body of the animal in Nautilus is attached to the shell by means of the two 

 adductor muscles, and by a continuous horny girdle around the mouth of the body-chamber. The sug- 

 gestion, therefore, of Yon Buch, that the function of the siphuncle was to hold the animal into its 

 shell loses much of its significance. But may it not have been the most important point of 

 attachment between the animal and its shell in the earlier forms of the Tetrabranchiata 1 

 In support of this view we may notice that in the fossil Nautili the siphuncle was a shelly tube of 

 considei-able size and thickness, whilst in Ortlwceras it attained to a great magnitude as, for instance, 

 in the genus Huronia, in which it is as large as a human vertebral column. In Actinoceras, 

 Gyroceras, and Phragmoceras the siphuncle is also very large, and contains in its centre a smaller 

 tu'ue, the space between the two being filled up with radiating plates, like the lamellae of a 

 coral. 



Speaking of the connection between the Nautilus pompilius and its shell, Professor Owen says : 

 " A third point of attachment is to the bottom of the shell, by the posterior extremity of the mantle, 

 which probably presents a conical form in the embryo Nautilus." If, then, the siphuncle in the young 

 stage forms the main point of attachment between the animal and its shell, we may reasonably argue 

 that the siphuncle in the adult Nautilus is simply the evidence of an aborted embryonal organ, whose 

 function is now fulfilled by the shell-muscles, but which, in the more ancient and straight-shelled 

 representatives of the group (the Orthoceratites), was not merely an embryonal, but an important 

 organ in the adult. 



The formation of the septa is undoubtedly due to the constant onward growth of the shell season 

 by season, and in the female to the periodic development of the ova within the ovary of the parent, 

 producing, when discharged from the shell, a corresponding reduction in the size of the soft parts of 

 the animal, and necessitating an equal reduction in the space of the body-chamber. In youth 

 these septa represent periods of rest in shell-growth, in middle life periods of fertility, in age 

 reduction of the shell to suit the reduced size of the Mollusc. In this respect the septa in 

 Nautilus agree with those found in other Mollusca. "The line of attachment of both the 

 muscles and the cincture progressively advances with the growth of the animal. A certain 

 portion of the funclus of the shell thus becomes vacated, and the Nautilus commences the formation 

 of a new plate for the suppoi't of the part of the body which has been withdrawn from the 

 vacated shell. The formation of the plate proceeds from the circumference to the centre, and there 

 meeting the conical process of the mantle, which retains its primitive attachment, the calcification 

 is continued backwards for a short distance around the process which now forms the 

 commencement of the membranous siphon, and requires the partial protection of the calcareous tube. 

 An air-tight chamber is thus formed, traversed by the siphon, which perforates its anterior wall or 

 septum. By a repetition of the same process a second chamber is formed, included within two 

 perforated septa ; and similar, but wider, partitions continue to be added concurrently with the 

 formation of the new layers, which extend and expand the mouth of the shell, until the animal 



