The otocysts of Nautilus were discovered by J.D. Macdonald. Each lies at the side of the head, ventral to the eye, resting on the capito-pedal cartilage, and supported by the large auditory nerve which apparently arises from the pedal ganglion but originates in the cerebral. It has the form of a small sac, 1 to 2 mm. in diameter, and contains whetstone-shaped crystals, such as are known to form the otoliths of other Mollusca.
The eye of Nautilus is among the most interesting structures of that remarkable animal. No other animal which has the same bulk and general elaboration of organization has so simple an eye as that of Nautilus. When looked at from the surface no metallic lustre, no transparent coverings, are presented by it. It is simply a slightly projecting hemispherical box like a kettle-drum, half an inch in diameter, its surface looking like that of the surrounding integument, whilst in the middle of the drum-membrane is a minute hole (fig. 3, u). Sir R. Owen very naturally thought that some membrane had covered this hole in life, and had been ruptured in the specimen studied by him. It, however, appears from the researches of V. Hensen that the hole is a normal aperture leading into the globe of the eye, which is accordingly filled by sea-water during life. There is no dioptric apparatus in Nautilus, and in place of refracting lens and cornea we have actually here an arrangement for forming an image on the principle of “the pin-hole camera.” There is no other eye known in the whole animal kingdom which is so constructed. The wall of the eye-globe is tough, and the cavity is lined solely by the naked retina, which is bathed by sea-water on one surface and receives the fibres of the optic nerve on the other (see fig. 14, A). As in other Cephalopods (e.g. fig. 33, Ri, Re, p), the retina consists of two layers of cells, separated by a layer of dark pigment. The most interesting consideration connected with this eye of Nautilus is found when the further facts are noted—(1) that the elaborate lens-bearing eyes of Dibranchiata pass through a stage of development in which they have the same structure as the eye of Nautilus—namely, are open sacs (fig. 34); and (2) that amongst other Mollusca examples of cephalic eyes can be found which in the adult condition are, like the eye of Nautilus and the developing eye of Dibranchs, simple pits of the integument, the cells of which are surrounded by pigment and connected with the filaments of an optic nerve. Such is the structure of the eye of the limpet (Patella), and in such a simple eye we obtain the clearest demonstration of the fact that the retina of the Molluscan cephalic eye, like that of the Arthropod cephalic eye and unlike that of the vertebrate myelonic eye, is essentially a modified area of the general epiderm, and that the sensitiveness of its cells to the action of light and their relation to nerve-filaments is only a specialization and intensifying of a property common to the whole epiderm of the surface of the body. What, however, strikes us as especially remarkable is that the simple form of a pit, which in Patella serves to accumulate a secretion which acts as a refractive body, should in Nautilus be glorified and raised to the dignity of an efficient optical apparatus. In all other Mollusca, starting as we may suppose from the follicular or pit-like condition, the eye has proceeded to acquire the form of a closed sac, the cavity of the closed vesicle being then filled partially or completely by a refractive body (lens) secreted by its walls (fig. 14, B). This is the condition attained in most Gastropoda. It presents a striking contrast to the simple Arthropod eye, where, in consequence of the existence of a dense exterior cuticle, the eye does not form a vesicle, and the lens is always part of that cuticle.
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| Fig. 14.—Diagrams of Sections of the Eyes of Mollusca. | |
A, Nautilus (and Patella). B, Gastropod (Limax or Helix). C, Dibranchiate Cephalopod(Oigopsid). Pal, Eyelid (outermost fold). Co, Cornea (second fold). Ir, Iris (third fold). Int 1,2,3,4, Different parts of theintegument. l, Deep portion of the lens. | l1, Outer portion of the lens Co.ep, Ciliary body. R, Retina. N.op, Optic nerve. G.op, Optic ganglion. x, Inner layer of the retina. N.S., Nervous stratum of theretina. (From Balfour, afterGrenacher.) |
The development of Nautilus is still entirely unknown. Dr Arthur Willey, during his sojourn in the East Indies, made special efforts to obtain fertilized eggs, both by offering rewards to the native fishermen and collectors and by keeping the living adults in captivity, but without success.
Phylogeny and Classification.—As Nautilus is the only living genus of the Tetrabranchiata, our knowledge of all the rest is based upon the study of their fossil shells. A vast number of species of shell similar in structure to that of Nautilus are known, chiefly from Primary and Secondary formations. These are divided into two sub-orders by differences in the form and structure of the initial chamber. In the Nautiloidea this chamber has the form of an obtuse cone, on the apex of which is a slit-like mark or cicatrix, elongated dorso-ventrally and placed opposite to the blind end of the siphuncle, which indents the front wall of the initial chamber but does not enter its cavity. In the Ammonoidea, on the other hand, the initial chamber is inflated, and is spheroidal, oval or pyriform in shape, with no cicatrix, and separated from the first air-chamber by a constriction. The siphuncle also commences with a dilatation which deeply indents the front wall of the initial chamber, called the protoconch, but does not penetrate into its cavity. Munier-Chalmas has shown that the cavity of the protoconch is traversed by a tubular organ, the “prosiphon,” which does not communicate with the true siphuncle, the place of which it is supposed to take in the early life of the animal. It is generally held, as suggested by Alpheus Hyatt, that the initial chamber of the Nautiloidea corresponds not to the protoconch of the Ammonoids, but to the second chamber of the latter, and that there existed in the young Nautiloids a true initial chamber, a protoconch which was either uncalcified or deciduous. The shell of the living nautilus does not decide this question, as its early stages are unknown, and there is a little vacuity in the centre of the spirally coiled shell which may have been originally occupied by the true protoconch.
The septa in the Nautiloidea are generally concave towards the aperture of the shell, their curvature therefore directed backwards (fig. I); in the Ammonoidea, on the other hand, the convexity is usually towards the aperture, the curvature therefore directed forwards. The lines along which the edges of the septa are united to the shell are known as “sutures,” and these in the Nautiloidea are simply curved or slightly lobed, whereas in the Ammonoidea they are folded in various degrees of complexity; the projections of the suture towards the mouth of the shell are called saddles, those in the opposite direction lobes. The siphuncle in the Nautilus pierces the centres of the septa, and in fossil Nautiloids it is usually central or sub-central. In a few cases it is marginal, and in that case may be external, i.e. ventral, or internal, i.e. dorsal. In Ammonoids the siphuncle is always marginal, and usually external. Its walls in the living Nautilus are strengthened by the deposit of calcareous granules, and in some fossil forms the wall is completely calcified. But this proper calcified wall is quite distinct from calcareous tubes surrounding the siphuncle, which are developed from the septa. In the pearly nautilus each septum is prolonged backwards at the point where it is pierced by the siphuncle, forming a shelly tube somewhat like the neck of a bottle. In many fossil forms these septal necks are continued from the septum from which they arise to the next, so that the siphuncle is enclosed in a complete secondary calcareous tube. In the majority of Nautiloids the septal necks are directed backwards, and they are said to be retrosiphonate. In the majority of the Ammonoids the septal necks are continued forwards from the septa to which they belong, and such forms are termed prosiphonate.
The Tetrabranchiata were most abundant in the Palaeozoic and Mesozoic periods. The Nautiloidea are the most ancient, appearing first in the Upper Cambrian, the genera being most numerous in the Palaeozoic period, and comparatively few surviving into the Secondary. On the other hand, the Ammonoidea are scarce in Palaeozoic formations, being represented in deposits earlier than the Carboniferous only by comparatively simple types, such as Clymenia and Goniatites. In the Secondary period Ammonoids were very abundant, both in genera and species and in individuals, and with few local exceptions none are known to have survived even to the commencement of the Tertiary. In the widest sense the genus Nautilus has existed since the Ordovician (Silurian) period, but the oldest types are not properly to be placed in the same genus as the existing form. Even with this qualification the genus is very ancient, shells very similar to those of the living Nautilus being found in the Upper Cretaceous.
It has been maintained by some zoologists that the Ammonoidea were Dibranchiate, though it would not follow from this that the shell was, therefore, internal. They are, however, generally classed with the Tetrabranchiata, and the absence of all evidence of the possession of an ink-sac is in favour of this view. There can be little doubt that they gave rise to the Dibranchiata.
About 2500 fossil species are included in the Nautiloidea, but only a few species of the genus Nautilus survive. Some of the fossil forms are very large, the shell reaching a length of 2 metres, or 6 ft. 6 in. Of the Ammonoidea more than 5000 species have been described, and some of the coiled forms are 70 cm., or nearly 2 ft. 6 in. in diameter.
Associated with various forms of Ammonoids there have been found peculiar horny or calcified plates, sometimes contained within the body-chamber of the shell, sometimes wholly detached. The most typical form of these structures has been named aptychus. It consists of two bilaterally symmetrical halves, of somewhat semicircular shape, and attached to one another by their straight inner margins, like a pair of doors. In some cases the aptychus is thin and horny, but more often it is thick and calcified, in which case the principal layer has a peculiar cellular structure. The surface may be smooth or sculptured, and one side is usually marked by concentric lines of growth. Another type is similar, except that the two halves are united in the middle line; bodies of this character are called synaptychus; they occur in the body-chamber of species of Scaphites. Another form called anaptychus consists of a thin horny undivided plate which is concentrically striated. This is associated with species of Ammonites and Goniatites.