Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/708

Rh 680 MOLLUSCA [CEPHALOPODA. Possibly the sense of taste resides in certain processes within the mouth of Nautilus and other Siphonopoda. Fio. 116. Cartilaginous skeleton of Siphonopoda (after Keferstein). A. Capito- pedal cartilage of Nautilus pompilius ; a points to the ridge which supports the pedal portion of the nerve-centre. B. Lateral view of the same, the large anterior processes are sunk in the muscular substance of the siphon. C. Cephalic cartilages of Sepia qfficinalis. D. Nuchal cartilage of Sepia ojfici- nalis. The otocysts of Nautilus were discovered by Macdonald (40). Each lies at the side of the head, ventral of the eye, resting on the capito-pedal cartilage, and supported by the large auditory nerve which arises from the pedal gan glion. It has the form of a small sac, 1 to 2 mm. in dia- meter, and contains whetstone - shaped crystals, such as are known to form the otoliths of other Mol- lusca. The otocysts of Dibranchiata are larger and deeply sunk in the cephalic cartilage. It has F IO. m. Minute structure of the cartilage of been shown bv Lan- Loligo (from Gegenbaur, after Furbringer). a, Kran Dy .an sim }e&amp;gt; t dividing, cells; c, canaliculi; d, an kester that they de- empty cartilage capsule with its pores ; e, canali- velop as open pits culi in section - (fig. 121, (5), (6), o), which gradually close up, the com munication with the exterior becoming narrowed into a fine canal, which is reflected over one end of the sac, and finally has its external opening obliterated. A single otolith only is found in all Dibranchiata. The eye of Nautilus is among the most interesting struc tures 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 pro jecting 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. 91, ). Owen very naturally thought that some membrane had covered this hole in life, and had been ruptured in the specimen studied by him. It, how ever, appears from the researches of Hensen (41) 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 refract ing lens and cornea we have actually here an arrangement for forming an image on the principle of &quot; the pin-hole camera.&quot; 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. 118, A). As in other Siphonopods (e.g., fig. 120, jRi, Re, ])), 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. 119) ; 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 Co.ep C Int- Fio. 118. Diagrams of sections of the eyes of Mollusca. A. Nautilus (and Patella). B. Gastropod (Limax or Helix). C. Dibranchiate Siphonopod (Oigopsid). Pal, eyelid (outermost fold) ; Co, cornea (second fold) ; Ir, iris (third fold) ; Infl, 2, 3, 4, different parts of the integument ; I, deep portion of the lens ; ?i, outer portion of the lens ; Co.ep, ciliary body ; It, retina ; N.op, optic nerve ; G.op, optic ganglion ; x, inner layer of the retina ; N.S, nervous stratum of the retina. (From Balfour, after Grenadier.) 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. Natural selection has had an altogether excep tional opportunity in the ancestors of Nautilus. 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. 118, 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. In the Dibranchiate division of the Siphonopoda the greatest elaboration of the dioptric apparatus of the eye is attained, so that we have in one sub-class the extremes of the two lines of development of the Molluscan eye, those two lines being the punctigerous and the lentigerous. The structure of the Dibranchiate s eye is shown in section in fig. 118, C, and in fig. 120, and its development in fig. 119 and fig. 123. The open sac which forms the retina of the young Dibranchiate closes up, and constitutes the posterior chamber of the eye, or primitive optic vesicle (fig. 123, A^oc). The