MOLLUSCA 



143 



I'ossilily (lie sense of taste resides in certain processes 

 within the mouth of Nautilus and other Siphonopoda. 



B 



Flo. llfi. Cartilaginous skeleton of Siphonopoda (after Keferstein). A. Capito- 

 pedal cartilage of Nautilus p&mpilius ; 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 officinalis. D. Nuchal cartilage of Sepia affici- 

 italis. 



The otocysts of Nautilus were discovered by Macclonald 

 (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 FIO. ll". Minute structure of the cartilage of 



been shown bv Lan- Loligo (from Gegenbaur, after Furbringer). a, 



simple, 6, dividing, cells : r, ranaliculi ; rf, an 



kester that they de- empty cartilage capsule witli its pores; c, cauali- 



velop as open pits cuU 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 "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 lathed by sea-water on one surface and 

 receives the fibres of the optic nerve on the other (see fig. 

 118, A). As in other Siphonopoda (<.</., tig. 120, AV, Jlc, 

 />), 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 



C 



Int* 



Coep 



Int 



Pal. 



Ir 



Int 3 



Jut 



Jf.Cif 



G.ep 



N.op 



Fro. 118. Diagrams of sections of the eyes of Mollusca. A. Nautilus (and 

 Patella). B. Gastropod (Limax or Helix). C. Dibranchiate Siphonopod 

 (Oigopsid). Pa/, eyelid (outermost fold) ; Co, cornea (second fold) ; Ir, iris 

 (third fold) ; /(!, 2, 3, 4, different parts of the integument ; I, deep portion 

 of the lens ; l\ outer portion of the lens ; Co.ep, ciliary body ; /;, retina ; 

 N.op, optic nerve ; G.op, optic ganglion ; ar, inner layer of the retina ; A r ..S, 

 nervous stratum of the retina. (From Balfour, after Grenacher.) 



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,poc). The 



