460 THE ANIMAL KINGDOM. 



visceral nerves. Nautilus alone has a right and left osphradial papilla, 

 supplied from the visceral ganglia, between the bases of the gills on each 

 side. A ciliated olfactory pit lies behind the eye in Dibranchiata, a 

 triangular olfactory (?) papilla below the eye in Nautilus. The nerve in 

 both instances originates from the spot where the pedal and optic ganglia 

 unite. The eye is a prominent stalked cone in Nautilus, with a flattened 

 end, which is pierced by a small central hole. This hole leads into the 

 cup-shaped interior of the eye, which is lined by the retina. The latter is 

 therefore bathed with sea-water. The eye in Dibranchiata is sunk in the 

 head, except in Procalistes, where it is stalked. The retinal chamber is 

 closed ; its anterior surface is occupied by a biconvex lens divisible into a 

 smaller outer and a larger semi-globular internal part, the two separated 

 by a membrane. The two parts are secreted by the outer and inner layers 

 respectively of an epithelial body (= ciliary body), which surrounds their 

 margins. In front of the closed retinal chamber is an iris, supported by a 

 cartilage, and containing a sphincter muscle ; in front of the iris a trans- 

 parent cornea, which is perforated in the centre by an aperture in certain 

 Decapoda, the Oegopsidae, e. g. Ommastrephes. The Octopoda have a 

 sphincter-like eye-lid ; Sepia and some other Decapoda a horizontal ventral 

 eye-lid. The retina of Nautilus consists of a single layer of cells, each 

 ending in a visual rod, and pigmented round the base of the rod, and an 

 underlying layer of ganglion cells (?). It is regarded in Dibranchiata as 

 composed either of a single layer of cells, each furnished with a visual rod, 

 and an underlying layer of ganglion cells, a delicate limiting membrane 

 separating the two (Carriere) ; or as composed of a single layer of retinal 

 cells, forming an inner layer of rods with swollen bases, and an outer layer 

 of nucleated cell-bodies, the two divided by a limiting membrane, which is 

 derived from a layer of small cells, ' Limitans-zellen,' which form a 

 prominent zone at the outer edge of the retina. The visual rods in this 

 case consist of two rhabdomeres, and two to four rhabdomeres fuse into a 

 rhabdome (Grenacher) 1 . The eye of the Dibranchiata is inclosed in 

 cartilage, which is pierced by the nerves. These come from a large optic 

 ganglion connected by a short nerve to the cerebral ganglion. A cellular 

 ' white ' body lies at the margin of the optic ganglion anteriorly. The oto- 

 cysts are a pair of vesicles imbedded in Dibranchiata in a cavity of the 

 cephalic cartilage, and attached to the pedal ganglion by a nerve, which is 



1 Patten regards the ' retinal cells ' as retinophorae, as they possess double rods and an axial 

 nerve-fibre. The 'Limitans-zellen ' are ganglionic cells, and are modified retinulae homologous with 

 the retinulae of Gastropod ommatidia, and with the inner layer especially of ganglionic cells in the 

 eye of Pecten. The closer juxtaposition of the broad ends of four retinophoral rods, has misled 

 Grenacher into supposing that these are rhabdomes. A unique feature in the Cephalopod eye is the 

 presence of pigment in the retinophorae round the axial nerve-fibre. See Patten, Mitth. Zool. Stat. 

 Naples, vi. pp. 623-25, and for explanation of terms, the note, p. 452 of this book. 



