﻿14 BRITISH FOSSIL CEPHALOPODA. 



(5) the lingual and (6) the oral. From the front border of the front ventral cord 

 (7) the tentacular and (8) the labial ; and from the hinder border of the hinder 

 ventral cord (10) the motor and (11) the visceral. The tentacular nerves are 

 numerous, as each tentacle has its own independent nerve. So, too, the labial : the 

 nerves to each tentacle of the external labial processes are distinct, but the inner 

 labial process of the female has a single nerve which subdivides only, after having 

 formed a ganglion. The nerves to the funnel (9) rise nearer the middle line: the 

 motor nerves pass without forming any ganglia direct into the great shell-muscles : 

 the visceral nerves rise near the middle line, pass up on each side of the vena cava, 

 give off a twig to the branchiEe and form a ganglion, which may perhaps be called 

 the " ganglion stellatum." 



The eye stands out on a pedicle which projects from the body, in front of the 

 sides of the funnel, near the dorsal side ; the hood in front rises up to cover one 

 half of the eye, and the mantle behind nearly covers the other half, leaving a small 

 interval, when either is slightly withdrawn, through which the pupil peeps. In 

 shape the eye is like a triangular pyramid, rapidly enlarging at the distal end ; the 

 surface is flat, coloured, and rather warty ; it is provided on the front and ventral 

 two-thirds with a free fold, from which another fold is continued to the minute 

 aperture, which is less than one-eighth the diameter of the eye. The interior 

 of the eye is one of the most remarkable in nature, from the absence of almost 

 all the structures which usually characterise that organ. The nerve, arising from 

 near the end of the upper commissure, expands after entering the pedicle into 

 a large mass at the base of the eye, where it divides and subdivides to form the 

 sentient layer. Externally the eye has a strong fibrous coat — the sclerotic ; within 

 this is a yellow layer of cellular tissue ; on the concave surface of this the nerves are 

 spread out to form the retina, which is overlaid by a layer of black pigment. The 

 nerves, however, pass into this layer to the ends of a number of rod-like cells which 

 stand vertically upon and in the pigment layer. These structures form the internal 

 lining of the cavity of the eye, in which there is nothing more. The pupil therefore 

 must simply act like a pinhole in a card, to produce an image of the exterior objects 

 upon the retina behind. 1 



On either side of the eye, in front and behind, is a tentacle distinct from those 

 already described, and obviously in connection with the organ of vision. These 

 have a similar structure to the others — except that they are more incised, and so 

 consist of a number of plates connected by a subcentral stem. They are supplied 

 by nerves directly from the commissure, and are probably altogether sentient in 

 function. * 



1 The histology of the eye of the Nautilus is admirably figured and described by Hensen, 

 Zeitschrift fur Wiss. Zool. vol. xv. 1865. See also Keferstein in Bronn's 'Classen und Ordnnngen,' 

 Band iii. 



