452 THE ANATOMY OF INVERTEBRATED ANIMALS. 



not adhering to its inner surface, in front, is the silvery tape- 

 turn, formed of two layers. These pass into one another at 

 the edges of the free prolongation of the tapetum, which forms 

 the iris. Longitudinal muscular fibres are interposed between 

 the two layers of the tapetum. Under the tapetum is a layer 

 of cartilage, which forms the inner capsule of the eye, extends 

 as far as the iris externally, and is perforated by the fibres of 

 the optic nerve on its inner side. The free edge of the inner 

 capsule gives attachment to a thick rim of connective tissue, 

 containing muscular fibres. This so-called ciliary body enters 

 the deep groove which surrounds the lens ; the latter is, in 

 fact, made up of layers of structureless membrane, which are 

 cuticular productions of the ciliary body. In shape, the lens 

 is elongated in the direction of the axis of the eye, so as to 

 be almost a cylinder with convex ends, and thus, with its deep 

 equatorial groove, into which the ciliary body fits, it has a 

 wonderful resemblance to a Coddington lens. The vitreous 

 humor is a transparent fluid. The retina lines the inner cap- 

 sule, and may be divided into an outer and an inner stratum, 

 separated by a pigment layer. The inner stratum is formed 

 of prismatic or cylindrical rods, the outer ends of which abut 

 upon the pigment, while their inner ends, turned toward the 

 cavity of the eye, are covered by a thick hyaloid membrane. 

 The outer stratum contains the plexus of the fibres of the 

 optic nerves, and numerous cells (ganglionic), supported by 

 connective tissue. The terminations of the nerves, therefore, 

 must traverse the pigment layer to reach the rods. 



It will be observed that the apparent resemblances between 

 the cephalopodous and the vertebrate eye are merely super- 

 ficial, and disappear on detailed comparison. 



In Nautilus, the eye has neither cornea, lens, nor vitreous 

 humor, but is a mere cup, lined by the retina. The aperture 

 for the admission of light is exceedingly small. 



The olfactory organs, the true nature of which was dis- 

 covered by Kolliker, 1 are sometimes pits, sometimes papillae 

 of the integument, situated behind or above the eyes. In the 

 Teuthidm and Sepiadce, they are depressions above the eyes; 

 in the Oetopoda, they are either depressions or papillae (Ar- 

 gonauta and Tremoctopus) in the same position, but nearer 

 the anterior face of the body. In Nautilus, thev are elon- 

 gated, tentaculiform, and situated immediately behind the 

 eyes. 



1 " Entwickelunjzscreschiehte der Cephalopodcn," 1841, p. 107. 



