772 ZOOLOGY SECT. 



immediately bounds the internal cavity of the eye is a layer . of 

 short narrow prismatic bodies the layer of rods (rd), while the 

 outermost is a layer of optic nerve-fibres connected with the nerve- 

 cells of the optic ganglion on the one hand, and with the other 

 elements of the retina on the other. 



In immediate contact with the eye, in addition to the optic 

 ganglion, is a large soft body of unknown function, the so-called 

 optic gland or white body, Bundles of muscular fibres bring about 

 limited movements of the eyeball in various directions. A pair of 

 integumentary folds of the character of eyelids are capable of 

 being drawn over the cornea. 



The statocyst (" otocyst ") (Fig. 656), though not of such compli- 

 cated structure as the eye, is very much more highly developed 

 than that of the Pelecypoda or Gastropoda. The two statocysts are 

 embedded in the cartilage of the posterior portion of the cranium 

 close to the pleuro-visceral ganglion. The cavities of the two 

 organs, which are about 3 mm. in diameter, are separated by a 

 median cartilaginous septum. The inner surface presents a 

 number of rounded and pear-shaped elevations, and is lined 

 with a flattened epithelium raised up on the posterior surface into 

 a ridge or crista dcustica and a macula acustica composed of 

 large cylindrical cells provided at their free extremities with short 

 cilia, and produced at their bases into processes continuous with 

 nerve-fibres derived from the statocyst-nerve. Enclosed in the 

 cavity of the statocj^st and attached to the macula is a large 



statolith (Fig. 669) of dense composition and 

 complicated form. The function of the stato- 

 cysts as organs of hearing is quite unproved ; it 

 has been shown by experiment that their re- 

 moval leads to a loss of the power of co- 

 ordinating the movements in such a way as to 

 maintain the equilibrium. 



Supposed to be olfactory in function is a 

 pair of ciliated pits, which open by slits on the 

 sm 'f ;lce behind each eye ; among the ciliated 

 highly inajvmtied. cells lining the pit are numerous narrow sensory 



cells connected at their bases with the fibres of 

 a nerve derived from a small ganglion situated close to the optic 

 ganglion. A small elevation (Fig. 660, yust), covered with papillae, 

 on the floor of the buccal cavity just in front of the odontophore, 

 is perhaps an organ of taste. 



The excretory organs or nephridia of Sepia (Figs. 670 and 

 671) are a pair of thin-walled sacs, which open into the mantle- 

 cavity by the conspicuous excretory apertures already described. 

 On either side is an aperture (ap. 1 ) placing the cavity of the sac in 

 communication with the pericardium, and the right and left sacs 

 communicate with one another anteriorly and posteriorly. From 



