xii PHYLUM MOLLUSCA 721 



looking body of a spherical shape. The lens consists of two plano- 

 convex lenses in close apposition ; it is supported by an annular 

 process the ciliary process (cil. proc) projecting inwards from the 

 sclerotic. The lens with the ciliary process divides the cavity of 

 the eye into two cavities, a smaller outer the cavity of the aque- 

 ous humour containing water, and a larger inner, containing a 

 gelatinous substance the vitreous humour. Over the wall of this 

 inner chamber extends the retina (ret), the sensitive part of the eye, 

 in which the optic nerve-fibres derived from the optic ganglion 

 terminate. The retina is of somewhat complicated structure, 

 consisting of a number of layers : of these that which 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 otocyst (Fig. 619), though not of such complicated structure 

 as the eye, is very much more highly developed than that of the 

 Pelecypoda or Gastropoda. The two otocysts 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 is raised up into a 

 number of rounded and pear-shaped elevations. The surface is 

 lined with a flattened epithelium raised up on the posterior surface 

 into a ridge 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 otocyst nerve. 

 Enclosed in the cavity of the otocyst is a large 

 otolith (Fig. 632) of dense composition and 

 complicated form. The function of the otocysts 

 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 Fl( trat^ 

 to maintain the equilibrium. highly magnified. 



Supposed to be olfactory in function is a 



pair of ciliated pits, which open by slits on the surface behind 

 each eye; among the ciliated cells lining the pit are numerous 

 narrow sensory cells connected at their bases with the fibres of 



VOL. I 3 A 



