VII 



MOLLUSGA—THE SENSORY ORGANS 



169 



numerous auditory colls, and in their midst, separated from the rest by 

 four supporting or isolating cells, one large central auditory cell. 

 On the larger remaining surface of the Avail of the otocyst, separated 

 by unditferentiated cells, are found flatter ciliated cells, which carry 

 very long cilia or setae, exhibiting peculiar movements. They some- 

 times lie flat along the inner wall of the vesicle, and at other times (it 

 is said in response to strong auditory stimuli) stand upright, projecting 

 towards the centre of the vesicle, and supporting the otolith. 



The auditory nerve, which enters the otocyst at a point exactly 

 opposite the central cell, at once radiates in the form of fibres over the 

 whole wall of the vesicle " as meridians radiate from the pole on a 

 globe," finally innervating the bases of the auditory cells. 



The two otocysts of the Cephalopoda are still more complicated ; 

 they lie in two spacious cavities of the cephalic cartilage. The sensory 

 epithelium is here found on a macula acustica and on a kind of ridge, 

 the crista acustica, which jjrojects inwards. Otoliths are only found 

 on the macula acustica. The auditory nerve divides into two branches, 

 one going to the macula, and the other to the crista acustica. Kolliker's 

 canal, above mentioned, which is internally ciliated and ends blindly, 

 runs out of the otocyst as the remains of the aperture of the original 

 invagination. 



Experiments made on Ceplialopods have shown that one of the 

 functions of the otocysts is to regulate the position of the animal 

 while swimming. 



C. Visual Organs. 



They are cup-shaped 



1. Optic Pits. 



These are the simplest form of visual organ 

 depressions of the body 

 epithelium, which at the 

 base of the cup. forms the 

 retina. The depression is 

 sometimes very shallow, at 

 other times deep, and like 

 a wide bottle with a short 

 narrow neck. The optic 

 nerve enters at the base of 

 the depression and spreads 

 out over it. The epithelial 

 wall or retina consists, ap- 

 parently in all Gastropoda, 

 of two kinds of long thread- 

 like cells: (1) clear cells 

 without pigment, and (2) 

 pigmented cells. Whether 

 either or possibly both of these kinds can be considered as retinal cells 



Fig. )4j.— Eye of Nautilus (after Hensen). 1, Optic 

 oavity (pit) ; -J, layers of rods ; 3, pigment layer ; 4, layer of 

 visual cells ; 5, layer of ganglion cells ; (J, branches of the 

 optic nerve. 



