iv VERMES SENSORY ORGANS 231 



dermis with its cuticle, which form the outer cornea over the centre 

 of the protruding eye. The eye itself is a vesicle whose posterior 

 thicker wall forms the retina, while the anterior thinner wall is the 

 inner cornea. The elements of the retina are long cells standing 

 closely pressed together, in which three parts can be distinguished : (1) 

 towards the brain, the cell body with a nucleus ; (2) the rod, which is 

 directed towards the hollow of the bulb ; and (3), between these two, 

 a thin layer of pigment. Under the cornea lies the spherical lens. 

 The rest of the eye is filled with fluid. The retinal cells are continued 

 into nerve fibres, which soon enter the ganglion cells of the optic lobe 

 (ganglion opticum) ; the latter is connected with the brain by a mass 

 of nerve fibres. 



3. The Chcetognathan eye (Fig. 154) is spherical, 

 of the sphere lie 3 bi-convex lenses 



imbedded in pigment ; to the out- 

 side of each of these 3 lenses a 

 third part of the whole retina is Jsr^** 



-"-- ' 1 . 4Bfc-"*"*.X> L 5 ^^** ""~**>Sfcl ' .. 



applied in such a way that the three f; 



parts together form the wall of the [ 



sphere. The retina consists of cells ; | 



the portion of each of these cells * 



which is in contact with the lens f 



is rod -like, and the part which f| 



is turned outwards is the cell-body Pia i54.-Section through the eye of sag- 



with its nucleus. At the circum- itta hexaptera, after O. Hertwig. ep, Body 



ference of the sphere, each retinal ^eiium ; z, lens ; p, pigment ; s rods;, 



retinal cells. 



cell is continued as a nerve-fibre. 



All the nerve fibres unite in the nervus opticus (Fig. 151, p. 227). 

 The Chcetognathan eye may be considered to have come from 3 simple 

 fused ocelli. 



A comparison of the three eyes just described shows how greatly 

 the eyes of worms may vary in structure. 



4. The eyes of Hirudo (Figs. 155, 156) lie in the anterior rings 

 of the body and vary in number. They are cylindrical and stand at 

 right angles to the somewhat modified hypodermis with which they 

 are in contact. The optic nerve enters at the base, its fibres passing 

 into long sensory cells which lie in the axis of the eye. Around 

 the axis are arranged large clear cells, each containing a nucleus and 

 a refractive substance. The whole organ is imbedded in strongly 

 pigmented connective tissue. Our present knowledge of the structure 

 of these organs hardly justifies us in calling them eyes ; morphologically 

 they are transformed tactile organs. 



C. Olfactory Organs (Ciliated Organs). 



In many worms of the Nemertian and Chcetopodan divisions there 

 are found, at the anterior end of the body, 2 lateral strongly ciliated 



