A Sketch of the Structure and Development of the Eye of Clepsine. 623 



the parenchyma; and by the local aggregation of such cells the eye 

 was imagined to have arisen. 



The Hirudo eye, having a double nerve, presented itself as a fusion 

 of two of these local aggregations ! and the Nephelis eye appeared to 

 anticipate the conditions of such a fusion. 



This double innervation of the eye of Hirudo is easily understood, 

 if we take the Clepsine eye as our starting-point. The eye-nerve is 

 composed of two parts, 1) optic fibres terminating in the visual cells, 

 and 2) tactile fibres ending in the hair-cells. If the optic branch, 

 which is extremely short in Fig. D, should separate from the main 

 nerve just behind and below the eye, and enter the base of the eye, 

 instead of passing to the front, as it now does, we should have the 

 relations shown in Maier's fig. 1, tab. 37. The resemblance would 

 be essentially complete, if the tactile cells w^ere changed into visual 

 cells. It seems to me most probable that that is precisely what has 

 happened in the history of the Hirudo eye. This view accounts for 

 the fact that the upper anterior part of the eye is not covered by 

 pigment, and for the further fact that the visual cells in this region 

 often spread out rather loosely in front of the proper eye-cylinder 

 (vide Maier, p. 562). It is this surplus of cells that I should regard 

 as the homologue of the tactile elements in the Clepsine eye. Of 

 course I understand that this theory stands in need of verification 

 through the study of the development. I venture to predict that both 

 parts of the Hirudo eye will be found to have a common origin, just 

 as in Clepsine. 



Functions of the Eyes and of the Sensillae. 



The structure of the eye makes it evident that it can form no 

 image of external objects. Leeches, however, have a very keen „phot- 

 esthetic" sensibility. The presence of visual cells in all, or nearly 

 all, the metameric sensillae, and in many of the scattered, non-meta- 

 meric sensillae, especially those of the head, enables us to see how 

 extensively the surface of the body is provided with incipient eyes, 

 and thus to account for its general sensibility to light. 



One must not conclude from a failure to get any manifest reaction 

 in experiments with these animals, that their sensibility is dull. No 

 response to the stimulus applied may mean merely that the animal 

 is afraid to move, or that it has no motive for moving. The habits 

 of the animal must be taken into consideration, and the stimulus 



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