OF THE ACALEPHjE OF NORTH AMERICA. 245 



tissue, which may be an accumulation of sensitive cells ; for the nervous tissue, or rather 

 the circular nerve, which follows the marginal circular tube, enters on both sides of the 

 bulb (Fig. 2, 4), within its substance. And as this nerve itself consists simply of a 

 string of ovate cells, I have scarcely any doubt that the transparent cells distributed 

 among the pigment-cells of the bulb are sensitive cells, as well as those which are 

 arranged in a distinct cord around the inner margin of the circular chymiferous tube. 

 In the centre and on the outside of the bulb, there is a heap of larger pigment-cells 

 grouped together so closely as to form a distinct, dark speck, which may be considered 

 as an eye proper, or rather as the apparatus which absorbs the light, and transmits it to 

 the sensitive elements of the tissue underneath. That such an eye, if it be an eye, 

 cannot properly be compared to the eyes of higher animals is plain, from the fact that 

 the dark pigment is placed here between the liglit and the nervous mass, and therefore 

 rather intercepts the image than receives it. But it may act as a condenser of light, 

 and give rise to different sensations in the dark, and under the direct influence of light. 

 We may, therefore, consider such an organ as a rudimentary eye, perhaps incapable of 

 perceiving distinct images, as there is no transparent lens to refract the rays of light, and 

 combine them in a focus. But in the transition between such dark specks of pigment 

 matter connected with nervous masses, and regular eyes or ocelli, there are so many in- 

 termediate stages of structure in the animal kingdom, that it were unpliilosophical to 

 deny the connection between the function which such an apparatus performs and the 

 functions of ordinary eyes. 



The difference, however, is sufficient to warrant the introduction of another distinct ap- 

 pellation, and justifies us in designating such imperfect apparatus of vision as " eye-specks." 

 It has been said, that they might be apparatus adapted for the perception of heat rather 

 than light ; but when we reflect how slight is the change of temperature of the medium 

 in which these animals dwell, we are less inclined to take such a view of this organ, and 

 we would only acknowledge that the perception of the calorific rays of light without a 

 distinct perception of the colorific rays might be none the less a sort of vision, if we 

 reflect upon the intimate connection there is in nature between heat and light, and if 

 we further consider, that, in the series of animal structures, we pass from apparatus clear- 

 ly constructed as eyes to those in which these organs, becoming more and more numer- 

 ous, appear on the surface of the body, rather like parts of the system of their colora- 

 tion, and therefore also as parts of their adaptation to be more or less influenced by 

 different sorts of rays of light. When describing Hippocrene and Pleurobrachia, I shall 

 have an opportunity to show how little ground there is for considering these dark specks 

 as organs of hearing. 



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