546 LOCK [Vol. XI. 



This view has been entertained by morphologists for upwards 

 of ten years, and has a firm foothold in philosophical discus- 

 sions. The evidence favoring such an interpretation has been 

 accumulating, and although still incomplete, the hypothesis 

 was never so well supported as at present. 



Professor Whitman's researches were among the first to 

 bring out this conception. Ten years ago he demonstrated in 

 leeches the genetic relation between eyes and tactile papillae, 

 and since that time it has no longer been a matter of pure 

 assumption to say that certain end-organs are modifications of 

 a common sensory basis. He shows that the sensory papillae 

 located on each body-ring in the leeches have a very interest- 

 ing relation. In the hinder part of the body the papillae are 

 purely tactile, but passing headward they become gradually 

 modified, and are at first mixed visual and tactile, and finally 

 the anterior ten pairs are purely visual. We have thus had 

 for upwards of ten years a well-authenticated case of the deri- 

 vation of sense-organs of a higher grade from those of a lower 

 order. 



In a more recent publication, " The Metamerism of Clep- 

 sine," 1892, Professor Whitman has added to the definiteness 

 of his earlier discoveries by observations on a new species, vis., 

 Clepsine hollensis. He shows by a comprehensive study of 

 the elements of the nervous system, the nerve distribution, and 

 the sensillae the complete homodynamy of all the segments. 



Regarding the relationship of the eyes to other metameric 

 sense-organs, he says : " One more peculiarity of this species 

 may be noticed. The median rows of sensillae are quite well 

 developed as eyes, at least on segments IV, V, and VI. The 

 visual elements in segment IV are quite numerous, and they 

 are placed in a pigment cup quite like that of the principal 

 eyes, only thinner and smaller, and directed backward instead 

 of forward. The sensilla of segment V is a little smaller, but 

 still presents the same general features. In segment VI the 

 organ is still eye-like, but its visual elements are fewer and the 

 pigment cup imperfectly represented by loose pigment. In 

 segment VII we find one or two visual cells and a little scat- 

 tered pigment. In the following segments we usually find the 



