50 MARINE BIOLOGICAL LABORATORY. 



almost incredible that cells devoted to one of these func- 

 tions could ever serve the other. Nevertheless, this 

 marvellous transformation and change of function have 

 actually taken place, and the fact still admits of ocular 

 demonstration in a very large group of annelids. Some- 

 times all the tactile cells are converted into visual cells ; 

 at other times only a part of the cells assume the new 

 function, while the rest continue to serve the old. The 

 result is that we have at one end of the series pure 

 visual organs, at the other end pure tactile organs, and 

 between the two extremes every grade of mixture repre- 

 sented in veritable compound sense-organs. The picture 

 is a revelation that gives swift wings to suggestion. If 

 such is the path of evolution in one case, the best ground 

 is given for suspecting that the same economy has been 

 practised elsewhere. The discovery of these facts in the 

 leeches, led naturally to the anticipation of a similar 

 origin for the eyes in other annelids and in those groups 

 that have had a common origin with the annelids, before 

 all the arthropods and vertebrates. The existence of 

 segmental sense-organs, as I have said, is well known in 

 other annelids than the leeches, and the origin of eyes 

 from them is fairly well indicated in many cases. It is 

 a most promising subject of investigation, which, like a 

 thousand others, still waits for the encouragement which 

 the wealth of this country will not long, I trust, refuse 

 to supply. 



The close relationship between the annelids and ar- 

 thropods rendered it probable that in the latter the eyes 

 were also derived from segmental sense-organs, and the 

 probability was strengthened by the arrangement of the 

 eyes in successive pairs, as in the larvae of many insects. 



