ß24 ^- ^- VVHITMAN, 



must be given in such a way as to tempt it to respond without 

 frightening it. The extreme sensitiveness of Clepsine, for example, to 

 the slightest tremor of the water about it, can be seen only under 

 well-chosen conditions. 



Let the animal be at rest and undisturbed by any jar; let 

 the point of a needle barely touch the surface of the water over it, 

 and the quick response will be a flattening out of the body, best seen 

 under a low magnification. 



The sensitiveness to light in Clepsine is so keen that the faint 

 shadow of the hand passed over the animal may — if the conditions 

 are favorable — arouse it to most energetic activity. The success 

 of the experiment may depend upon whether the leech is hungry or 

 not, and again upon whether the stimulus is one it is accustomed to 

 respond to under natural conditions. We have several species of 

 Clepsine that appear to depend upon turtles for their food. These 

 species, if hungry, respond to the shadow of the hand, as if it were 

 the shadow of a turtle. 



Other species, as C. sexoculata, which obtain their food from 

 fresh-water snails, although probably equally sensitive to light, do not 

 respond to this stimulus. The host they seek does not apprise them 

 of its presence by a shadow cast as it swims over them; and hence, 

 allowing that it is recognized, it only serves to warn them to keep 

 still in order to avoid danger. 



In this connection, a paragraph may be added from a lecture of 

 1889 („Biological Lectures" of the Marine Biological Laboratory of 

 Wood's Holl, 1890, p. 49—50): 



„The sensory impressions received by a visual organ differ so 

 radically from those received by a tactile organ, that is seems almost 

 incredible that cells devoted to one of these functions could ever serve 

 the other. Nevertheless, this marvelous transformation and change 

 of function have actually taken place, and the fact still admits of 

 demonstration in a very large group of Annelids. Sometimes 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 represented in veritable com- 

 pound sense-organs. The picture is a revelation that gives swift 

 wings to suggestion. If such is the path of evolution in one case, 



