50 THE WHENCE AND THE WHITHER OF MAN 



These are little sacks in the skin, lined with neuro- 

 epithelial cells and having in the middle a little con- 

 cretion of carbonate of lime hung on rather a stiffer 

 hair, like a clapper in a bell. Such organs serve in 

 higher animals as organs of hearing, for the sensory 

 hairs are set in vibration by the sound-waves. It is 

 quite as probable that they here serve as organs for 

 feeling the slightest vibrations in the surrounding 

 water, and thus giving warning of approaching food or 

 danger. The animal has also eyes, and these may be 

 very numerous. They are not able to form images of 

 external objects, but only of perceiving light and the 

 direction of its source. A little group of these eyes 

 lies directly over the brain, near the front end of the 

 body ; the others are distributed around the front or 

 nearly the whole margin of the body. 



The turbellaria, doubtless, have the sense of smell, 

 although we can discover no special olfactory organ. 

 This sense would seem to be as old as protoplasm itself. 



This distribution of the eyes around a large portion 

 of the margin, and certain other characteristics of the 

 adult structure and of the embryonic development, are 

 very interesting, as giving hints of the development 

 of the turbellaria from some radiate ancestor. The 

 mouth is in a most unfavorable position, in or near 

 the middle of the body, rarely at the front end, as the 

 animal has to swim over its food before it can grasp it. 

 The animal onlv slowly rids itself of old disadvanta- 



V J 



geous form and structure and adapts itself completely 

 to a higher mode of life. 



By far the most highly developed system in the 

 body is the reproductive. It is doubtful whether any 

 animal, except, perhaps, the mollusk, has as cornpli- 



