134 WORMS. 



. In most of the Annulata (worms), the eyes, so far as 

 they have yet been described, are very simple, and 

 probably in most cases not capable of giving more than 

 a mere impression of light. In some species the eye- 

 spot is merely a group of pigmented epithelial cells. 

 In many (Fig. 87) there is, besides the pigment, a 

 well-marked Icds. At the same time, it is probable 

 that in some cases this supposed simplicity is more 

 apparent than real. The dioptric part is often cellular, 

 consisting sometimes of one cell, sometimes of several. 

 They are generally, but not always, situated on the 

 head. The Polyophthalmians (Fig. 90), as already 

 mentioned, have a series along the sides of the body, 

 in pairs from the seventh to the eighteenth segments. 

 I agree with Carriere that there is no sufficient reason 

 for considering the supposed "eyes" of the leech as 

 organs for the perception of light, but other species 

 of the same group (Clepsine) possess well-marked, 

 though rudimentary eyes.* 



Certain leeches — for instance, Fiscicola respirans — in 

 addition to the pigmented spots on the head, have also 

 some on the posterior sucking disc. These somewhat 

 resemble the supposed organs of touch, but are larger, 

 and surroimded by pigment. There is no lens, but the 

 large cells are very transparent. It is not supposed 

 that they give any distinct image, or can do more than 

 distinguish light from darkness — as Leydig says, 

 " feel " the light. Still, I must confess that the deter- 

 mination of these curious organs as eyes seems to me 

 very doubtful. 



Fig. 88 represents the anterior extremity of a small 

 freshwater worm (Bohemilla). 



* Graber, " Morph. Unt. iiber die Augen der frei-lebenden Borsten- 

 wurnier," Arch.fur Mic. Anat,, 1880. 



