138 



MOLLUSCS. 



feelers, as in the Prosobrancliiata ; or on the back. In 

 some cases they are deeply sunk, even into the brain. 



Fig. 92.— Perpendicular section through the eye-pit of a limpet (Patella) ; after 

 Carriere. 1, Epithelial cells ; 2, retina cells ■■, 3, vitreous body. 



The mussels are generally deficient in eyes; and 

 some which are, as larvae, provided with an eye, lose 

 their eyes when mature. 



Fig. 93.— Eye of Trochus magus (after Hilger).* Gl, Vitreous body ; No, nerve. 



In the limpet (Patella),* on the outer side of the 

 tentacles, where the eyes are situated in more highly 

 organized species, are certain spots, which may be 



* '*Fraisse. TJeber Molluskenaugeii," Zett. fur Wiss. Zool, 188L 

 t " Beit, zur Kennt. der Gastropodenaugen," Gegenbaur's Morph. 

 Jahrhuch, 1885. 



