138 



MOLLUSCS. 



feelers, as in the Prosobranchiata ; or on the back. Id 

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



€ 



m 



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

 Cai-iiere. 1, p;;pithel)al 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 ; jVc, nerve. 



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

 tentacles, where the eyes are situated in move highly 

 organized species, are certain spots, which may be 



• "Fraisse. Uelier Molhiskenaugeii," Zei't.fur Wi's. Zml, 1881. 

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

 Jahrhuch, 1885. 



