MOLLUSCA 



197 



are light -sensitive, containing an ocellus composed of a deep retinal cup surrounded 

 by pigment lying beneath a lens, the whole organ being covered by a cornea. It 

 is to be remembered, however, that Crozier (1920) could find merely a general 

 photosensitivity in Chiton, most pronounced where ocelli are lacking. Among 

 SOLENOGASTKES, these organs are replaced by simple epithelial papillte. In the 

 SCAPHOPODA (" tusk-shells "), a small class of molluscs which burrow in the sand 

 {Dentalium, elephant's-tooth shell, etc.) the sensory organs are represented only 

 by statocysts. 



Most members of the large class of gastropods, the eyes of which 

 were studied at an early date by J. Miiller (1831), are provided with 

 ocelh of a relatively primitive kind often associated with the tentacles. 

 In the extremely passive limpet, Patella , the eyes at the base of the 

 tentacles are very elementary, being merely lepresented by simple 



Dentalium 



Fig. 185. — The Common Whelk or Buckie, 

 buccisum vsdatum. 

 Note the two simple eyes (e) at the base of 

 the tentacles, s, respiratory siphon ; o, oper- 

 culum ; /, foot (Thomson's Zoology, James 

 Ritchie ; Oxford University Press). 



Fig. 184. — The Limpet, 

 Patella vulgata 



Ventral surface. Note the 

 simple eyes (appearing as 

 black dots) at the base of tlie 

 2 tentacles. The star-shaped 

 median structure is the 

 mouth (Thomson's Zoology, 

 James Ritchie ; Oxford Uni- 

 versity Press). 



cupiilate depressions of sensory and pigmented cells (Figs. 97 and 184). 

 More usually, however, the eyes are vesicular in type. These are typified 

 in the two simple vesicular eyes of the grey slug, Limax, or the snail, 

 Helix (Fig. 110), perched on the tips of the two longer (and jjosterior) 

 tentacles (" horns '") and innervated from the cerebral ganglion 

 (Galati-Mosella, 1915) ; on exjDosure to light the tentacle is capable of 

 retraction like the finger of a glove so that the eye can be drawn within 

 it (Figs. 186 to 188). The common whelk, Bticcinum, has eyes of a 

 somewhat similar vesicular type situated near the base of the tentacles 

 (Fig. 185), as also has Murex. 



The most elaborate eye of this type, however, is seen in the spider-shell, 

 Pterocera lambis, a gastropod found in quantity on tropical reefs. According to 



Shell of Murex 



