298 Introduction to Animal Morphology. 



behind. Tethys and other Opisthobranchs have a 

 velum moved by separate muscles. 



The eyes rarely absent (Chiton, Vermetus, Jan- 

 thina ?), are seated either on the summits (Helix, Te- 

 rebra), middle (Murex, Conus), or bases (Littorina, &c.) 

 of ommatophores, or else are sessile (Opisthobranchi- 

 ata). In Strombus the ommatophores bear flagelli- 

 form tentacles. The eyes are globular or oval, covered 

 with a cuticular cornea, a firm lamellar dermal sclero- 

 tic, lined with a pigmentary choroid layer, which is 

 continued in front as an iris. The bilaminar retina 

 consists of crystal cones, fibres, and granules, spread 

 on a vitreous humour, in front of which is embedded 

 a lens. 



eves are most complex in Ilcteropoda, enclosed in 

 irregular < ules, which freely communicate with the 



budy cavity. Tin -re is also a protractor and a retractor 

 muscle for the eveball in some (Pterotrachea), in which also 

 the retina ha> a vehicular layer under the rods. In some 

 ! l-ranchs the optic nerves are united at their origin. In 



Pulmonates a layer of pigment separates the two laminae of 

 the retina. In ( )pisthol>ranchs they are rarely in front of the 

 >f the tentacles (Aplysia, Dolabella), and are sometimes 

 far apart (Haminea). Neither tentacles nor eyes are ever 

 seated on the proboscis. The optic nerves often form a gan- 

 glion behind the eyeball. 



The sense of smell may be seated in the tentacles of Xu- 

 dibranchs, which are lamellar, screw-like (in Japus), or 

 branched. In some there are radiately folded ciliary sur 

 like the wheel organs of Pneumodermon. In Pulmonates this 

 sense is probably seated in a large lobate grey organ on each 

 side of the mouth, in contact with the skin, and only covered 

 by epithelium : within are many nucleated cells, and over 

 its surface is a shallow groove (organ of Semper}. The 

 ciliary organ of Heteropods is probably sensory, but of un- 



