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 r), 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. 



The eyes are most complex in Heteropoda, enclosed in 

 irregular eye capsules, which freely communicate with the 

 body cavity. There is also a protractor and a retractor 

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

 the retina has a vesicular layer under the rods. In some 

 Prosobranchs the optic nerves are united at their origin. In 

 Pulmonates a layer of pigment separates the two laminae of 

 the retina. In Opisthobranchs they are rarely in front of the 

 bases of 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 Nu- 

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

 branched. In some there are radiately folded ciliary surfaces, 

 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 Seviper). The 

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



