RETINAL PIGMENT OF PLANORBIS 



365 



DESCRIPTION OF THE EYE OF PLANORBIS 



Planorbis is a representative of the group of pulmonate gas- 

 teropods which is characterized by the possession of but one 

 pair of non-retractile tentacles, at the bases of which the eyes 

 are located. Although the tentacles are not retractile after 

 the manner of an introvert, they nevertheless are capable of 

 exhibiting a high degree of muscular shortening whereby their 

 surface becomes thrown into transverse ridges (fig. A, ta.). 



fbr.mus. 



oc. n.opt. 



Fig. A An axial section of both the eye and the tentacle of Phinorbis, show- 

 ing the position and mutual relation of these structures (X 60). fbr. mus., mus- 

 cle fibers; n. opt., optic nerve; oc, eye; ta., tentacle. 



The eye (fig. A, oc), which lies just bene<ath the surface 

 epithelium, has the shai:)e of a cone or pear whose base, cor- 

 responding to the corneal portion, points outward. The base 

 has a diameter of about 150 m, whereas the axial measurement 

 is 200 M- 



An axial section of the eye (fig. B) shows the following parts : 

 (1) optic capsule; (2) cornea; (3) retina; (4) optic nerve; (5) 

 lens ; (6) vitreous humor. 



The optic capsule (fig. 3, cps. opt.) is a thin, connective- 

 tissue sheath surrounding the eye, to which the retinal and cor- 

 neal elements are attached. It is continuous with the sheath 

 of the optic nerve. 



