23G 
VISUAL ORGANS — EXERCISE AND POWER. 
Ill the Styloiiiiiiatophora the eyes tlioiigh in tlie adult bume 
obliciuely at the extremity of tlie iipjier or dorsal tentacles, are lateral 
during developnient, only act|iiiring the apical position at a later 
perioil, a situation wliich enables them to he readily protruded or 
invaginated within the tentacle for iirotection, hut they are only 
functional when the tentacle is fully protruded. In this invagina- 
tion or retraction the relative position of the crystalline lens and 
the ocular bulb are reversed, the retractor muscle causing the lens to 
descend the tentacular tube in advance of the body of the eye. The 
genus VertKjo, in which, though bitentaciilate, the eyes are borne at 
the tip of the tentacles, is also Stylommatophorous. 
In the Basonmiatoi)hora the eyes are sessile and placed at the 
internal base of the tentacles, and often contain a blood s])ace between 
the outer cornea and inner cornea or pellucida. They have generally 
veiy feeble ocular powers, for although Stiebel considers Lhnnwn, 
.'(fiKjiiiili.'! to be better endowed in this respect than the Helices, Lespes 
has shown that the allied genus Pl'()i(>rl>is only perceive objects when 
in very close proximity to their visual organs. 
In the Pelecypoda, although the cei)halic eyes are absent in the 
adult stage, they are retained by throughout the free- 
swimming larval stage, and are transitorily present in other species at 
an early stage of their development. In comi)ensation for the suh- 
sei|uent loss of these cephalic visual organs, })rimarily due to their 
enclosure within the valves of the .shell, numerous pallial eyes are in 
l)rocess of development along the mantle mai'gius, which are more or 
less acutely sensible to light and shade. 
All our land and freshwater mollusca are umpiestionahly myoitic : 
they do not perceive the ultra violet rays, but are especially adapted 
fur vision in dim creimscular light, as exposure to bright sunlight 
ai)pears to have a dazzling or l)linding effect upon them. Must 
(tastrojiods .shun the full noon-day glare, and seldom voluntarily 
leave their retreats except during twilight or total darkness. The 
a(puitic Pulmonates have very feeble ocular powers and cannot dis- 
criminate the form of objects even in immediate ];)roximity to their 
eyes, but the terrestrial species have a much keener perception, experi- 
ment demonstrating that in a dim light Helices perceive large and 
bulky objects at a distance of six centimetres, but in a bright light 
their power of vision is much impaired, and this distance is so greatly 
reduced that it may not even exceed four millimetres. Vk'ipdra 
