UERMATOPTIC VISION. 
235 
power, the various species, according to their habits, exhibiting a 
more or less striking response to sudden illumination or shading, the 
reaction being more or less apparent in accordance with the intensity 
of the light or depth of the shadow, this perceptive power being due 
to the sensibility of the peripheral neuro-epithelial cells; the Rev. 
J. E. Tenison-Woods, how'ever, affirms the presence in the mollusca 
generally of a multiplicity of minute eyes, corresponding to the 
Aesthetes of the Isopleura, these organs being said to exist, not only 
on the mantle-lobes of the animal, but even upon the opercula and 
the outer surface of the shell itself, while in addition there are large 
isolated eyes within the shell substance. 
Such species as markedly react to light are distinguished as 
Phot-optic, those which are strikingly afi'ected by shade as Skioptic, 
while the species which perceptibly respond to both light and shade 
are known as Photoskioptic. 
According to Nagel, Helix pomatia and Helix hortensis, if left for 
a time undisturbed, are acutely skioptic, but Helix arhustorum is 
less markedly affected, especially if the animal be a darkly pigmented 
one, while the slugs generally are said to be only slightly responsive, 
although Mr. E. J. Lowe, who had charge of the “Himalaya 
Eclipse Expedition” of 1860, stationed near Santander in Spain, has 
recorded his observation there of the intense skioptic sensibility of 
Avion uter and also of Helix pisana, as shown by their almost 
immediate response to the advent of the gloom at 3 p.m., due to the 
obscuration of the sun’s disc, both .species being observed to be actively 
moving about as at dusk, even before the eclipse became total. 
Unio pictorum, Unio margaritifer , and other species are photo- 
skioptic, for, in addition to a perception of a sudden accession of 
light, they are also keenly responsive to sudden .shading, as is evidenced 
by the rapid withdrawal of the mantle and closure of the shell, the 
latter species immediately closing its shell if the sun becomes over- 
cast, the water muddied or even if a boat passes above at a scarcely 
perceptible speed. 
In the Streptoneures the eyes are elevated upon slightly contractile 
special eye stalks or ommatophores, external to and more or less 
closely fused with the tentacle, and not invaginahle. In some of the 
more archaic marine Rhipidoglossa the eye is not always closed by 
the cornea, but is merely a simple tegumentary invagination, lined 
with sensory and pigmented epithelium. 
