120 THE GASTROPODA 
extremities of the tentacles in the sub-genera Drillia and Clavatula. 
The ocular tubercle is better developed than the tentacle in the 
Strombidae (Fig. 75 #), and finally the tentacle may be aborted and 
the eye appear to be placed on 
its summit (Terebellum). It is 
really placed on its summit in 
Assiminea and in the adult ter- 
restrial Pulmonates or Stylom- 
matophora (Figs. 172, 177), 
but during the development of 
these forms it is some distance 
removed from it. In the basom- 
matophorous Pulmonates, and 
= in the Opisthobranchia the eye 
Rreeaae: is at the base of the tentacle, 
Axial section of the eye of Trochus wmbilicaris and in the latter group some- 
fyctustalie lens: AL, retina; 1H, eptle nerve; times at some distance from it 
and often buried beneath the 
integuments, especially in the Nudibranchia. As regards its struc- 
ture, the Gastropod eye typically consists of a retina or invagination 
of the tegumentary epithelium, in which sensory and pigment cells 
may be distinguished. The former are known as retinophora and 
are colourless ; their free extremities are much contracted, and their 
opposite extremities are continuous with prolongations of nerve- 
fibres. The latter, or retinulae, have expanded free extremities, and 
surround the retinophora. As these two kinds of cells arise by the 
differentiation of normal epithelial cells, they may not in all cases 
possess sharply defined characters, and may pass insensibly into 
one another: the colourless cells actually appear to be absent in 
the eyes of certain Opisthobranchia that are buried beneath the 
integuments. The visual organ is completed by accessory structures, 
of cuticular nature, secreted by the epithelium, and are more dis- 
tinct from one another in proportion as the eye is more highly 
specialised. These cuticular structures comprise the layer of rods 
and the refracting bodies properly so called. The layer of rods, or 
retinidia, caps the epithelial cells of the retina. These rods, little 
developed in the Aspidobranchia (Fig. 100, IV), attain their highest 
degree of specialisation in certain Rachiglossa (Strombidae) and in the 
Heteropoda (Fig. 101, B, VII). In the last named they are disposed 
in furrows perpendicular to the optic axis, an arrangement analogous 
to that found in another pelagic Gastropod, Gustropteron. ‘The 
refractive elements are the crystalline lens—a spheroidal body 
formed of concentric layers, which does not as a rule fill the cavity 
of the eye—and a less dense cuticular substance, known as the 
vitreous body, which surrounds the crystalline lens. In its most 
primitive condition the visual organ consists simply of an entirely 
