120 THE GASTROPODA 



extremities of the tentacles in the sub-genera Drillia and Clavaiula. 

 The ocular tubercle is better developed than the tentacle in the 

 Strombidae (Fig. 75 t), 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 

 Flo 100 is at the base of the tentacle, 



Axial section of the eye of Trochu* wnbilicaris and in the latter group SOmC- 

 I, crystalline lens; II, retina; III, optic nerve; ti^pa of cnmp rlistflnp0 frnm it 



IV, retini.lian layer or rods. 



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 pigmont 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, Gastropteron. 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 



