THE GASTROPODA 117 
forms, such as the terrestrial Pulmonata, the majority of the naked 
Opisthobranchia, Cyclostrema, Xenophorus, and all the Gastropods 
without an osphradium, these ramifications issue from a rhinophoric 
ganglion situated at the extremity of the olfactory nerve. The 
olfactory end-cells are frequently localised in a tract of higher 
epithelium at the extremity of the tentacle, or in a furrow ex- 
cavated in the surface of the tentacle (Pyramidellidae, Fig. 137, te, 
Solarium), and in many Opisthobranchia the sensitive surface of 
this olfactory prominence or cavity is increased by the development 
of numerous parallel pleats or foliations (Fig. 163, ¢). In terrestrial 
Pulmonates—e.g. Helirv—the sense of smell does not extend for a 
greater distance than half a metre, and then only in the case of 
exceptional odours ; the most usual distance at which odours are 
recognised is from one to three centimetres, but certain carnivorous 
marine Streptoneura—for example, Nassa—are able to recognise 
odours at a distance of more than two metres. 
The Osphradia are the sensory organs of the pallial or respiratory 
cavity, and exist in diverse forms. There is a pair of osphradia 
in all the bictenidiate Aspidobranchia and in the Docoglossa: in all 
other Gastropods the osphradium is unpaired. It disappears only 
in some terrestrial Streptoneura (Helicinidae and Cyclophoridae), 
in the Pleurobranchidae, the Nudibranchia, and all the Stylommato- 
phorous or terrestrial Pulmonates ; nevertheless in all the stylom- 
matophora (Limaz, Helix, ete.) the osphradium is present during 
development and during the first few days after hatching. To sum 
up, then, the osphradium is absent in aerial species or in aquatic 
forms devoid of a respiratory cavity, and when it is absent a rhino- 
phoric ganglion is present. An osphradium consists of a specialised 
and usually elevated and ciliated region of the epithelium, in which 
there is an accumulation of sensory cells. In the ctenidiate Gastro- 
pods the organ is situated on the outer side of the ctenidium (Fig. 99, 
XVI). The most simple form of osphradium is seen in the Strepto- 
neura, in which it is not differentiated into a definite organ, but is 
merely a localisation of neuro-epithelial cells on the course of the 
branchial nerve along the two supporting margins of the ctenidia, 
as in the Fissurellidae, or on an osphradial nerve running along the 
support and formed by a differentiation of the branchial nerve, as 
in other Rhipidoglossa, or again on a ganglion placed on the 
extremity of this special nerve at the base of the ctenidium. In 
other Gastropods the osphradium becomes a distinct terminal organ at 
the base or at the left (external) side of the single ctenidium, athwart 
the current of water which supplies the latter organ. The osphradium 
may persist in this place after the disappearance of the ctenidium, 
but only in aquatic forms such as the Patellidae, Gymnosomata, 
and basommatophorous Pulmonates. In the most archaic Taenio- 
glossa, viz. Paludina, Littorina (Fig. 85, p.br), Cyclostoma, Vermetus, 
