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, t). In terrestrial 

 Pulmonates e.g. Helix 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 (Limax, Helix, etc.) 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 \T 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), Cydostoma, Vermetus, 



