366 



MOLLUSCA. 



mantle (dialyneury). This arrangement is sometimes modified (more often on 

 the right side), in that the mantle-nerve of the pleural passes direct to the 

 ganglion of the visceral commissure of its own side (supra- or suhintestinal), 

 from which ganglion all the mantle-nerves of that side appear to be given off 

 (zygoneury, right side of Fig. 286). 



In some Strcptoncura there is, in addition to the stomatogastric commissure, 

 a labial commissure (Fig. 283, 3) given off from the cerebral ganglia and 

 passing veutrally to the oesophagus. 



Sense organs. Tactile organs are represented by the tentacles, the 

 edges of the lips, which are often folded (labial palps), and tentacular 



and lobe-like prolongations which are found here 



and there on the head, mantle, and foot (see 



these organs). 



The rhinophores are the organs which subserve 



the olfactory sense ; they are placed upon the 



cephalic tentacles, and on the posterior tentacles 



when there are two pairs. They are epithelial 



structures, and may be localised in a patch of 



high epithelium at the end of 



the tentacles, or in a pit, the 



Avails of which may be even 



folded (some Opisthobranchs). 



The olfactory nerve arises from 



the cerebral ganglion, some- 

 times in common with the 



optic, and may have a ganglion 



on its course. The olfactory 



sense is well developed in 



most Gastropods. 



The osphradium (Fig. 280) 



is also supposed to be an 



olfactory organ. It lies in the mantle-cavity close to the ctenidium 

 and consists of a special patch of epithelium generally columnar and 

 ciliated, and frequently placed upon a special ganglion (Fig. 284, 

 branchial ganglion, see above). It is innervated by a nerve from the 

 visceral commissure (supra-intestinal ganglion if it is present). In 

 some forms its surface is much folded, and it has the appearance of a 

 bipectinate ctenidium (Fig. 280), for which it was formerly mistaken 

 (e.g., Natica, Buccinum, Cypraea, Strombidae, etc.) ; in some 

 Eutliyneura it is invaginated into the ganglion, and has the form 

 of a pit. In the Zygobranchiate forms there are two osphradia 

 the right one supplied by the sub-intestinal, and the left by the 



FIG. 287. Lamellar 

 rhinophore of Eolis 

 coronata (from Per- 

 rier, after Alder 

 and Hancock). 



FIG. 288. Rhinophore of 

 Hermaea bifida (from 

 Perrier, after Alder 

 and Hancock). 



