PHYLUM MOLLUSCA 



757 



fused together in the middle line : on each side the enlarged 

 sheaths of a second pair of tentacles are closely applied to, though 

 not completely coalescent with, the hood, being separated from the 

 latter by a narrow groove. The hood, with these two enlarged 

 sheaths, is covered with a thickened tuberculated skin, and acts 



sh- 



w.f - 



FIG. 685. -Nautilus macromphalus, adhering to the substratum in a vertical position by 

 means of its tentacles, e. eye ; h. hood ; n. m. nuchal membrane detached from coil of 

 shell ; o. t. ophthalmic tentacles ; sh. shell ; w. f. wing of funnel. (After Willey.) 



after the manner of an operculum for protecting the tentacles 

 and other soft parts about the head. Altogether there are forty- 

 two tentacles of the outer series, including four ophthalmic tentacles, 

 one situated on the oral and another on the aboral side of each 

 eye. The latter (ophthalmic) differ from the rest in being highly 



