Hickson and others, that in the bivalves Pecten and Spondylus, which also have eyes upon the mantle quite distinct from typical cephalic eyes, there is the same relationship as in Oncidiidae of the optic nerve to the retinal cells.
In both Oncidiidae and Pecten the pallial eyes have probably been developed by the modification of tentacles, such as coexist in an unmodified form with the eyes.
The Oncidiidae are, according to K.
Male and female genital apertures open into a common vestibule, except in Vaginulidae and Oncidiidae.
The families Limacidae, Arionidae and Oncidiidae of the same sub-order, include nearly all the slugs.
The Oncidiidae are entitled to the name "sea-slugs," as they are shell-less Pulmonates living on the seashore, though not actually in the sea.
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