. Animal life as affected by the natural conditions of existence. Animal ecology. ON MOLLUSCA. 211 bend up its large foot and withdraw it completely into the shell like other univalves; even when it is plunged at once alive into spirit, the foot remains rigid and almost straight. Thus an operculum placed on the hinder surface of the foot, as in Neritina, would be useless, since it cannot serve to close the opening and to protect the soft parts, if the foot cannot be withdrawn; but the Navicella, as has been said, need any such protection, since it clings more closely to the rock when
. Animal life as affected by the natural conditions of existence. Animal ecology. ON MOLLUSCA. 211 bend up its large foot and withdraw it completely into the shell like other univalves; even when it is plunged at once alive into spirit, the foot remains rigid and almost straight. Thus an operculum placed on the hinder surface of the foot, as in Neritina, would be useless, since it cannot serve to close the opening and to protect the soft parts, if the foot cannot be withdrawn; but the Navicella, as has been said, need any such protection, since it clings more closely to the rock when threatened with danger, and we might expect to a. Fig. 61.—Diagram of section through Nerilina and Navicella to show the position oi: the operculum in each genua, a, Mavkella (op, operculum), 6, operculum of ^^avi- celta ; c, Nerilina {op, operculum), d, operculum of Nerilina. find, on examination, that it had no operculum. In this, how- ever, we should be quite mistaken; in all the Navicellse, without exception, there is a true operculum placed, exactly as in all the Operculata, on the back or upper side of the foot. It agrees too in structure with that of Neritina, but it is much smaller than the mouth of the shell (fig. 61), and the little hook which server to attach it to the muscle of the foot, and. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Semper, C. (Carl), 1832-1893. New York, D. Appleton
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Keywords: ., bookcentury1800, bookdecade1880, bookpublishernewyo, bookyear1881
