. An introduction to zoology, with directions for practical work (invertebrates). 128 INTRODUCTION TO ZOOLOGY Structure of Fig. 81.—Loiigituilinal Section through the tip of one of the long tentacles of Hdix to show the structure of the eye. e, Epidermis ; L, lens ; j), pigmented layer; r, retina; oji, optic nerve. The colour of the body is a dark brown or grey, speckled with the milky-white spots which give to it its specific name, " aspersa," signifying "; The foot has a yellow band round it. The form of the body of this snail is very similar to that of L
. An introduction to zoology, with directions for practical work (invertebrates). 128 INTRODUCTION TO ZOOLOGY Structure of Fig. 81.—Loiigituilinal Section through the tip of one of the long tentacles of Hdix to show the structure of the eye. e, Epidermis ; L, lens ; j), pigmented layer; r, retina; oji, optic nerve. The colour of the body is a dark brown or grey, speckled with the milky-white spots which give to it its specific name, " aspersa," signifying "; The foot has a yellow band round it. The form of the body of this snail is very similar to that of Limnaea, although in Helix there are four tentacles on the head instead of two, and at the tip of each of the longer upper pair is a fairly complex eye, which can be readily seen as a little black dot. This eye contains a solid lens, behind this a pigmented layer, and then a layer of sensory cells in connection with the optic nerve (see Fig. 81). In spite, however, of the considerable complexity of the organ, the sense of sight does not seem at all keen. The tentacles are hollow, and can be retracted by means of a special muscle which is visible through the semi-transparent wall of the tentacle, running inside it from base to apex (see Fig. 82). When this muscle con- tracts, the tip of the tentacle, (;) with the eye, is drawn down inside the tentacle, and so (1, Long tentacle extended; «2, long tentacle the eye is protected. The withdrawn; ^, (^, short tentacles; m, ., ."^ - , , muscles; 71, nerves. positioii of the eye and optic nerves, and the muscles which cause by their contraction the turning " outside in". 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 Lulham, Rosalie Blanche Jermaine. London, Macmillan
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Keywords: ., bookcentury1900, bookdecade1910, bookpublisherlondo, bookyear1913
