. An introduction to zoology, with directions for practical work (invertebrates). 112 INTRODUCTION TO ZOOLOGY chap. teeth in one row of the radula of Planoiiifi, a very common pond snail, is shown in Fig. 63. When the snail is feeding, the lips are retracted, and the cushion and rasp pushed forwards until the latter can be scraped against the surface on which the snail is feeding. If the snail is feeding on the Algae which have covered the sides of the tank, the passage of the tongue over the glass is indicated by the clean tracks left where the Algae have been scraped away. Naturally the teet
. An introduction to zoology, with directions for practical work (invertebrates). 112 INTRODUCTION TO ZOOLOGY chap. teeth in one row of the radula of Planoiiifi, a very common pond snail, is shown in Fig. 63. When the snail is feeding, the lips are retracted, and the cushion and rasp pushed forwards until the latter can be scraped against the surface on which the snail is feeding. If the snail is feeding on the Algae which have covered the sides of the tank, the passage of the tongue over the glass is indicated by the clean tracks left where the Algae have been scraped away. Naturally the teeth on the rasp rapidly become worn down by this scraping action, but just as rapidly are they replaced by the pushing forward of fresh teeth from behind. The formation and growth of the new teeth occur at the hinder end of the rasp, which is lying in the regenerative " tongue sac"; the whole structure, as it is renewed from behind,. Fig. 63.âRadula of Planorbis. A few of tlie teeth from one transverse row; c, central tooth of the row. gradually grows forward, and so a constant supply of teeth is forthcoming. The sense of sight, in spite of the presence of eyes, does not seem at all keen, the keenest senses being those of touch and of smell (the tactile and olfactory senses). The whole body seems sensitive to touch, parti- cularly the tentacles, which are also the seat of the special sensory cells thought to be olfactory in function.^ The sense of taste is probably located in the upper lip, which, in some forms, is curiously lobed. There is no proof that a purely auditory sense is present, apart from the tactile sense, which would be affected by any strong vibration due to a sound. The sensitiveness of water snails to various stimuli has not, however, been very fully worked out. 'â J. W. Taylor, Monograph of the Land awi Fre^i WC'ier Mollusca of the. British Isles, vol. i. pp. Please note that these images are extracted from scanned page images that may have
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