CHAPTER V. 



UNIVALVES, 



LIKE all things else the mollusca are more interesting the more 

 we know of them. They are far from being the mere masses 

 of flesh or jelly they may seem at first sight. Their structure is 

 anything but simple ; their organs are well defined ; their senses are 

 in full complement. 



They can see. The Razor-shell will pop down into its hole at 

 your near approach ; the Oyster will snap its valves as the shadow 

 of your boat goes over it; and Cydostoma will shut itself in with 

 its operculum if you hold a stick within a foot of it. The old 

 rhyme of beating the snail is not so meaningless as it appears, 

 but the snail will retire into its shell instead of coming out of it if 

 the stick be held within its range of vision, which has been found 

 to be two inches, the Helicidse being among the most short-sighted 

 of their class. To say nothing of the great orbs, fifteen inches 

 across, of the giant cuttle, it is difficult to forget the range of gems 

 along the mantle edges of the Pecten, who requires a good look-out 

 as he drives himself along, hydraulically, by clapping his valves 

 together and squirting the water from each side of his hinge. 



They can hear. The Swan Mussel will shut its shell at the 

 sound of a whistle or of a creaking door, and Anomia will close up 

 as soon as you favour it with a musical note of a certain pitch. Yet 

 the ears are not external, and in only one family, the Nuculidse, has 

 a free communication been discovered between the otocyst (that is, 

 the organ of hearing), and the exterior. 



They can smell. All the whelks in the neighbourhood will gather 

 round a lobster pot ; hang a piece of meat over sand in which a 

 Nassa is buried, and he will come up in a hurry to see what he can 

 get ; a Helix will come a hundred yards after a strawberry, and 

 retreat fifty from a whiff of turpentine; an Arion has been seen 

 making for a bean-pod in a road, the pod has been picked up and 

 the Arion has stopped and gone round and round waving his 

 tentacles; the pod has been placed on the road again and the Arion 

 has made straight for it, and again it has been shifted, so that the 

 unfortunate slug has been led backwards and forwards at will. 



They can feel. Nothing is more noticeable than the varied forms 

 of their organs of touch the tentacles, palps, mantle-lobes, arms, 

 and crowns, which are found amongst them. They can taste, and 

 are particular in their choice of food, except in respect of the few 

 that are omnivorous. And some of them, when hungry, are un- 

 expectedly bold in their choice of prey, as Limnaa peregra in 

 feeding on minnows, and Limncea stagnalis in choosing newts and 



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