48 UNIVALVES, 



sticklebacks ; and some are cannibalistic, or nearly so, as Helix 

 pisana or Helix ericetorum. 



They have the sense of locality. Allowing for his limited means 

 of locomotion, a snail is as good a homer as a pigeon. He will go 

 out every evening and be found at home again every morning ; and, 

 what is more, he can find food over a garden wall and return to tell 

 his mate and take her back with him to have a meal, and then escort 

 her home again ; and for months he has been known to live in the 

 same crevice, from which, in one case at least, he has been taught 

 to come out and show himself when spoken to. And it is not only 

 the land species that go foraging, even the limpet will go for a cruise 

 as soon as the rising tide covers it, and returns to its pit before the 

 ebb has left it dry again. 



Slow as a snail may be, he is not weak. He can drag vertically 

 nine times his own weight, and there is one experiment of Sandford's 

 in which a specimen weighing a third of an ounce dragged along a 

 smooth table twelve reels of cotton, a pair of scissors, a screw- 

 driver, a key, and a knife, all tied on one behind the other, the 

 weight of the load being seventeen ounces, or more than fifty times 

 the weight of the drawer, the proportion being the same as if a 

 twelve-stone man were to pull along 3 tons 15 cwt. 



The strength of a mollusc lies in its so-called foot, an organ which 

 has been described as a thickening of a portion of the integument, 

 modified to give different forms of motion. In some cases the motion 

 occurs only during infancy, and is very slight ; in others it lasts 

 through life, and is of even a violent character, as in the case of 

 the cockles, which move in a succession of long hops. Some, like 

 Natica, use the foot -as a sand plough ; some, as the Tectibranchs, 

 as a fin to swim with ; some, as Mya, as a spade to dig with ; some, 

 as Pholas, as a drill to bore with. Sometimes it is comparatively 

 large, as in the slugs; sometimes it is almost aborted, as in the 

 oysters ; but more or less it is always present, and it is the 

 characteristic organ of the mollusca. 



Attached to the foot in the gastropods is the operculum, the 

 plate with which when the animal withdraws into his shell he closes 

 the mouth. It is not always present and not always solitary ; some- 

 times in specimens of Buccinum undatum there are two or three 

 opercula. It is absent in all the British land shells, except 

 Cyclostoma and Acicula. It used to be considered as representing 

 the second valve in the bivalves, but it is not produced by the 

 mantle ; by others it was regarded as corresponding to the byssus, 

 that bunch of horny threads used as a means of attachment by the 

 mussels, etc., but it differs from it in not being due to a special 

 gland. It varies in composition, being of almost every intermediate 

 grade between horn and shell, and it is of all degrees of thickness 

 and of many shapes, ranging from a mere thin flake to what looks 

 like a well-formed discoidal shell. In the land shells it is repre- 

 sented by the epiphragm, which is a stopper of hardened mucus 

 secreted by the liver cells, and formed only during periods of 

 inactivity as a protection from the weather or the enterprising 

 enemy. 



The shell is secreted by the mantle, which is an expansion of the 

 integument on the upper side. Each layer of the shell was once a 



