338 



GASTEP.OrnpA.- 



-3I0LLUSCA.- 



-RoSTniFERA. 



ill their haljits ; many of them seem to prefer spots 

 where they can be covered only at high water, and 

 myriads of onr common species may be seen, wlien 

 yoiing, clustered in hollows of rocks that are many feet 

 above the highest tides. Some of the species found 

 on the shores of South America are stated by M. 

 D'Orbigny to prefer dry land to sea-water. L. Pcru- 

 rhiua, he informs us, is abundant in the hollows of 

 rocks, so high above the highest water-mark that they 

 can only be wetted by the spray during tempests. If 

 placed in sea water they develope themselves, and 

 quickly search for means of escaping from it, and reach 

 f-ome dry place. Though marine, they are found in 

 tlie Baltic, within the influence of fresh water, in which 

 f itnations they often become distorted. The Conmion 



Fig. 22U. 



Llltorina Uttorca— Bhell, operculLim, and teeth of animal. 



Tcriwinkle {L. liUorea)—{is. 220, and Plate 1, fig, 1 

 (;inimal) — is oviparous, and is well known as affording 

 an article of food. In times of scarcity, limpets and 

 periwinkles are the daily fare of the poor inhabitants 

 of several of tlie western isles of Scotland. An allied 

 species, L. ritdis, " is viviparous, and the young have 

 a hard shell before their birth, in consequence of which 

 the species is not eaten." — {Woodwai-d.) 



Family— LACUNID.a:. 



This family, composed of only one genus, Lacuna, 

 have a turbinated shell, with a short spire, a half- ovate 

 mouth, and the pillar lip flattened, with a deep umbili- 

 cal groove. In one of the species, L. pallidula, the 

 animal is said to be furnished witli a long cylindrical 

 proboscis. None of the animals as yet examined 

 possess jaws, but all the known species are found to 

 feed on sea- weed. They eat " the surfixce of the frond 

 or stem, often forming holes through the former, and 

 deep holes in the latter, into which many specimens 

 ciflen crowd themselves. They are also often found 

 crowdfd together in the crevices among the roots of 

 the plant." — {Gray.) Lov^n observed that when the 

 eea-weed upon which they feed is of a brown colour, 

 the animals become green; but if red, they assume a 

 rosy tint. They are chiefly northern species, and are 

 natives both of Great Britain and North America. 



Family— TRUNCATELLID.a: {Looping snails.) 



The family of Looping snails have little inlercst 

 as regards their shells. The animals, however, are 

 interesting little creatures, from the manner in which 

 they walk. Tliey progress by means of their foot and 

 the fore part of their muzzle, fixing one as a point of 

 support, and then drawing the other after it, as tlie 

 caterpillars of some lepidopterous insects (the geometric 

 caterpillars) do ; hence the name of Looping snails. 



In the eight succeeding families, unlike the three 

 preceding, the respiratory organs or gills are composed 

 of a few lamince (see page 299), twelve to fifteen in 

 number, disposed in regular descending spiral series on 

 the left side of the mantle cavity. 



Family— PLAN A5ID^. 



This family consists of shells bearing a resemblance 

 to those of Liltoriiia, but tlie animals differ from them 

 in their gills, and in their mantle furnished with a pro- 

 duced siphon in front. 



Genus Litiopa. — This is the most interesting genus 

 in the family. The species are quite pelagic, and found 

 floating in the open ocean. The animals have the sides 

 of the foot and the operculigerous lobes furnished with 

 tentacular filaments, and they have a horny operculum. 

 The shell is tliin, horny, semipellucid, and conoidal, 

 with an acute spire and an oval aperture, emargi- 

 nate anteriorly. They are all very small, and arc 

 found living on the gidf-weed. The animals are 

 remarkable for having the power of spinning a byssus 

 of glutinous threads, by which they attach themselves 

 to the stems of the fuci on which they reside. Dr. 

 Johnston gives a very interesting description of the 

 habits of this little creature — " This is a small snail," 

 he says, " born amid the gidf-weed, where it is des- 

 tined to pass the whole of its life. The foot, though 

 rather narrow and short, is of the usual character, and 

 having no extra hold, the snail is apt to be swept ofl 

 its weed ; but the accident is provided against, for the 

 creature, like a spider, spins a thread of the viscous 

 fluid that exudes from the foot, to cheek its downward 

 fall and enable it to regain its pristine site. But sup- 

 pose the shock has severed their connection, or that 

 the Litiopa finds it necessary to remove, from a 

 deficiency of food, to a richer pasture, the thread is 

 still made available to recovery or removal. In its fall, 

 accidental or purposed, an air-bubble is emitted, pro- 

 bably from the branchial cavity, which rises slowly 

 through the water, and as the snail has enveloped it with 

 its slime, this is drawn out into threads as the bubble 

 ascends ; and now, having a buoy and ladder whereon 

 to climb to the surface, it waits suspended until that 

 bubble comes into contact with the weeds that every- 

 where float around." 



Family- PvISSOID^. 



This family consists of numerous species of smnll, 

 generally white, spiral, more or less tuiTCtcd shells, 

 with a simple ovate aperture, furnished with a very 



