338 Gasteropoda. MOLLUSCA. Rostrifera. 
in their habits; many of them seem to prefer spots 
where they can be covered only at high water, and 
myriads of our common species may be seen, when 
young, 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. Peru- 
viana, 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 
some dry place. Though marine, they are found in 
the Baltic, within the influence of fresh water, in which 
situations they often become distorted. The Common 
Fig. 220. 
Littorina littorea— shell, operculum, and teeth of animal. 
Periwinkle {L. littorea) — fig. 220, and Plate 1, fig. 1 
(animal) — is oviparous, and is well known as aflbrding 
an article of food. In times of scarcit}'’, limpets and 
periwinkles are the daily fare of the poor inhabitants 
of several of the western isles of Scotland. An allied 
species, L. rudis, “is viviparous, and the young have 
a hard shell before their birth, in consequence of which 
the species is not eaten.” — {Woodward.) 
Family— LACUNIDiE. 
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 with 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 surface of the frond 
or stem, often forming holes through the former, and 
deep holes in the latter, into which many specimens 
often crowd themselves. They are also often found 
crowded together in the crevices among the roots of 
the plant.” — {Gray.) Loven observed that when the 
sea-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 — TRUNCATELLIDj®! {Looping snails.) 
The family of Looping snails have little interest 
as regards their shells. The animals, however, are 
interesting little creatures, from the manner in which 
they walk. They 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 the 
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 laminae (see page 299), twelve to fifteen in 
number, disposed in regular descending spiral series on 
the left side of the mantle cavity. 
Family— PLANAXIDJE. 
This family consists of shells bearing a resemblance 
to those of Littorina, but the 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 thin, horny, semipellucid, and conoidal, 
with an acute spire and an oval aperture, emargi- 
nate anteriorly. They are all very small, and are 
found living on the gulf-weed. The animals are 
remarkable for having the power of spinning a byssus 
of glutinous threads, by which thej' attach themselves 
to the stems of the fuci on which thej"^ 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 gulf-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 off 
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 check 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— EISSOID^. 
This family consists of numerous species of small, 
generally white, spiral, more or less turreted shells, 
with a simple ovate aperture, furnished with a very 
