PATELLIDiE. MOLLUSCA. Ghitomid^e. 
adhesion, “ They adhere very lirmly by atmospheric 
pressure (fifteen pounds per square inch), and the diffi- 
culty of detaching them is increased by the form of tlie 
shell.” — {Woodward.') Dr. Johnston, on the strength 
of Reaumur’s experiments upon this subject, affirms 
that this cohesion entirely depends on a glue, or kind 
of paste, which, though invisible, produces a very con- 
siderable effect. Reaumur cut the animal from top to 
bottom in two halves, as it stood fixed perpendicularly 
upon the rock ; and he made other deep incisions in a 
horizontal direction, destroying in this manner all the 
muscular power of its base, and all supposable vacuity 
between it and the stone : but the adhesion continued 
as firm as before the experiment. Even the death of 
the animal does not destroy its cohesion. There seems 
to be some doubt as to how far these animals are sta- 
tionary on the rocks, or whether they have the power 
of moving to some distance and returning to their roost. 
Mr. Clark, in his “ Mollusca,” says, “ How some Patellae 
live is a mystery they are often fixed for months, 
perhaps years, on rocks, at altitudes where they can, 
rarely, if at all, be aspersed by the sea, and are debarred 
access to marine vegetables ; their recorded descents 
from high levels, and periodical exits from and returns 
to the identical hollows they have made, after feeding 
on algae, have almost a fabulous complexion ; zones of 
sand fifty yards wide often intervene between them and 
such food, and their exceeding slow locomotion is op- 
posed to such manoeuvres. Mr. Lukis of Guernse}', 
however, in a paper published in Loudon's Magazine, 
has apparently shown that these sluggish creatures 
have the power, and exercise it too, of moving to some 
distance from their resting-place and returning to the 
same favourite spot. The times for these excursions 
being during night, and when the rocks are covered by 
the tide, has led superficial observers to suppose that, 
because the same individuals are oftcm found in the 
same spots for a great length of time, months or even 
years, they therefore never leave these sites at all. Mr. 
Lukis appears from his paper to have marked indi- 
viduals, to avoid mistake, and has watched their roam- 
ings and followed the tracks left by them for several 
yards. These tracks are very peculiar, and when once 
seen are not to be mistaken. 
This Common limpet, of which we have spoken so 
much (see fig. annexed), is much used as an article of 
Fig. 226. 
Patella vulgata and animal. 
food. In the north of Ireland, Mr. Patterson gives a 
very interesting account of the use made by the poor 
of this, to them, valuable article of subsistence. Many 
tons weight are annually collected near the town of 
347 
Larne alone. In Guernsey, Mr. Lukis tells us, great 
quantities are daily consumed ; “ scarcely a cottage on 
the coast is seen, where a heap of empty shells does 
not form a prominent feature near the door.” Patella 
vulgata, he adds in a note, “ seems to have been used 
by the inhabitants of these islands from the earliest 
times, as appears from masses of its shells now found 
in ground which has lain waste and unturned for cen- 
turies.” 
This shell is much used by fishermen as bait. In 
the Berwickshire Club Transactions, Dr. Johnston in- 
forms us that, on the coast near Berwick, there were for 
many years nearly twelve million collected j^early for 
that purpose ; but they have decreased in number so 
much of late that they no longer repay the trouble of 
gathering them. 
Family — CLIITONIDH5 [Sea Wood-lice). 
In this family the shell consists of a series of eight 
transverse valves situated on the middle of the animal’s 
back, each valve being inserted into the mantle, and 
having a deep lateral notch on each side. The border 
of the mantle into which those valves are inserted, is 
of a coriaceous texture, and is either smooth and bare, 
or covered with minute scales, spines, or hairs. There 
are several anatomical peculiarities possessed by the 
animals of this family, of such a nature as to induce 
some naturalists to remove them from the Mollusca 
altogether. “A Chiton,” says Dr. Williams, “has a 
carapace like an isopod crustacean ; a dorsal vessel like 
an annelid; bilateral, symmetrical, reproductive viscera 
like an acephalous mollusc; a head and foot like a patel- 
loid gasteropod ; a posterior anus like the Fissure Hid ce, 
and branchim like those of the brachyurous Crustacea ! 
Such manifold affinities at once unite and sever this 
odd group from several most dissimilar classes.” As 
Cuvier, however, has shown, their gills are like those 
of the Patellidce, their foot is that of a true gasteropod, 
and in their lingual dentition they resemble the Cteno- 
hrancliiata. The hinder valve of the shell was at one 
time considered by Dr. Gray as homologous with the 
shell of the Patellidce, the other valves being only so 
many portions successively detached from it. The 
species of Ckitonidce are very numerous, more than two 
hundred having been described. They are world-wide 
in their distribution, occurring in all climates. Though 
by far tbe greater number are found on rocks at low 
water, they are occasionally taken by the dredge in 
from ten to twenty-five fathoms water ; and some of 
our small British species range as low as one hundred 
fathoms. “ In the tropics,” says Mr. A. Adams, “ the 
Chitons appear to be more vivacious than those found 
farther north. If turned over on their backs, they will 
gradually bend their calcareous jointed bodies in every 
direction, contracting and dilating their ventral disk 
until they assume their natural position. Their pro- 
gressive motion is scarcely perceptible, however ; the 
principal object, apparently being again to fix them- 
selves to the surface of the rocks which Nature has 
given them to inhabit. Their food consists of fuci and 
other algae with which the rocks and stones are covered.” 
In the West Indies there are several species which grow 
