GASTROPODS. 
397 
with a hole between the front edge and the apex ( Rimula , Puncturella). In 
Scutum the shell is less conical, and more elongate, and only very faintly sinuated 
at the front margin. It is white, and almost wholly concealed beneath the mantle. 
The holes and slits in these shells serve excretory purposes. The animals are 
remarkable for their two symmetrical gills, and certain points in their internal 
anatomy, which offer some resemblance to the bivalves. More than a hundred 
species of Fissurella are known from all seas, except the Arctic. The west coast 
of South America produces some of the handsomest kinds; but the giant of the 
genus, F. ( Lucapina ) crenulata inhabits the shores of California. 
SECTION DOCOGLOSSA. 
The second group of Scutibranchs comprises the families Acmceidce, Patellidce, 
and Lepetidce, all being limpet-like molluscs, with a peculiar form of dentition, 
as already described. The shells of these 
families are alike in construction, but the 
animals differ essentially in their respira¬ 
tory organs. In Acmcea there is a single 
branchial plume, or ctenidium, over the neck; 
in Patella the gills are arranged in almost a 
circle around the foot; and in Lepeta the gill 
is entirely wanting. In addition to these 
differences the radula affords further distin¬ 
guishing characters. Limpets are generally 
attached to stones and rocks, but when the 
tide is up they quit their resting-place in quest 
of food, which consists of various kinds of 
Algae. As the tide ebbs they creep home 
again to the spot which each appropriates as 
its own location. The common limpet is one 
of the molluscs eaten on certain parts of the 
British coasts. With regard to the tenacity 
with which the limpet holds on, experiments 
have been made showing that a force of 
sixty-two lbs., or one thousand nine hundred and eighty-four times its own 
weight, is required to detach it from the rock. 
UNDER SURFACE OF LIMPET. 
a, Foot; b, Mantle ; c, Gills ; d, Mouth ; 
e. Tentacles. 
EDGAR A. SMITH. 
