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, PateUidce, 

 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 Acmwa 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 

 Alga3. 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 Ibs., or one thousand nine hundred and eighty-four times its own 



weight, is required to detach it from the rock. 



EDGAR A, SMITH. 



UNDER SURFACE OF LIMPET. 



a, Foot ; b, Mantle ; c, Gills ; d, Mouth ; 

 e, Tentacles. 



