FRONT-GILLED GROUP 



3367 



anatomy of Pleurotomaria shows that it has close relationship with Haliotis. Like 

 Trochus, it is furnished with a horny spiral operculum. A magnificient specimen 

 of P. adamsoniana is exhibited in the shell gallery at the Natural History Museum. 

 Sdssurella is another genus of this family, containing a number of very minute shells 

 with a slit in the lip like Pleurotomaria; S. crispata being a pretty little mollusk 

 found (in Britain. The members of the so-called keyhole limpets (Fissurellidce) 

 mostly have shells like those of the limpets in form, but either perforated at the 

 apex, as in Fissurella, slit at the front margin (Emarginula}, or with a hole between 

 the front edge and the apex (Rinmla, 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 mollusks, 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 respiratory 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 distinguishing characteristics. 

 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 mollusks 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 pounds, or one thousand 



nine hundred and eighty-four times its own weight, is required to detach it from 

 the rock. 



UNDER SURFACE OF UMPET. 

 a. Foot ; *. Mantle ; c. Gills ; d. Mouth 

 e. Tentacles. 



