. Elements of conchology / Prepared for the use of schools and colleges. Mollusks. the warm parts of both continents : the second are subdivided into the genera of Planorbis, Limn^a, Auricula, &c. 22. The mollusks of the genus Planorbis have a thin shell rolled upon the same plane, which is consequently discoid, and the whorls very gradually increase in size {jig. 29). Their tentacles are long and filiform, and the eyes are placed at the internal side of their base. These gasteropods feed on vege- table substances, and in- habit stagnant waters; m winter they bury them- V / "^^ ^^^ se
. Elements of conchology / Prepared for the use of schools and colleges. Mollusks. the warm parts of both continents : the second are subdivided into the genera of Planorbis, Limn^a, Auricula, &c. 22. The mollusks of the genus Planorbis have a thin shell rolled upon the same plane, which is consequently discoid, and the whorls very gradually increase in size {jig. 29). Their tentacles are long and filiform, and the eyes are placed at the internal side of their base. These gasteropods feed on vege- table substances, and in- habit stagnant waters; m winter they bury them- V / "^^ ^^^ selves in the mud, and ^^—^ lie torpid. Fig. 29.—PLANORBIS. 23. The Limn^a {Jig, 18, page 33) have the same habits, and are almost always found in the same localities as the planorbis, from which they are readily distinguished by the form of their shell, the spire of which is oblong; their tenta- cles are thick and triangular. In the laying season, they are often found in great numbers joined together, so as to form a sort of chaplet. They swim on the back with the ventral disk extend- ed on the surface of the water, and in winter they become torpid like the planorbis. 24. We give the name of Physa to small mollusks found in foun- tains, the shell of which {fig. 30) is similar to that of the limnsea, but Fig. 30. PHYSA. very thin, without a thick lip, and without a fold on the columella. The animal, when it crawls or swims, covers its shell with the two lobes of its mantle. In other respects it closely re- sembles the limnsea. 25. The Auricula differ from the pre- ceding by having la-rge oblique grooves upon the columella of their shell, which is oval or oblong {fig, 31). The name is derived. Fig. 31. AURICULA. 22. What are the characters of the genus Planorbis ? Upon what do ani- mals of this genus feed ? Where are they found ? 23. How is the genus Limnse'a distinguished from the genus Planorbis ] What are the habits of this genus ? 24. What are the characters of the genus P
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Keywords: ., bookcentury1800, bookdecade1840, booksubjectmollusks, bookyear184
