"_Acera_ and the following genus have each but one species, _Bulla carnosa_ and _B. aperta_ of Linnæus. The original genus _Bulla_ was composed of an assemblage of shells of various characters, having little resemblance except in their gibbous form. _Bulla naucum_ is an example of the genus of Lamarck's system; so is _B. lignaria_. (See Frontispiece.) _B. ampulla_ and _B. striata_ are common in collections. * * * * * [Sidenote: APLYSIA. DOLABELLA.] "_Aplysiana_ is a small family, containing _Aplysia_, or Sea-hare. _Dolabella._ "One species of _Aplysia_ is found on the Devonshire coast: the name Sea-hare marks the singularity of the two tentacula, which resemble the ears of the hare. The body is folded up in a loose skin, or mantle: upon the middle of the back it carries a circular shield, thin, transparent, and yellowish, in which it resembles the slug. These animals swim with ease. "_Dolabella_ resembles the _Aplysia_ in some degree; the genus is foreign, and one species is known to inhabit the bays of the Isle of France, where it covers itself with a portion of mud." "I cannot understand why animals related to the slugs should find a place here," said Charles. "Have not slugs the characteristics of the _Mollusca_ class?" asked his father. "And are you quite sure that they are without a shell? [Sidenote: ONCHIDIUM. LIMAX.] "Our next family, the _Limacina_, has _Onchidium._ _Parmacella._ _Limax_, slug. _Testacella._ _Vitrina._ "_Onchidium_ is a genus from the shores of the Indian seas. The animals have a shield: they live near the sea, and some are known to swim, often coming to the surface to breathe the air. "_Parmacella_ was found by an English traveller in Mesopotamia. It has a shell covered by a shield. But you have not answered my question respecting the _Limax_, or slug." "I do not recollect," replied Charles; "yet how often we see slugs!" "If I may be allowed to answer," said Lucy, "I think that the slug has what I now understand to be a _shield_. I have often watched the animal contract itself, and seen a broad, flat piece upon the back, which I thought was a kind of shelter for it." "The _Limax_, or slug," continued Mr. Elliot, "is, in fact, provided with a coriaceous escutcheon, or shield, beneath which the animal partly conceals itself. The _Limax agrestis_, or spinning-slug, has the power of suspending itself by a kind of thread, formed of the viscid substance that covers the body. [Sidenote: TESTACELLUS. COLIMACEA.] "_Testacellus_ is a very interesting genus, lately found in England: the animal has a resemblance to the common slug: it carries the shield on the hinder part of the body. "_Testacellus scutellum_ feeds on earth-worms, and can so much lengthen the body that it follows them under-ground. "Our next order will show great alterations in the very large genus _Helix_. I shall name to you those of Lamarck.