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Rh surfaces of the nerve-end cells nearer the lens instead of with the opposite end. The significance of this arrangement is not known, but it is important to note, as shown by V. Henson, S. J. Hickson and others, that in the bivalves Pecten and Spondylus, which also have eyes upon the mantle quite distinct from typical cephalic eyes, there is the same relationship as in Oncidiidae of the optic nerve to the retinal cells. In both Oncidiidae and Pecten the pallial eyes have probably been developed by the modification of tentacles, such as coexist in an unmodified form with the eyes. The Oncidiidae are, according to K. Semper, pursued as food by the leaping fish Periophthalmus, and the dorsal eyes are of especial value to them in aiding them to escape from this enemy.

Sub-order 1.—. Pulmonata with an external shell. The head bears a single pair of contractile but not invaginable tentacles, at the base of which are the eyes. Penis at some distance from the female aperture, except in Amphibola and Siphonaria. All have an osphradium, except the Auriculidae, which are terrestrial, and it is situated outside the pallial cavity in those forms in which water is not admitted into the lung. There is a veliger stage in development, but the velum is reduced.

Fam. 1.—Auriculidae. Terrestrial and usually littoral; genital duct monaulic, the penis being connected with the aperture by an open or closed groove; shell with a prominent spire, the internal partitions often absorbed and the aperture denticulated. Auricula. Cassidula. Alexia. Melampus. Carychium, terrestrial, British. Scarabus. Leuconia, British. Blauneria. Pedipes.

Fam. 2.—Otinidae. Shell with short spire, and wide oval aperture; tentacles short. Otina, British. Camptonyx, terrestrial.

Fam. 3.—Amphibolidae. Shell spirally coiled; head broad, without prominent tentacles; foot short, operculated; marine. Amphibola.

Fam. 4.—Siphonariidae. Visceral mass and shell conical; tentacles atrophied; head expanded; genital apertures contiguous; marine animals, with an aquatic pallial cavity containing secondary branchial laminae. Siphonaria.

Fam. 5.—Gadiniidae. Visceral mass and shell conical; head flattened; pallial cavity aquatic, but without a branchia; genital apertures separated. Gadinia.

Fam. 6.—Chilinidae. Shell ovoid, with short spire, wide aperture and folded columella; inferior pallial lobe thick; visceral commissure still twisted. Chilina.

Fam. 7.—Limnaeidae. Shell thin, dextral, with prominent spire and oval aperture; no inferior pallial lobe. Limnaea, British. Amphipeplea, British.

Fam. 8.—Pompholygidae. Shell dextral, hyperstrophic, animal sinistral. Pompholyx. Choanomphalus.

Fam. 9.—Planorbidae. Visceral mass and shell sinistral; inferior pallial lobe very prominent, and transformed into a branchia. Planorbis, British. Bulinus. Miratesta.

Fam. 10.—Ancylidae. Shell conical, not spiral; inferior pallial lobe transformed into a branchia. Ancylus, British. Latia. Grundlachia.

Fam. 11.—Physidae. Visceral mass and shell sinistrally coiled; shell thin, with narrow aperture; no inferior pallial lobe. Physa, British. Aplexa, British.

Sub-order 2.—. Pulmonata with two pairs of tentacles, except Janellidae and Vertigo; these tentacles are invaginable, and the eyes are borne on the summits of the posterior pair. Male and female genital apertures open into a common vestibule, except in Vaginulidae and Oncidiidae. Except in Oncidium, there is no longer a veliger stage in development.

Tribe 1.—. Jaw simple, without a superior appendage.

Fam. 1.—Selenitidae. Radula with elongated and pointed teeth, like those of the Agnatha; a jaw present. Plutonia. Trigonochlamys.

Fam. 2.—Zonitidae. Shell external, smooth, heliciform or flattened; radula with pointed marginal teeth. Zonites, British. Ariophanta. Orpiella. Vitrina. Helicarion.

Fam. 3.—Limacidae. Shell internal. Limax, British. Parmacella. Urocyclus. Parmarion. Amalia. Agriolimax. Mesolimax. Monochroma. Paralimax. Metalimax.

Fam. 4.—Philomycidae. No shell; mantle covers the whole surface of the body; radula with squarish teeth. Philomycus.

Fam. 5.—Ostracolethidae. Shell largely chitinous, not spiral, its calcareous apex projecting through a small hole in the mantle. Ostracolethe.

Fam. 6.—Arionidae. Shell internal, or absent; mantle restricted to the anterior and middle part of the body; radula with squarish teeth. Arion, British. Geomalacus. Ariolimax. Anadenus.

Fam. 7.—Helicidae. Shell with medium spire, external or partly covered by the mantle; genital aperture below the right posterior tentacle; genital apparatus generally provided with a dart-sac and multifid vesicles. Helix, British. Bulimus. Hemphillia. Berendtia. Cochlostyla. Rhodea.

Fam. 8.—Endodontidae. Shell external, spiral, generally ornamented with ribs; borders of aperture thin and not reflected; radula with square teeth; genital ducts without accessory organs. Endodonta. Punctum. Sphyradium. Laoma. Pyramidula.

Fam. 9.—Orthalicidae. Shell external, ovoid, the last whorl swollen, aperture oval with a simple border; radular teeth in oblique rows. Orthalicus.

Fam. 10.—Bulimulidae. Jaw formed of folds imbricated externally and meeting at an acute angle near the base. Bulimulus. Peltella. Amphibulimus.

Fam. 11.—Cylindrellidae. Shell turriculated, with numerous whorls, the last more or less detached. Cylindrella.

Fam. 12.—Pupidae. Shell external, with elongated spire and numerous whorls, aperture generally narrow; male genital duct without multifid vesicles. Pupa, British. Eucalodium. Vertigo, British. Buliminus, British. Clausilia, British. Balea. Zospeum. Megaspira. Strophia. Anostoma.

Fam. 13.—Stenogyridae. Shell elongated, with a more or less obtuse summit; aperture with a simple border. Achatina. Stenogyra. Ferussacia, British. Cionella. Caecilianella. Azeca. Opeas.

Fam. 14.—Helicteridae. Shell bulimoid, dextral or sinistral; radular teeth, expanded at their extremities and multicuspidate. Helicter. Tornatellina.

Tribe 2.— No jaws; teeth narrow and pointed; carnivorous.

Fam. 1.—Oleacinidae. Shell oval, elongated, with narrow aperture; neck very long; labial palps prominent. Oleacina (Glandina). Streptostyla.

Fam. 2.—Testacellidae. Shell globular or auriform, external or partly covered by the mantle. Streptaxis. Gibbulina. Aerope. Rhytida. Daudebardia. Testacella. Chlamydophorus. Schizoglossa.

Fam. 3.—Rathouisiidae. No shell, a carinated mantle covering the whole body; male and female apertures distant, the female near the anus. Rathouisia. Atopos.

Tribe 3.— Jaw with a well-developed dorsal appendage.

Fam. 1.—Succineidae. Anterior tentacles much reduced; male and female apertures contiguous but distinct; shell thin, spiral, with short spire. Succinea, British. Homalonyx. Hyalimax. Neohyalimax.

Fam. 2.—Janellidae. Limaciform, with internal rounded shell; mantle very small and triangular; pulmonary chamber with tracheae; no anterior tentacles. Janella. Aneitella. Aneitea. Triboniophorus.

Tribe 4.— Male and female apertures distant.

Fam. 1.—Vaginulidae. No shell; limaciform; terrestrial; female aperture on right side in middle of body; anus posterior. Vaginula.

Fam. 2.—Oncidiidae. No shell; limaciform; littoral; female aperture posterior, near anus; a reduced pulmonary cavity with a distinct aperture. Oncidium. Oncidiella, British. Peronia.

—L. Boutan, “La Cause principale de l’asymétrie des mollusques gastéropodes,” ''Arch. de zool. expér.'' (3), vii. (1899); A. Lang, “Versuch einer Erklärung der Asymmetrie der Gastropoder,” ''Vierteljahrsschr. naturforsch. Gesellschaft'', Zürich, 36 (1892); A. Robert, “Recherches sur le développement des Troques,” Arch. ''de zool. expér.'' (3), x. (1903); P. Pelseneer, “Report on the Pteropoda,” ''Zool. “Challenger” Expedit.'' pts. lviii., lxv., lxvi. (1887, 1888); P. Pelseneer, “Protobranches aériens et Pulmonés branchifères,” ''Arch. de biol.'' xiv. (1895); W. A. Herdman, “On the Structure and Functions of the Cerata or Dorsal Papillae in some Nudibranchiate Mollusca,” ''Quart. Journ. Mic. Sci.'' (1892); J. T. Cunningham, “On the Structure and Relations of the Kidney in Aplysia,” ''Mitt. Zool. Stat. Neapel'', iv. (1883); Böhmig, “Zur feineren Anatomie von Rhodope veranyi, Kölliker,” ''Zeitschr. f. wiss. Zool.'' vol. lvi. (1893).

—S. P. Woodward, Manual of the Mollusca (2nd ed., with appendix, London, 1869); E. Forbes and S. Hanley, History of British Mollusca (4 vols., London, 1853); Alder and Hancock, Monograph of British Nudibranchiate Mollusca (London, Roy. Society, 1845); P. Pelseneer, ''Mollusca. Treatise on Zool.'', edited by E. Ray Lankester, pt. v. (1906); E. Ray Lankester, “,” in 9th ed. of this Encyclopaedia, to which this article is much indebted.

 GASTROTRICHA, a small group of fairly uniform animals which live among Rotifers and Protozoa at the bottom of ponds and marshes, biding amongst the recesses of the algae and sphagnum and other fresh-water plants and eating organic débris and Infusoria. They are of minute size varying from one-sixtieth to one-three-hundredth of an inch, and they move by means of long cilia. Two ventral bands composed of regular transverse rows of cilia are usually found. The head bears some especially large cilia. The cuticle which covers the body is here and there raised into overlapping scales which may be prolonged into bristles. An enlarged, frontal scale may cover the head, and a row of scales separates the ventral ciliated areas from one 