The Gastropoda Anisopleura are classified as follows (Ray Lankester):

I. Streptoneura (Prosobranchia Minus Placophord)

Visceral nerve-loop twisted; sexes separate.

(1) Zygobranchia

Both ctenidia persistent or aborted. A left rudimentary nephridium present in some instances. No accessory generative organs. The visceral dome relatively small; lying close upon the foot and co-extensive with its prolongation in an aboral direction. Marine.

Ctenidiobranchia, including Haliotidae and Fissurellidae; Phyllidiobranchia, including the Patellidae with secondarily developed gill-lamellae.

(2) Azygobranchia

Left ctenidium, with corresponding osphradium and right nephridium retained (see p. 479, and note). Osphradium often large and lamellate = parabranchia. Hypobranchial and adrectal gland often present. Shell usually large and spiral; operculum often present. Visceral dome large. Anus on the right side. Radular teeth vary much in form.

(A) Reptantia

Foot or mesopodium a creeping disc. Includes (1) Holo-chlamyda; with mantle edge entire; with mouth, as a rule, at the end of a non-introversible snout, i.e. rostriferous; vegetarian; marine, brackish, freshwater, or terrestrial, e. g. Trochus, Ianthina, Paludina, etc.: (2) Pneumonochlamyda, no ctenidium; pallial chamber, a lung sac; rostriferous; terrestrial, e. g. Cyclostoma: (3) Siphonochlamyda, a pallial siphon; shell always spiral; usually an operculum, horny, and lamelliform; rostriferous, or a proboscis; mostly carnivorous; marine, e. g. Strombus, Cypraea, Conus, Murex, Buccinum.

(B) Natantia (Heteropodd)

Pelagic; foot a swimming organ; carnivorous. Atlantacea, with large visceral dome and shell: Carinariacea, visceral dome greatly reduced; shell small, cap-like, hyaline; ctenidium projecting freely; mesopodium with a sucker, fin-like; metapodium pointed, forming posterior end of body: Ptero-tracheacea, no shell; visceral dome, an oval sac imbedded in the posterior dorsal region of body; mesopodial sucker absent in female.

II. Euthyneura

Visceral nerve loop not twisted, often very short; shell generally light, little calcified; often lost in adult; operculum often present in embryo but lost in adult; radular teeth usually fine and similar: anus not so far forwards as in Azygobranchia, may become median and posterior; left ctenidium and right nephridium only retained (see p. 479, and note); ctenidium and mantle-fold sometimes lost. Hermaphrodite.

(1) Opisthobranchia

Visceral dome always small; shell rarely present; a nauti-loid shell in the Veliger stage. Foot large in archaic forms, and mantle-fold projecting over right side. Anus then placed far back beyond mantle-edge, behind the ctenidium, the free end of which is turned backwards. Heart in front of base of ctenidium. Epipodia commonly present and often reflected over shell or dorsal surface. Marine.

(A) Palliata (Tectibranchid)

Mantle-fold present. Includes (i) Ctenidiobran-chia, a ctenidium present; shell, with rare exceptions, delicate, sometimes small or completely enclosed within the reflected mantle-edge; epipodia common, e. g. Bulla, Aplysia, Pleurobranchus: (ii) Phyllidiobranchia, ctenidium. atrophied, replaced by a lateral row of lamellae on each side within the pallial fold, e. g. Phyllidia.

(B) Non-Palliata (Dermatobranchia)

Mantle atrophied in adult; and shell lost; body slug-like, visceral dome and foot co-extensive; anus usually median and posterior. Includes (i) Pygobranchia, ctenidium (?) a circlet of pinnate processes round the anus; epipodial-fold may be large and simulate a mantle-fold; cerata sometimes present, e. g. Doris, Polycera) (ii) Ceratonota, ctenidium aborted; simple or branched cerata, into which liver processes usually pass; anus on right side, or dorsal and median: e. g. Tethys, Eolis; (iii) Haplomorpha, no ctenidium; no cerata; foot also sometimes lost; degenerate and small, e. g. Phyllirhoe, Elysia.

(2) Pulmonata

Ctenidium aborted; dorsal wall of branchial cavity formed by mantle, respiratory; osphradium on left side (Planorbis, Auricula) or right (Limnaeus); usually lost; visceral dome small and co-extensive with foot, e. g. Limax, Arion, or large and spirally twisted, e.g. Limnaeus, Helix. Includes (i) Basommatophora; eyes median at base of tentacles; male and female genital orifices separate but near one another; aquatic; e. g. Limnaeus, Planorbis, Ancylus, Auricula; (ii) Stylommatophora, eyes elevated on invaginable tentacles; shell either large or spirally coiled, or minute, or absent; edge of branchial fold of mantle always fused to neck, leaving a pulmonary aperture; mostly terrestrial, e. g. Helix, Pupa, Clausilia; Testacella, Limax, Arion; Onchidium.

See literature, pp. 109, 112, 118, 123, 283.

Haliotis, Wegmann, A. Z. Expt. (2), ii. 1884. Patella, Harvey Gibson, Trans. Roy. Soc. Edinburgh, xxxii. pt. 3, Session 1884-85; development of, Patten, Arb. Zool. Inst. Wien, vi. 1886. Anatomical points in Fissurella, Haliotis, Tro-chidae, Haller, marine Rhipidoglossa, M. J. ix. 1883. Vermetus, de Lacaze Duthiers, A. Sc. N. (4), xiii. i860. Pleurobranchus, Id. op. cit. (4), ix. 1859; cf. A. G. Bourne, Q. J. M. xxv. 1885. Non-palliate Opisthobranchia, Bergh, Challenger Reports, x. 1884 (with lit. referred to); Trinchese, Atti Acad. Lyncei (3), xi. 1880-81; cf. British Nudibranchiata, Alder and Hancock, Ray Soc. 1845. Onchidium, marine, slug-like Pulmonate, Bergh, Challenger Reports, x. 1884, Appendix, p. 126; and M. J. x. 1885. Structure and development of O. celticum, Joyeux-Laffuie, A. Z. Expt. x. 1882.

Glands of mantle margin in Aplysia, Blochmann, Z. W. Z. xxxviii. 1883.

Purple gland = hypobranchial gland of Purpura, de Lacaze Duthiers, A. Sc. N.

(4), xii. 1859. Sucker of Natantia = Heteropoda, Niemiec, Recueil Zool. Suisse, ii. 1885.

Nerve eminences of Fissure/la, etc, gustatory organs, Haller, M. J. ix. 1883, pp. 44, 73. Tactile bodies on tentacles, Flemming, A. M. A. xxiii. 1884.

Eyes of Onchidium, Semper, Reisen im Archipel. d. Philippinen, (2), iii. Erganzungsheft, 1877, and A. M. A. xiv. 1877.

Histology of nervous system, Haller, M. J. ix. 1883.

On formation of ctenidium, Osborn, Studies from Biological Laboratory Johns Hopkins University, iii. pt. i. 1884; of Fasciolaria, Id. ibid. pt. 3, 1886.

Nephridia; structure, in marine Rhipidoglossa, M. J. xi. 1886; of Patella, Cunningham, Q. J. M. xxiii. 1883; of Aplysia, Id. Mittheil. Zool. Stat. Naples, iv. 1883. See a note on function, Joliet, C. R. 97, 1883.

Two forms of spermatozoa, Von Brunn, A. M. A. xxiii. 1884; Z. A. vii. 1884.

Mimicry in Mollusca, Osborn, Science, vi. 1885.