Chapter XXXII: Section 2: GYMNOSOMATA.--Mantle and shell absent in the adult, fins not

connected by a lobe, no branchial chamber, head well developed, with two pairs of tentacles, eyes on the posterior pair; cerebral ganglia above the oesophagus; buccal cavity provided with a pair of protrusible “hook-sacs,” radula generally with 4 to 12 hooked laterals, central tooth triangular, jaw in one piece, composed of horny plates, no horny plates in stomach, anus on the right side.

The Gymnosomata are carnivorous, feeding on Thecosomata and other pelagic animals, being provided for this purpose with a formidable buccal armature of hook-sacs and suckers. The intestine, as usual in carnivorous groups, passes straight from the stomach to the anus; the fins are not attached to the head, but to the anterior part of the body. The larva has a straight shell, which disappears in the adult. About 21 species are known, belonging to 7 genera.

FAM. 1. _Pneumodermatidae._--Animal fusiform, fins rather small, head prominent, anterior part of buccal cavity protrusible, with suckers on the ventral side, hook-sacs well marked; branchia on right side, skin soft, pigmented. Genera: _Dexiobranchaea_, no posterior gill, hook-sacs short; _Spongiobranchaea_, posterior gill circular; _Pneumoderma_, gill tetraradiate, hook-sacs long.

FAM. 2. _Clionopsidae._--Body barrel-shaped, proboscis three times the length of the body, no buccal appendages; hook-sacs short, no lateral gill, posterior gill tetraradiate, skin not pigmented. _Clionopsis_ is the single genus.

FAM. 3. _Notobranchaeidae._--Body ovate, buccal appendages conical, no lateral gill, posterior gill with three radiating crests, skin pigmented. _Notobranchaea_ is the single genus.

FAM. 4. _Clionidae._--Body long, angulated behind, proboscis short, mouth with two or three pairs of appendages, no jaw, no gills.

_Clione limacina_ is so abundant in Arctic seas as at times to colour the surface for miles. Each of the cephalic appendages has about 60,000 minute pedicellated suckers.

FAM. 5. _Halopsychidae._--Body ovate, thick, rounded behind, no gill or proboscis, fins long, narrow, broadened at the ends, epidermis sub-cartilaginoid.

_Halopsyche_ (= _Eurybia_) has the power of withdrawing its head completely into a sort of pocket, which is closed by an anterior fold of the mantle. There are two long non-retractile buccal appendages.

=Order IV. Pulmonata=

Gasteropoda with two pairs of tentacles, visceral loop euthyneurous, ganglia concentrated round the oesophagus; breathing air by a pallial cavity formed by the union of the front edge of the mantle with the cervical region, sexes united, shell present or absent, no operculum[407] (except in _Amphibola_).

=Sub-order I.= =Basommatophora.=--Eyes generally at the base of the tentacles, which are not retractile, male and female genital orifices separate, radula (p. 235) multiseriate, shell always present, external. Fresh water or quasi-marine.

FAM. 1. _Auriculidae._--Breathing organ a pulmonary sac or true lung; shell spiral, conoidal, internal partitions usually absorbed, aperture more or less strongly toothed. Jurassic----. Genera: _Auricula_, _Carychium_, _Scarabus_, _Alexia_, _Tralia_, _Plecotrema_, _Cassidula_, _Melampus_, _Leuconia_, _Pedipes_ (Fig. 292).

FAM. 2. _Otinidae._--Shell auriform, spire very short. Genera: _Otina_, _Camptonyx_.--Recent only.

FAM. 3. _Amphibolidae._--A pulmonary sac on right side of neck, eyes almost pedunculate, shell turbinate, rudely sculptured, operculate.--Recent. Genus, _Amphibola_ (Fig. 293); subg. _Ampullarina_.

FAM. 4. _Limnaeidae._--Pulmonary sac protected by an external lobe; shell variable, fragile. Jurassic----. (i.) _Ancylinae_, shell more or less limpet-shaped. Genera: _Ancylus_, _Gundlachia_, _Latia_. (ii.) _Limnaeinae_, shell spiral. Genera: _Limnaea_, _Amphipeplea_, _Erinna_, _Lantzia_, _Pompholyx_, _Choanomphalus_ (with _Carinifex_). (iii.) _Planorbinae_, shell sinistral, spire flattened or elevated. Genera: _Planorbis_, _Isidora_ (= _Bulinus_).

FAM. 5. _Physidae._--Mantle more or less reflected over the shell (radula, Fig. 141C, p. 235); shell sinistral, lustrous. Jurassic----. Genera: _Physa_, _Aplecta_.

FAM. 6. _Chilinidae._--Lobe of pulmonary sac large, tentacles broad; shell ventricose, rather solid; columella plicate. Miocene----. Single genus, _Chilina_.

=Sub-order II.= =Stylommatophora.=--Two pairs of retractile tentacles (except in _Janella_), eyes at the tip of the upper pair, male and female orifices united (except in Vaginulidae and Onchidiidae), no distinct osphradium.

FAM. 1. _Testacellidae._--Animal carnivorous, slug-like or spirally coiled, no jaw (whence the name _Agnatha_, often given to this group), radula with usually few, large, sickle-shaped teeth (p. 232), shell variable, rarely absent, usually external. Cretaceous----. Principal genera: _Chlamydephorus_ (shell a simple plate, internal), _Apera_, _Testacella_ (slug-like, shell terminal), _Strebelia_, _Streptostyla_, _Glandina_, _Salasiella_, _Petenia_, _Pseudosubulina_, _Streptostele_, _Tomostele_, _Streptaxis_ (Fig. 203), _Gibbus_, _Ennea_, _Daudebardia_ (Fig. 193), _Schizoglossa_, _Guesteria_, _Aerope_, _Paryphanta_, _Rhytida_ (subg. _Diplomphalus_, _Elaea_ and _Rhenea_).

FAM. 2. _Selenitidae._--Shell internal, external, or absent; jaw present, radula Testacellidan, central tooth present. Tertiary----. Genera: _Selenites_, _Plutonia_, _Trigonochlamys_, _Pseudomilax_ (?), _Rathouisia_ (?).

FAM. 3. _Limacidae._--Shell present or absent, internal or external, spiral or not, tail often with a mucus pore, jaw (Fig. 107 A, p. 211) with projecting rostrum on cutting edge, radula with central tooth tricuspid, laterals bi- or uni-cuspid, marginals aculeate. Eocene----. Genera: _Otoconcha_, _Urocyclus_, _Mariaella_ (subg. _Tennentia_), _Parmarion_, _Helicarion_, _Cystopelta_, _Aspidelus_, _Estria_, _Vitrinopsis_ (subg. _Vitrinoidea_, _Parmella_), _Damayantia_, _Nanina_ (= _Ariophanta_, including _Pachystyla_, _Rhysota_, _Hemiplecta_, _Trochonanina_, _Euplecta_, _Orpiella_, _Xesta_, _Macrochlamys_, _Microcystis_, _Sitala_, _Kaliella_, _Durgella_, _Austenia_, _Girasia_, _Parmacochlea_, _Africarion_, _Sesara_, _Macroceras_, and others), _Vitriniconus_, _Parmacella_, _Limax_ (subg. _Amalia_ and many sections), _Vitrina_ (subg. _Vitrinozonites_, _Velifera_), _Zonites_ (subg. _Stenopus_, _Moreletia_, _Mesomphix_, _Hyalinia_, _Gastrodonta_, _Pristiloma_, _Poecilozonites_, _Thyrophorella_).

FAM. 4. _Philomycidae._--Shell absent, jaw limacidan, radula helicidan, shield covering all the body. Single genus, _Philomycus_ (= _Tebennophorus_), with subg. _Pallifera_.

FAM. 5. _Helicidae._--Shell present or absent, internal or external; jaw of various types, radula with central tooth tricuspid, equal in size to the first laterals, laterals bi- or tricuspid, marginals smaller, cusped. Eocene----. Principal genera: _Oopelta_ (no shell), _Arion_ (shell absent or formed of calcareous granules), _Ariolimax_, _Geomalacus_, _Anadenus_ (subg. _Prophysaon_), _Hemphillia_, _Cryptostracon_, _Binneya_, _Helix_ (see below), _Cochlostyla_, _Bulimus_ (subg. _Borus_, _Orphnus_, _Dryptus_, _Strophochilus_, _Pachyotus_, and possibly _Caryodes_, _Leucotaenia_, _Liparus_, _Livinhacea_, _Pachnodus_, _Rachis_, _Atopocochlis_, _Cerastus_, _Clavator_ belong here, or with _Buliminus_), _Berendtia_, _Rhodea_. Pilsbry proposes[408] to group _Helix_ as follows:

A. Eggs or young very large at birth:

(1) _Macroön_, incl. _Acavus_, _Pyrochilus_ (= _Phania_), _Stylodonta_, _Helicophanta_.

B. Eggs or young smaller or minute at birth:

(2) _Belogona._--Female genital system with dart sac and mucus gland. _Helix_ [restricted] (with sections _Arionta_, _Campylaea_, _Chilotrema_, _Pomatia_, _Macularia_, _Tachea_, _Iberus_, _Leptaxis_, _Eulota_, _Fruticicola_, _Xerophila_; _Dorcasia_, _Acusta_, _Plectotropis_, _Aegista_, _Cathaica_, _Satsuma_, _Euhadra_; _Lysinoe_), _Gonostoma_, _Leucochroa_, _Allognathus_, _Cochlostyla_, _Polymita_, _Hemitrochus_ (with sections _Plagioptycha_, _Dialeuca_, _Coryda_, _Jeanerettia_), _Glyptostoma_, _Acanthinula_, _Vallonia_.

(3) _Teleophalla._--Female system without accessories, male with flagellum and appendix on penis; no epiphallus. _Sagda_, _Cysticopsis_.

(4) _Epiphallophora._--Female system without accessories, male with epiphallus on penis; no appendix. _Caracolus_ (with sections _Lucerna_, _Dentellaria_, _Isomeria_, _Labyrinthus_, _Eurycratera_, _Parthena_, _Polydontes_, _Theliodomus_, _Cepolis_), _Camaena_ (incl. _Phoenicobius_), _Obba_, _Chloritis_ (incl. _Hadra_), _Papuina_, _Planispira_ (subg. _Cristigibba_).

(5) _Haplogona._--All accessory organs absent, jaw soldered into one piece. _Polygyra_ (incl. _Daedalochila_, _Triodopsis_, _Mesodon_, _Stenotrema_), _Endodonta_ (incl. _Libera_, _Charopa_, _Gerontia_, _Therasia_, and others), _Patula_, _Trochomorpha_, _Anoglypta_.

(6) _Polyplacognatha._--All accessory organs absent, jaw composed of 16–24 separate plates. _Punctum_, _Laoma_.

Genera of doubtful position: _Strobilops_, _Ampelita_, _Pedinogyra_, _Polygyratia_, _Macrocyclis_, _Solaropsis_.

FAM. 6. _Orthalicidae._--Radula, p. 233. Shell external, large, bulimoid. Single genus, _Orthalicus_; subg. _Liguus_, _Porphyrobaphe_, _Corona_.

FAM. 7. _Bulimulidae._--Radula, p. 233; jaw, p. 211; shell usually external. Genera: _Bulimulus_ (incl. _Plecochilus_, _Goniostomus_, _Drymaeus_, _Liostracus_, _Otostomus_, _Navicula_, _Scutalus_, _Peronaeus_, _Eurytus_, _Eudioptus_, _Plectostylus_, _Mesembrinus_, _Mormus_, etc.; _Thaumastus_, _Nesiotis_), _Placostylus_ (incl. _Charis_), _Amphidromus_, _Partula_, _Calycia_ (?), _Peltella_ (animal limaciform, shell internal), _Pellicula_, _Amphibulimus_ (incl. _Simpulopsis_).

FAM. 8. _Cylindrellidae._--Radula, p. 233; shell many whorled, long turriculate, last whorl often detached, apex often truncated (Fig. 169, p. 260). Eocene----. Genera: _Cylindrella_ (with sections _Callonia_, _Thaumasia_), _Leia_, _Macroceramus_, _Pineria_.

FAM. 9. _Pupidae._--Radula, p. 233; shell external, spire usually long, aperture often narrowed, more or less toothed, often with internal lamellae. Carboniferous----. Genera: _Anostoma_ (Fig. 154, p. 248), _Hypselostoma_ (Fig. 202A, p. 302); _Anastomopsis_ (Cretaceous), _Lychnus_ (Cretaceous), _Boysia_, _Odontostomus_ (incl. _Tomigerus_), _Buliminus_ (incl. _Petraeus_, _Napaeus_, _Zebrina_, _Mastus_, _Chondrula_, _Ena_, and perhaps _Rachis_, _Pachnodus_, _Hapalus_, and others), _Pupa_ (incl. _Torquilla_, _Pupilla_, _Sphyradium_, _Leucochila_, etc.), _Zospeum_, _Vertigo_, _Megaspira_, _Strophia_, _Holospira_, _Eucalodium_ (incl. _Coelocentrum_), _Coeliaxis_, _Perrieria_, _Balea_; _Rillya_ (Eocene), _Clausilia_ (with many sub-genera), _Rhodina_ (?).

FAM. 10. _Stenogyridae._--Radula, p. 234; shell long, spiral, shining, more or less translucid, apex blunt, sometimes decollated. Eocene----. Genera: _Stenogyra_ (subg. _Rumina_, _Obeliscus_, _Opeas_, _Melaniella_, _Spiraxis_, _Leptinaria_, _Nothus_, _Subulina_, _Glessula_), _Ferussacia_ (subg. _Cionella_, _Azeca_), _Caecilianella_ (subg. _Geostilbia_). _Achatina_ (shell large, ventricose, columella strongly truncate), with the sub-genera _Perideris_, _Limicolaria_, _Columna_, _Pseudachatina_, _Homorus_, probably belongs to a distinct family.

FAM. 11. _Achatinellidae._--Radula, p. 234; shell small, bulimoid, indifferently dextral or sinistral. Genera: _Achatinella_ (subg. _Auriculella_, _Amastra_, _Carelia_), _Tornatellina_.

FAM. 12. _Succineidae._--Radula, p. 234; lower pair of tentacles wanting or small; shell internal or external, thin, spiral or not, last whorl large. Eocene----. Genera: _Succinea_, _Homalonyx_, _Hyalimax_, (?) _Lithotis_, (?) _Catinella_.

FAM. 13. _Janellidae._--Radula, p. 234; animal slug-like, no lower tentacles, shell an internal plate. Single genus, _Janella_ (= _Athoracophorus_), with subg. _Aneitea_.

FAM. 14. _Vaginulidae._--Radula, p. 234; animal slug-like, covered with a coriaceous mantle, lower tentacles bifid, genital orifices widely separated, male behind the lower right tentacle, female on inferior median part of right side, anus and pulmonary orifice nearly terminal; shell absent. Single genus, _Vaginula_ (= _Veronicella_).

FAM. 15. _Onchidiidae._--Body oval, mantle thick, often warty, sometimes set with “eyes” (p. 187), two tentacles, genital orifices widely separate, anus and pulmonary orifice as in _Vaginula_; no shell. Genera: _Peronia_, _Onchidium_, _Onchidiella_. The family appears to be an instance of Pulmonata reverting to marine habits of life.