CHAPTER XV

CLASS GASTEROPODA (_continued_): OPISTHOBRANCHIATA AND PULMONATA

=Order III. Opisthobranchiata=

Visceral loop not twisted (except in _Actaeon_) in a figure of 8 (Euthyneurous type, p. 203), auricle usually behind the ventricle, ctenidium often replaced by secondary branchiae, pallial cavity, if existing, more or less open, shell present or absent, operculum absent (except in _Actaeon_), animal hermaphrodite, with separate sexual openings, marine only.--Carboniferous to present time.

The character of their nervous system decisively removes the Opisthobranchiata from the Prosobranchiata, and approximates them to the Pulmonata. _Actaeon_, however, which is streptoneurous, as well as possessing an operculate shell with prominent spire, forms an interesting link with the Prosobranchiata. At the opposite extreme to _Actaeon_ stand forms like _Siphonaria_ and _Gadinia_, which are probably close links with the Pulmonata (p. 19). The generative system of the whole group, which is, as in the Basommatophora, of the hermaphrodite type, without mutual fecundation, is another link of connexion with the Pulmonata. The respiratory organs present the most varied forms, sometimes consisting of one ctenidium (never two), sometimes of secondary branchiae, variously placed, while sometimes no special organ exists.

The prolongation of the foot into lateral epipodia or parapodia (possibly to aid in swimming), and the effect of the epipodia upon the shell, according as they involve it completely or partially, are among the most instructive features of the Opisthobranchiata. If the epipodia are developed on the anterior portion of the body, and do not become reflected, they may, as in most Pteropoda Thecosomata, not directly affect the shell. But when, as in the Tectibranchiata, the epipodia are medio-lateral, and tend to envelope the shell, their effect may be traced by a series of forms varying in proportion to the amount of shell-surface covered by the epipodia. The two principal lines along which modification takes place are the gradual reduction of the spiral nature of the shell, and the gradual lessening of its solidity. Both these changes are the direct result of the additional protection afforded to the visceral mass by the reflected epipodia, which renders the existence of a shell less and less necessary. A precisely similar line of change is seen in the Pulmonata, culminating in forms like _Arion_ (p. 174).

[Illustration: FIG. 286.--Illustrating the transition of form in the shell of Tectibranchiata from the pointed spiral to the almost flattened plate: =A=, _Actaeon_; =B=, _Aplustrum_; =C=, _Cylichna_; =D=, _Atys_; =E=, _Philine_; =F=, _Dolabella_; =G=, _Aplysia_; =H=, _Pleurobranchus_. (Not drawn to scale.)]

[Illustration: FIG. 287.--Illustrating the gradual covering of the shell in the Tectibranchiata by the epipodia and mantle: =A=, _Haminea_; =B=, _Scaphander_; =C=, _Aplustrum_; =D=, _Aplysia_; =E=, _Philine_; _c.d_, cephalic disc; _ep_, _ep_, epipodia; _sh_, shell. (Not drawn to scale.)]

The habits of life of the Opisthobranchiata are very varied. Some, especially the heavier types, burrow in sand, and are then usually furnished with a broad cephalic disc, as a digging apparatus; some (certain _Bulla_) flit about in shallow pools on mud flats; others (_Phyllirrhoe_ and the Pteropoda) swim freely in the open sea; others (most Nudibranchiata) crawl slug-like on sea-weeds or corallines, and in colour singularly harmonise with their environment (p. 71 f.); others again (_Siphonaria_, _Gadinia_), stick limpet-like to rocks between tide marks. As a rule, they occur only in clean salt water, but _Embletonia_ has been found in the Victoria Docks at Rotherhithe, as well as in parts of the Baltic, where the water has only 7 parts of salt in 1000, while _Limapontia_ occurs in nearly fresh water at Bornholm and Gothland.

Their food varies greatly. As a rule, they are frugivorous, but many cases of carnivorous habit occur. _Scaphander_ has been seen to swallow _Dentalium_ six at a time, and in six hours the shells of all were reduced to tiny fragments. _Glaucus_ devours the soft portions of the pelagic _Porpita_ and _Velella_; _Idalia elegans_ eats its way into the test of Ascidians, and completely buries itself in the body of its prey.[403]

The Opisthobranchiata may be classified as follows:--

{ _Bulloidea_ { 1. TECTIBRANCHIATA { _Aplysioidea_ { { _Pleurobranchoidea_ { { _Siphonarioidea_ { =Opisthobranchiata= { 2. ASCOGLOSSA { { 3. NUDIBRANCHIATA { _Cladohepatica_ { { _Holohepatica_ { { 4. PTEROPODA { _Thecosomata_ { { _Gymnosomata_

=Sub-order I.= =Tectibranchiata.=--Right ctenidium usually present, more or less concealed by the mantle fold, visceral ganglia united by a very long commissure, shell variable in form, more or less enveloped in folds of the mantle and foot, often becoming rudimentary.

SECTION I. BULLOIDEA.--Shell more or less spiral, internal or external, epipodia more or less developed, a broad cephalic disc, distinct from the dorsal region, usually no tentacles, eyes sessile.

FAM. 1. _Actaeonidae._--Shell spiral, solid, entirely covering the animal; spire generally prominent, operculum corneous, visceral loop streptoneurous, no epipodia, radula multiseriate, teeth numerous, very small. Carboniferous ----. Genera: _Actaeon_ (Fig. 286a); _Volvaria_ (Tertiary), _Fortisia_ (Eocene) _Actaeonina_ (Carboniferous), _Cylindrites_ (Secondary strata), _Actaeonella_ (Cretaceous).

FAM. 2. _Tornatinidae._--Shell spiral, cylindrical, entirely covering the animal; spire concealed, cephalic disc with two large tentaculiform appendages behind, no radula. Genera: _Tornatina_ (= _Utriculus_), _Volvula_.

FAM. 3. _Scaphandridae._--Shell more or less external, covering all or nearly all the animal, spire concealed, cephalic disc simple or notched behind, epipodia well developed, radula with first lateral very large, stomach sometimes with powerful gizzard. Genera: _Scaphander_ (Fig. 287 B); _Sabatia_ (Pliocene), _Smaragdinella_, _Atys_ (Fig. 286 D), _Cylichna_ (Fig. 286 C), _Amphisphyra_.

FAM. 4. _Bullidae._--Shell external or partly internal, spire quite or nearly hidden, cephalic disc broad, without appendages, epipodia often large; radula usually multiseriate. Genera: _Bulla_ (subg. _Haminea_), _Acera_, mantle with long filiform appendage, epipodia touching over the shell; _Cylindrobulla_, _Volvatella_.

FAM. 5. _Aplustridae._--Shell partly internal, overlaid by the posterior part of the cephalic disc, spire not prominent, epipodia reflected, tentacles auriform. Single genus, _Aplustrum_ (Fig. 286 B; subg. _Hydatina_).

FAM. 6. _Ringiculidae._--Shell small, solid, covering all the animal; spire somewhat prominent, aperture narrow, plicated; peristome thick, sometimes channelled, cephalic disc with a kind of posterior siphon. Genera: _Ringicula_; _Avellana_ (Cretaceous).

FAM. 7. _Gastropteridae._--Shell completely internal, nautiloid, small; epipodia very large, rounded, united behind; cephalic disc simple. Single genus, _Gastropteron_.

FAM. 8. _Philinidae._--Shell completely internal, thin, slightly spiral; epipodia thick, cephalic disc large, thick, simple; stomach usually with powerful gizzard. Genera: _Philine_ (Fig. 287 E), _Colpodaspis_, _Colobocephalus_, _Chelinodura_, _Phanerophthalmus_, _Cryptophthalmus_.

FAM. 9. _Doridiidae._--Shell completely internal, a mere pellicle with a small spiral nucleus, mantle with two posterior lobes and a caudal filament, epipodia reflected. Single genus, _Doridium_.

SECTION II. APLYSIOIDEA.--Shell small, usually not spiral, sometimes absent, no cephalic disc, head prominent, with two pairs of tentacles, epipodia large, more or less reflected.

FAM. _Aplysiidae._--Characters those of the section. Genera: _Aplysia_ (Fig. 287 D), shell arched, flattened, animal large (the “sea hare”); _Dolabella_, shell subtriangular (Fig. 286 F); _Dolabrifer_, shell sub-quadrangular, not spiral; _Notarchus_, shell microscopic, spiral; _Phyllaplysia_, body very depressed, oval, no shell.

SECTION III. PLEAUROBRANCHOIDEA.--Dorsal region protected by a wide _notaeum_ or dorsal covering, or by a shell; no epipodia, ctenidium large, external, between the right under surface of the notaeum or shell and the foot; head short, shell present or absent.

FAM. 1. _Pleurobranchidae._--Shell internal or absent, notaeum with spicules, radula multiseriate. Genera: _Pleurobranchus_ (Fig. 286 H), (?) _Haliotinella_, _Pleurobranchaea_, (?) _Neda_.

FAM. 2. _Runcinidae._--Branchial lamellae few, under the posterior right notaeum, no shell. Single genus, _Runcina_.

FAM. 3. _Umbrellidae._--Shell external, depressed patelliform, not covering all the animal; foot very thick, ctenidium large, head depressed, small; radula multiseriate, teeth innumerable, very small. Genera: _Umbrella_ (Fig. 5A, p. 10), _Tylodina_.

SECTION IV. SIPHONARIOIDEA.--Shell patelliform, branchia replaced wholly or in part by a pulmonary sac, pulmonary orifice closed by a small lobe, radula multiseriate, teeth very small.

FAM. _Siphonariidae._--Characters those of the section. Genera: _Siphonaria_ (branchia as well as pulmonary sac), _Gadinia_ (no branchia). These genera, hitherto placed among the Pulmonata, have been recently shown (see p. 19) to be modified Opisthobranchiata.

=Sub-order II.= =Ascoglossa.=[404]--Branchia, mantle cavity, and shell generally wanting, liver ramified, rami enclosed in external papillae (cerata) or beneath the dorsal surface, kidney not compact, branched; radula with one series of strong teeth (Fig. 288), worn out teeth at the front end not dropping off, but preserved in a special sac (ἀσκός).

According to Bergh, the Ascoglossa form a link between the Tectibranchiata,--especially the Aplysiidae and Bullidae--and the Cladohepatic Nudibranchs, while the Pleurobranchidae form a somewhat similar link between the Holohepatic Nudibranchs and the other Tectibranchiata.

FAM. 1. _Oxynoeidae._[405]--Animal long, tentacles auriform, epipodia large, simple, or wing-like, a ctenidium and branchial chamber on right side, shell small, thin, slightly spiral, not covering much of the body. Genera: _Oxynoe_ (= _Lophocercus_), _Lobiger_.

FAM. 2. _Hermaeidae._--Body depressed, cerata in several rows, no branchiae, no shell. Genera: _Hermaea_, _Phyllobranchus_, _Stiliger_, _Alderia_.

FAM. 3. _Elysiidae._--Body depressed, head rather elevated, tentacles auriform, sides of body dilated into two large wings, which enclose branches of the liver and sometimes fold over the dorsal surface, no branchiae, no shell. Genera: _Elysia_, _Thridachia_, _Placobranchus_.

[Illustration: FIG. 288.--Radula of one of Ascoglossa (_Elysia viridis_ Mont. × 40).]

FAM. 4. _Limapontiidae._--Body slug-like, liver scarcely ramified, no branchiae, shell, or appendages. Genera: _Limapontia_, _Actaeonia_, _Cenia_.

=Sub-order III.= =Nudibranchiata.=--Shell absent in the adult, no ctenidium proper, or osphradium, cerata dorsal or dorso-lateral, nervous system concentrated, kidney not compact, ramified, penis retractile, jaws and radula usually present.

SECTION I. CLADOHEPATICA.--Cerata usually latero-dorsal, elongated, or arborescent, buccal mass strong, jaws present, liver generally ramified, rami generally entering the cerata.

FAM. 1. _Aeolidiidae._--Body slug-like, head with tentacles and rhinophores, dorsal area with rows of cerata, which usually contain sting-cells, radula variable. Genera: _Aeolis_, _Cratena_, _Tergipes_, _Coryphella_, _Favorinus_, _Facelina_, _Flabellina_, _Fiona_, _Glaucus_, _Janus_, _Hero_, with many sub-genera.

FAM. 2. _Tethymelibidae._--Body slug-like, large, cerata very large, no sting-cells, head large, cowl-shaped, no tentacles, rhinophores much foliated, no radula. Genera: _Tethys_, _Melibe_. The cerata of _Tethys_, which are capable of independent movement when severed, have been described as parasitic worms. _Tethys_ feeds on molluscs and Crustacea.

FAM. 3. _Lomanotidae._--Body slug-like, dorsum prominent, undulating or lobed, with one row of small cerata, no tentacles, rhinophores much foliated, radula with uncinated dentate laterals. Single genus, _Lomanotus_.

FAM. 4. _Dotonidae._--Body slug-like, small, two rows of cerata, each ceras surrounded by a ring of tubercles, rhinophores simple, radula uniseriate. Single genus, _Doto_.

FAM. 5. _Dendronotidae._--Body slug-like, somewhat compressed, two rows of arborescent cerata, no tentacles, frontal margin with arborescent papillae, rhinophores arborescent, radula multiseriate. Genera: _Campaspe_, _Dendronotus_.

FAM. 6. _Bornellidae._--Two rows of dorsal papillae, with branchiform appendages at the base, rhinophores foliate, radula multiseriate. Single genus, _Bornella_.

FAM. 7. _Scyllaeidae._--Body oblong, compressed, two large foliated cerata with branchial appendages on the inner side, no tentacles, rhinophores large, radula multiseriate. Single genus, _Scyllaea_.

FAM. 8. _Phyllirrhoidae._--Body much compressed, with bovine head and neck, tail tapering, no tentacles, rhinophores simple, teeth few, no marginals. Single genus, _Phyllirrhoe_.

FAM. 9. _Pleurophyllidiidae._--Body elongate-oval, snout broad, covered by an arched shield with lateral angles prolonged, branchiae consisting of two rows of lamellae placed between the notaeum and the foot, no tentacles, rhinophores short, hidden, radula multiseriate. Single genus, _Pleurophyllidia_.

FAM. 10. _Pleuroleuridae._--Animal resembling _Pleurophyllidia_, but without the branchial lamellae. Single genus, _Pleuroleura_.

FAM. 11. _Tritoniidae._--Body long, two rows of unequal arborescent cerata, rhinophores with ramose appendages, liver not prolonged into the cerata. Genera: _Tritonia_, _Marionia_.

SECTION 2. HOLOHEPATICA.--Cerata medio-dorsal, retractile or not, usually paucifoliate, liver never ramified, usually no jaws.

FAM. 1. _Dorididae._--Branchia consisting of a circle or semicircle of pinnate leaves united at the base, surrounding the anus, almost always retractile into a cavity, rhinophores foliate, no suctorial proboscis, radula multiseriate. Genera: _Bathydoris_, _Hexabranchus_, _Archidoris_ (Fig. 289), _Discodoris_, _Diaulula_, _Cadlina_, _Centrodoris_, _Platydoris_, _Chromodoris_, _Miamira_, with many sub-genera.

[Illustration: FIG. 289.--_Doris_ (_Archidoris_) _tuberculata_ L., Britain: _a_, anus; _br_, branchiae surrounding the anus; _m_, male organ; _rh_, _rh_, rhinophores. × ⅔.]

FAM. 2. _Doriopsidae._--Branchia and rhinophores as in _Dorididae_, oral aperture pore-shaped, suctorial, no radula. Single genus, _Doriopsis_.

FAM. 3. _Phyllidiidae._--Body oval, depressed, leathery, a ring of branchial lamellae, only interrupted by the head and genital papilla, under the pallial edge, oral aperture pore-shaped, suctorial, no radula. Genera: _Phyllidia_, _Fryeria_. Bergh unites this and the preceding family in the group _Porostomata_, which, with Fam. 1, form the group _Dorididae cryptobranchiatae_.

FAM. 4. _Polyceridae._--Body slug-like, branchiae not retractile, usually surrounding the anus, rhinophores foliate, tentacles simple, radula variable, central tooth generally wanting. Genera: _Notodoris_, _Triopella_, _Aegires_, _Triopa_, _Issa_, _Triopha_, _Crimora_, _Thecacera_, _Polycerella_, _Palio_, _Polycera_, _Ohola_, _Trevelyana_, _Nembrotha_, _Euplocamus_, _Plocamopherus_, _Kalinga_.

FAM. 5. _Goniodoridae._--Body oval, depressed, branchia multifoliate, usually disposed in shape of a horse-shoe, rhinophores foliate, retractile or not, mouth with a large suctorial proboscis, radula variable. Genera: _Akiodoris_, _Doridunculus_, _Acanthodoris_, _Adalaria_, _Lamellidoris_, _Calycidoris_, _Goniodoris_, _Idalia_, _Ancula_, _Drepania_.

FAM. 6. _Corambidae._--Body otherwise _Doris_-like, but with two posterior branchiae under the mantle edge, jaws present, no central tooth, about five laterals. Single genus, _Corambe_ (= _Hypobranchiaea_). Bergh unites this and the two preceding families in the group _Dorididae phanerobranchiatae_.

=Sub-order IV.= =Pteropoda.=--The Pteropoda are pelagic animals in which the lateral portions of the foot are modified into fins, which are innervated by the pedal ganglia. Their systematic position has undergone recent revision. It has been the custom to regard them as an Order of equivalent value to the other four, while some have held them to be a subdivision of Cephalopoda. Modern authorities, chief among whom is Pelseneer, regard the Pteropoda not as a primitive, but as a derived and recent group. They are “Gasteropoda in which the adaptation to pelagic life has so modified their external characters as to give them an apparent symmetry.”

The principal point which relates the Pteropoda to the Gasteropoda is the asymmetry of the visceral organs, intestine, heart, kidney, and genital gland, which results from their development on one side only of the body. Their hermaphroditism and the structure of their nervous system relate them to the Euthyneura rather than to the Streptoneura. Resemblances in the organs of circulation and generation approximate them to the Opisthobranchiata rather than to the Pulmonata, while of the two groups of the former, they tend to closer relationship with the Tectibranchiata than with the Nudibranchiata. The two sections of Pteropoda have been considered of distinct origin, the Thecosomata being derived from the Bulloidea, the Gymnosomata from the Aplysioidea.[406]

Thus the Pteropoda are a group whose true relations are masked by the special conditions of their existence, which have tended towards the development of certain organs, the so-called “wings” and the shell, which give them an apparent symmetry; this symmetry disappears on a closer investigation of the internal organs. They are hermaphrodite; the genital gland has a single efferent duct (except in some _Cavolinia_), a seminal groove leading to the copulatory organ, which in the Thecosomata is on the right side of the head, in the Gymnosomata on the right side of the foot. The genital system resembles that of the Opisthobranchiata and of the “digonoporous” Pulmonata.

SECTION 1. THECOSOMATA.--Shell or cartilaginoid test always present, fins united by an intermediate lobe, ctenidia as a rule absent, replaced by secondary branchiae, no very distinct head or eyes, one pair of tentacles; cerebral ganglia on the sides of and under the oesophagus; radula with three rather large teeth in a row, generally unicuspid, jaw in two pieces, stomach with horny plates, anus generally on the left side.

The Thecosomata feed on Protozoa and the lower Algae; they have no proboscis, and the intestine is flexured. The fins are always closely connected with the head, or what answers to it. About 42 species are known, belonging to 8 genera.

FAM. 1. _Limacinidae._--Fins very large, branchial chamber dorsal, anus on right side; shell spiral, sinistral (ultra-dextral, see p. 249), operculate. Genera: _Limacina_, shell helicoid, deeply umbilicated (_L. helicina_ swarms in Arctic seas and furnishes food for many Cetacea); _Peraclis_, spire turreted, aperture large, elongated, produced anteriorly, no umbilicus; operculum sinistral, in spite of the shell being ultra-dextral.

FAM. 2. _Cavoliniidae._--Fins large, branchial chamber ventral, shell a non-spiral cone, angular or round, very thin, embryonic portion distinct, or formed of two separate plates.

[Illustration: FIG. 290.--Illustrations of Pteropoda Thecosomata: =A=, _Limacina australis_ Eyd.; =B=, _Cleodora cuspidata_ Bosc. (shell only); =C=, _Cuvierina columnella_ Rang; =D=, _Creseis virgula_ Rang; =E=, _Clio balantium_ Rang; _f_, _f_, fins; _l_, liver; _o_, ovary; _sh_, shell. (After Souleyet.)]

In _Cavolinia_ (= _Hyalaea_, Fig. 5 B, p. 10) the shell consists of two plates, the ventral being convex, with one to three sharp spines at the posterior end, the dorsal flatter, without spines. The aperture is broad, contracted dorso-ventrally. Two long pointed prolongations of the mantle project from the lateral slits of the shell, and probably serve to balance the bulky body when swimming. Fins trilobed at the margin. _Cleodora_ has only rudimentary lateral prolongations, fins bilobed, shell triangular, angles greatly produced, aperture very wide, dorsal side keeled. In _Cuvierina_ the shell is straight, sub-cylindrical, with a median partition, slightly expanding towards the apex, which is truncated in the adult. The principal sub-genera of _Clio_ are _Creseis_, which has an elongated sub-cylindrical shell, sometimes slightly curved, smooth or grooved; and _Clio_ proper, in which the shell is long, angular, with a dorsal rib, apex (= embryonic shell) rounded, constricted. _Styliola_ and _Hyalocylix_ also belong to this group.

FAM. 3. _Cymbuliidae._--Test (which is not homologous with the shell of other Thecosomata) slipper-shaped, cartilaginoid, simply a thickening of the mantle; embryo with a calcareous, spiral, operculate shell. Genera: _Cymbulia_, _Cymbuliopsis_, _Gleba_.

Three other families, Hyalithidae, Pterothecidae, and Conulariidae, from Palaeozoic strata, are generally added to the Thecosomata. All are fossil only, and it is doubtful whether they are really Molluscan. Pelseneer holds that no true fossil Pteropoda occur until the lower Tertiaries.

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.

[Illustration: FIG. 291.--=A=, Anterior portion of _Pneumoderma_; =B=, _Clione limacina_ Phipps; =C=, _Halopsyche Gaudichaudi_ Soul.; _f_, _f_, fins; _h.s_, _h.s_, hook-sacs; _l.f_, lobe of the foot; _s_, _s_, suckers; _o_, posterior genital orifice; _t_, _t_, tentacles. (After Souleyet.)]

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).

[Illustration: FIG. 292.--Examples of the _Auriculidae_: =A=, _Auricula Judae_ Lam., Borneo; =B=, _Scarabus Lessoni_ Blainv., E. Indies; =C=, _Cassidula mustelina_ Desh., N. Zealand; =D=, _Melampus castaneus_ Mühlf., S. Pacific; =E=, _Pedipes quadridens_ Pfr., Jamaica.]

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_.

[Illustration: FIG. 293.--_Amphibola avellana_ Chem.]

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_).

[Illustration: FIG. 294.--=A=, _Ennea_ (_Gibbulina_) _palanga_ Fér; =A´= young of same; =B=, Gibbus lyonetianus Pall.]

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_).

[Illustration: FIG. 295.--Example of the _Macroön_ group of _Helix_. _Helicophanta Souverbiana_ Fisch., Madagascar, showing large embryonic shell. × ⅔.]

(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_.

[Illustration: FIG. 296.--_Odontostomus pantagruelinus_ Moric., S. Brazil. × ½.]

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_ (?).

[Illustration: FIG. 297.--=A=, _Clausilia crassicosta_ Ben., Sicily; =B=, _Clausilia macarana_ Zieg., Dalmatia; =B´=, clausilium of same.]

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_).

[Illustration: FIG. 298.--_Achatina zebra_ Lam., S. Africa. × ½.]

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.

##