[Ill.u.s.tration: Fig. 8. _Spirula_.]

CLa.s.s II.--GASTEROPODA (Belly-footed) comprises those mollusca which, like the common snail, creep on the under-surface of the body, and with one exception (_Chiton_, Fig. 20) their sh.e.l.ls are univalve (_i.e._, composed of one piece). But before we go further, it may be well to point out the names given to different parts of a univalve sh.e.l.l. The aperture whence the animal issues is called the _mouth_, and its outer edge the _lip_; each turn of the sh.e.l.l is a _whorl_; the last and biggest, the _body-whorl_, the whorls, from the point at the top, or _apex_, down to the mouth form the _spire_; and the line where the whorls join each other is called the _suture_. The axis of the sh.e.l.l around which the whorls are coiled is sometimes open or hollow, and the sh.e.l.l is then said to be _umbilicated_ (as in Fig. 3_b_); when closely coiled, a pillar of sh.e.l.l, or _columella_, is left (as in Fig.

9). Sometimes the corner of the mouth farthest from the spire and next the columella, is produced into a channel, the _anterior ca.n.a.l_ (as in Fig. 9); whilst where the mouth meets the base of the spire there may be a kind of notch which is termed the _posterior ca.n.a.l_.

Most Gasteropods are _dextral_, that is to say, the mouth is to the right of the axis as you look at it; a few, however, are _sinistral_, or wound to the left (like _Physa_); whilst reversed varieties of both kinds are met with.

Gasteropods of the first order have comb-like gills placed in advance of the heart, and are hence termed PROSOBRANCHIATA. They are divided into two groups: (_a_) _Siphonostomata_ (Tube-mouthed), in which the animal has a long proboscis, and a tube, or siphon, from the breathing-chamber that pa.s.ses along the anterior ca.n.a.l of the sh.e.l.l, which in this group is well developed. They have a h.o.r.n.y operculum, or lid, with which to close the aperture. (_b_) _Holostomata_ (or Whole-mouthed). In these the siphon is not so produced, and does not want to be protected; accordingly the mouth of the sh.e.l.l is _entire_, _i.e._ has no ca.n.a.l. The operculum is h.o.r.n.y or sh.e.l.ly. The former (group _a_) includes several families:

1. _Strombidae_, comprising sh.e.l.ls, like the huge _Strombus_, or "Fountain-sh.e.l.l," which is so often used to adorn the mantelpiece or rockery, and from which cameos are cut.

2. The _Muricidae_, of which the _Murex_ (an extraordinary form of this is the "Venus' comb," _Murex tenuispina_, Fig. 9), the Mitre-sh.e.l.ls, and the Red-Whelks (_Fusus_) are examples.

[Ill.u.s.tration: Fig. 9. _Murex tenuispina._]

3. The _Buccinidae_, taking its name from its type, the Common Whelk (_Buccinum undatum_), and including such other forms as the Dog-Whelk (_Na.s.sa_), the _Purpura_, the strange _Magilus_, and the lovely Harp-Sh.e.l.ls and Olives (Fig. 10).

[Ill.u.s.tration: Fig. 10. _Oliva tessellata._]

4. The _Ca.s.sididae_, or "Helmet-Sh.e.l.ls." _Ca.s.sis rufa_, from West Africa, is noted as the best species of sh.e.l.l for cameo engraving; with it are cla.s.sed the "Tun" (_Dolium_) and the great "Triton"

(_Triton tritonis_), such as the sea-G.o.ds of mythology are represented blowing into by way of trumpet, and which are used by the Polynesian Islanders to this day instead of horns.

5. The _Conidae_, whose type, the "Cone-sh.e.l.l" (Fig. 11), is at once distinctive and handsome, but which in the living state is covered by a dull yellowish-brown periostrac.u.m that has to be carefully removed before the full beauties of the sh.e.l.l are displayed.

[Ill.u.s.tration: Fig. 11. Conus vermiculatus.]

6. The _Volutidae_, embracing the Volutes and "Boat-sh.e.l.ls" (_Cymba_).

7. The _Cypraeidae_, or Cowries (Fig. 12), which owe their high polish to the size of the sh.e.l.l-secreting organ (mantle), whose edges meet over the back of the sh.e.l.l, concealing it within its folds. With these is cla.s.sed the "China-sh.e.l.l" (_Ovulum_).

[Ill.u.s.tration: Fig. 12. Cypraea oniscus.]

The second group, or _Holostomata_, is divided into nineteen families, beginning with--

1. The _Naticidae_, whose type, the genus _Natica_, is well known to all sh.e.l.l-collectors through the common _Natica monilifera_ of our coasts.

2. The _Cancellariadae_, in which the sh.e.l.ls are cancellated or cross-barred by a double series of lines running, one set with the whorls, and the other across them.

3. The _Pyramidellidae_, which are high-spired, elongated, and slender sh.e.l.ls, with the exception of the genus _Stylina_, which lives attached to the spines of sea-urchins or buried in living star-fishes and corals. 4. The _Solaridae_ or "Staircase-sh.e.l.ls," whose umbilicus is so wide that, as you look down it, the projecting edges of the whorls appear like a winding staircase. It is a very short-spired sh.e.l.l.

5. The _Scalaridae_, "Wentle-traps" or "Ladder-sh.e.l.ls," which may be readily recognised from their white and l.u.s.trous appearance and the strong rib-like markings of the periodic mouths that encircle the whorls.

6. The _Cerithiadae_, or "Horn-sh.e.l.ls," which are very high-spired, and whose columella and anterior ca.n.a.l are produced in the form of an impudent little tail, the effect of which, however, in the genus _Aporrhais_, or "Spout-sh.e.l.ls," is taken away by the expanded and thickened lip.

7. In the next family, the _Turritellidae_, or "Tower-sh.e.l.ls," the type Turritella is spiral; but in the allied form _Vermetus_, though the spire begins in the natural manner, it goes off into a twisted tube resembling somewhat an ill-made corkscrew. The mouth in this family is often nearly round.

8. The _Melaniadae_, and 9. The _Paludinidae_, are fresh-water sh.e.l.ls.

The former are turreted, and the latter conical or globular. Both are furnished with opercula, but the mouth in the first is more or less oval and frequently notched in front, while in the latter it is rounded and entire.

10. The _Litorinidae_, or Periwinkles, need no word from us.

11. The _Calyptraeidae_ comprise the "Bonnet-limpet," or _Pileopsis_, and "Cup-and-saucer-limpets" (_Calyptraea_). They may be described briefly as limpets with traces of a spire left. The genus _Phorus_, however, is spiral, and resembles a _Trochus_. They have been called "Carriers" from their strange habit of building any stray fragments of sh.e.l.l or stone into their house, thus rendering themselves almost indistinguishable from the ground on which they crawl.

12. The _Turbinidae_, or "Top-sh.e.l.ls," are next in order, and of these the great _Turbo marmoreus_ is a well-known example, being prepared as an ornament for the whatnot or mantelpiece by removing the external layer of the sh.e.l.l in order to display the brilliant pearly nacre below. These mollusca close their mouths with a h.o.r.n.y operculum, coated on its exterior by a thick layer of porcelain-like sh.e.l.ly matter. With them are cla.s.sed the familiar _Trochus_ and other closely allied genera.

13. The _Haliotidae_ offer in the representative genus _Haliotis_, or the "Ear-sh.e.l.l," another familiar mantelpiece ornament.

14. The _Ianthinidae_, or "Violet-snails," that float about in mid-Atlantic upon the gulf-weed, and at certain seasons secrete a curious float or raft, to which their eggs are attached, are next in order, and are followed by--

15. The _Fissurellidae_, or "Key-hole" and "Notched limpets," whose name sufficiently describes them. To these succeed--

16. The _Neritidae_, an unmistakable group of globular sh.e.l.ls, having next to no spire and a very glossy exterior, generally ornamented with a great variety of spots and bands.

17. The _Patellidae_, or true Limpets, are well known to every sea-side visitor: large species, as much as two inches across, are to be found on the coast of Devon, but these are pigmies compared with a South American variety which attains a foot in diameter.

18. The _Dentaliadae_, represented by the genus _Dentalium_, or "Tooth-sh.e.l.l," are simply slightly curved tubes, open at both ends and tapering from the mouth downwards, and cannot be mistaken.

19. Lastly, we have the _Chitonidae_, whose single genus _Chiton_ possesses sh.e.l.ls differing from all other mollusca in being composed of eight plates overlapping each other, and in appearance reminding one of the wood-louse. This animal is not only like the limpet in form but also in habits, being found adhering to the rocks and stones at low-water.

Order II.--PULMONIFERA. Contains the air-breathing _Gasteropods_, and to it consequently belong all the terrestrial mollusca, though some few aquatic genera are also included. The members of this order have an air-chamber instead of gills, and are divided into two groups, (_a_) those without an operculum, and (_b_) those having an operculum.

Foremost in the first group stands the great family--

1. _Helicidae_, named after its chief representative, the genus _Helix_. It also includes the "Gla.s.s-sh.e.l.l" (_Vitrina_), the "Amber-sh.e.l.l" (_Succinea_), and such genera as _Bulimus_, _Achatina_, _Pupa_, _Clausilia_ (Fig. 13), etc., which differ from the typical _Helix_ in appearance, possessing as they do comparatively high spires.

[Ill.u.s.tration: Fig. 13. _Clausilia biplicata._]

2. The _Limacidae_, or "slugs," follow next; of these only one, the genus _Testacella_, has an external sh.e.l.l stuck on the end of its tail; the rest have either a more or less imperfect sh.e.l.l concealed underneath the mantle, or else none at all.

3. The _Oncidiadae_ are slug-like, and devoid of sh.e.l.l.

4. The _Limnaeidae_ embrace the "Pond-snails," chief of whom is the well-known, high-spired _Limnaea stagnalis_. Other sh.e.l.ls of this family a.s.sociated with _Limnaea_ are, however, very different in shape; for instance, _Physa_ has its whorls turning to the left instead of to the right; _Ancylus_ (Fig. 24), or the freshwater limpet, is of course limpet-like; while _Planorbis_, or the "Coil-sh.e.l.l," is wound like a watch-spring.

5. The _Auriculidae_ includes both spiral sh.e.l.ls, such as _Auricula_ and _Charychium_, and a limpet-like one _Siphonaria_.

At the head of group _b_ stands 1, _Cyclostomidae_. _Cyclostoma elegans_ is a common sh.e.l.l on our chalk-downs, and well ill.u.s.trates its family, in which the mouth is nearly circular. Foreign examples of this genus are much esteemed by collectors. The other two families are, (2) _Helicinidae_ and (3) _Aciculidae_.

Order III.--OPISTHOBRANCHIATA. These animals carry their gills exposed on the back and sides, towards the rear of the body. Only a few have any sh.e.l.l. 1. The _Tornatellidae_, which have a stout little spiral sh.e.l.l. 2. The _Bullidae_, in which the spire is concealed (Fig. 26). 3.

The _Aplysiadae_, where the sh.e.l.l is flat and oblong or triangular in shape. The remaining families are slug-like and sh.e.l.l-less.

[Ill.u.s.tration: Fig. 14. _Bulla ampulla._]

Order IV.--NUCLEOBRANCHIATA. Derives its name from the fact that the animals const.i.tuting it have their respiratory and digestive organs in a sort of nucleus on the posterior part of the back, and covered by a minute sh.e.l.l. As they are pelagic, the sh.e.l.ls are not readily to be obtained. They are divided into two families, _Firolidae_ and _Atlantidae_.

CLa.s.s III.--PTEROPODA. Like the last, these pretty little mollusca are ocean-swimmers. The members of one division of them, to which the _Cleodora_ belongs, is furnished with iridescent external sh.e.l.ls.