Part 2
The “Keyhole Limpets” and “Slit Limpets” (_Fissurellidæ_) resemble in external shape ordinary Limpets, but are perforated at or near the apex, or more or less slit at the front margin. The hole or slit gives passage to a tubular fold of the mantle, through which the water apparently flows to the gills. The largest species are from California and South America, and others are found, but not abundantly, on most shores. The animal of the large _Lucupina crenulata_ from California is eight or ten inches in length, and almost conceals the shell, and the shell of the South-African _Pupillæa aperta_ is also all but hidden beneath the mantle of the animal.
[Illustration:
Fig. 5.[3]
_Pleurotomaria adansoniana._ Case 9.
⅓ natural size. ]
[Sidenote: Case 9.]
The _Pleurotomariæ_ are extremely rare in recent times, only five species being known, whereas over a thousand fossil forms have been described. The specimens of _P. adansoniana_ and _P. beyrichi_ exhibited in case 9, are among the finest acquisitions to the shell collection of recent years.
[Sidenote: Cases 9–11.]
The “Ear-shells” or “Ormers” (_Haliotidæ_) are found adhering to rocks in most parts of the world, with the exception of South America. They are lined with _pearl_, and many exhibit splendid colours and sculpture externally. Like the Limpets they hold on to the rocks with such tenacity that it is absolutely impossible to remove some of the larger species by force without injuring the shell. Boiling water or mustard and water poured over them will, however, soon compel them to relinquish their hold. The shell of _Haliotis_ is pierced by a series of holes parallel with the left margin. Through such of them as are open the animal protrudes a slender filament or feeler, and the water also finds its way through them to the gills beneath.
The Single British species (_H. tuberculata_) is not actually found on the English coast, but common on rocks and stones at low-water in the Channel Islands. It is frequently eaten by the poor of those islands and the north of France; other species in New Zealand, China, Japan, West Africa, and elsewhere, constitute a common article of diet among the natives. _Haliotis_-shells are largely used in the manufacture of pearl ornaments, and in all kinds of inlaid work.
[Illustration:
Fig. 6.
Top-shell (_Turbo petholatus_). (From the Indo-Pacific Ocean.)
_a._ Inner surface of operculum. _b._ Exterior of ditto. ]
[Sidenote: Cases 12–18.]
The _Trochidæ_, and _Turbinidæ_ are two extensive families, the animals of which are very much alike, and mainly distinguished by the operculum, which in the former is horny, and shelly in the latter. The shells of these families are beautifully pearly within, and the external shelly coat is generally brightly coloured and highly ornamented. Several very pretty species are found on our own shores. The opercula of _Turbo petholatus_ (Fig. 6), from the Indian and Pacific Oceans, are frequently mounted in gold and silver as scarf-pins, ear-rings, &c.
[Sidenote: Cases 18–20.]
The _Nerites_ are mostly found in tropical countries, and, like the Winkles, are very strongly made, to resist the force of the breaking waves. The _Neritinas_ are partly found in the sea, and partly in fresh water, and are less solid shells. The third section of _Neritidæ_, the _Septariæ_, are shaped very much like Limpets, except that the apex is at one end instead of central. They are, however, very different animals, and furnished with a shelly operculum imbedded in the foot.
Order 2.—PECTINIBRANCHIA.
[Sidenote: Cases 22–94.]
In most cases the molluscs of this order have an attached monopectinate gill and a single osphradium. A few are fresh or brackish water forms, but the majority are marine.
[Sidenote: Case 22.]
The “River-Snails” (_Viviparidæ_) might be termed freshwater Periwinkles, as the animals of both are very similar. The true _Viviparæ_ are viviparous. They are rather sluggish, and found at the bottom of ponds and rivers feeding on decaying animal and vegetable matter.
[Illustration:
Fig. 7.
The Common British River-Snail (_Vivipara vivipara_).
_a_, head; _b_, tentacles; _c_, eyes; _d_, foot; _e_, operculum. ]
[Sidenote: Cases 23–25.]
The _Cyclophoridæ_ are land-shells, which, however, cannot properly be considered true lung-breathers like ordinary snails. They have not the closed lung-chamber of the Pulmonates, their eyes are placed at the base of the tentacles instead of at their tips, they have a long proboscis armed with a different rasping tongue (_radula_), a spiral operculum, and the sexes are distinct, whereas the true Snails are hermaphrodite.
The operculated air-breathers have been divided into many sections, chiefly on account of differences in the apertures of the shells and in the opercula. They most abound in hot countries, but a few species are met with in temperate regions.
[Sidenote: Cases 25–26.]
The “Apple-Snails” (_Ampullariidæ_) live in the rivers and marshes of tropical regions, and, although represented by a large number of species, exhibit comparatively slight variations in form and colour. The animal has both a pectinated gill and a lung cavity, being thus enabled to breathe either water or air.
[Illustration:
Fig. 8.
_Ampullaria canaliculata._ ]
[Sidenote: Cases 27–28.]
The “Periwinkles” (_Littorinidæ_) are found almost on every known shore; they feed upon all kinds of marine vegetation. Some species are met with at low-water mark, others on rocks almost beyond the reach of the sea, and some have been discovered inland nearly half a mile away from the shore. It is calculated that 1900 tons of the “Common Periwinkle” (_Littorina littorea_), of the value of £15,000, are annually consumed in London alone.
[Sidenote: Cases 31–32.]
The family of _Calyptræidæ_ includes the “Slipper-Limpets” (_Crepidula_) and the “Cup-and-saucer Limpets” (_Crucibulum_). Although furnished with a foot, they rarely crawl about, but remain attached to rocks, stones, or other shells, sometimes forming a shelly plate under the foot by which they become fixed to the spot where they have taken up their abode.
[Sidenote: Cases 32–35.]
The “Cowry-shells” (_Cypræidæ_) are remarkable for their varied markings and splendid polish, which is produced and preserved by two flaps of the mantle, one on each side, which fold over the back, a line down the centre of which usually marks where the flaps meet. The animals are even more brilliantly coloured than the shells. They have no operculum, but a large foot, which they can withdraw entirely within their shell, although the aperture is usually very narrow. Cowries, as is well known, are sold as ornaments; and a small yellow species, “the money-cowry” (_C. moneta_), which is very common in the Indian and Pacific Oceans, passes current as coin among the negro tribes of certain parts of Africa. The specimen of _Cypræa leucodon_ figured on p. 16 is extremely valuable and supposed to be the only one hitherto discovered. The “orange cowry” (_Cypræa aurora_) is worn by chiefs in the Friendly Islands, and is considered the highest order of dignity. Only one small species, _Trivia europæa_, is found on the British coast, and about 100 fossil forms have been discovered in the Chalk.
[Illustration:
Fig. 9.
The Tiger Cowry (_Cypræa tigris_). (From the Indo-Pacific Ocean.)
_a_, the shell; _b_, the mantle; _c_, foot; _d_, siphon; _e_, proboscis; _f_, tentacles; _g_, eyes. ]
[Sidenote: Case 35.]
Of the _Ovulidæ_, the most curious is the “Weaver’s-shuttle” (_Radius volva_), in which the shell is peculiarly beaked at both ends. It is found living on barked corals (_Gorgoniidæ_), and some of the smaller species exhibit differences of coloration, resembling the tints of the Gorgonias upon which they are found.
[Sidenote: Cases 36–37.]
The Naticas are mostly blind, and have a very large foot, suitable for burrowing in the sand when in quest of bivalves. They are very voracious. This is one of the groups of shells that have continued to exist from Palæozoic times.
[Sidenote: Case 38.]
The “Violet Snails” (_Ianthinidæ_) are found floating about in every ocean, excepting in cold regions, with the spire of the shell downwards, and the bottom, being more exposed to the action of light, is more deeply tinted than the upper part. They feed upon Jelly-fish, and construct a gelatinous raft, filled with air-bubbles, beneath which the females attach their eggs.
[Illustration:
Fig. 10.
_Cypræa leucodon_. Case 32. ]
[Sidenote: Cases 38–41.]
The _Melaniidæ_ are freshwater Snails which abound in most tropical and subtropical countries; about 1000 species are known. They are mostly of dark colours, and are fond of muddy places.
[Sidenote: Cases 42–43.]
The _Cerithiidæ_ are chiefly marine forms, some, however, entering brackish water. About five hundred fossil species have been described, some of them gigantic in comparison with any now living, of which more than two hundred are known.
[Sidenote: Case 44.]
The _Scala scalaris_ was formerly considered a great rarity, as much as £40 having been given for a single specimen, which might now be purchased for as many pence.
[Illustration:
Fig. 11.
_Scala scalaris._
Case 44. ]
[Sidenote: Cases 46–47.]
The “Worm-shells” (_Vermetidæ_) are a very peculiar family. Their shells can scarcely be distinguished from the shelly tubes which are formed by certain species of marine worms, _Serpula_, &c. They are free and spiral in early life, but afterwards become distorted and generally attached to rocks, stones, &c. A foot for walking purposes therefore would be of no use; consequently it is more or less obsolete, serving only as a support to the operculum.
[Sidenote: Case 48.]
The “Screw-shells” (_Turritellidæ_) have elongate tapering shells; about 100 recent and 200 fossil species are known. One species only (_Turritella communis_) is now found living on the British coasts.
[Sidenote: Case 49.]
The _Xenophoridæ_ have the singular habit of cementing to the exterior of their shell, stones, pieces of coral, and fragments of other shells; hence they have been called “Carrier-shells,” and, according to the kind of material chosen, have been named “Conchologists” and “Mineralogists.” Beyond acting as a disguise, and consequently as a protection, there does not appear to be any special utility in thus adding to the weight of their own shells. The animals do not glide like most other molluscs, but scramble along like the Strombs, the form of their foot being small, divided into a front, expanded, and a hind, tapering portion admirably adapted to the nature of the ground on which they live, which usually consists of broken and dead shells.
[Sidenote: Cases 49–52.]
The “Wing-shells” (_Strombidæ_) are the largest of the Gastropods with a proboscis or non-retractile snout. They do not crawl like most other Gastropods, but progress by a sort of hopping movement. They act as scavengers, feeding on decomposing animal matter.
The _Strombus gigas_, or “Fountain-shell,” occurs in great numbers in the West Indies, and is a very heavy solid shell. It is a favourite ornament for rockwork and fountains in gardens, and, like the Helmet-shells, is used for cameo-carving. It is also employed in the manufacture of porcelain, as many as 300,000 having been imported into Liverpool in one year for that purpose.
[Sidenote: Cases 51–52.]
The Scorpion-shells, or “Spider-claws,” as they are sometimes called (_Pterocera_), possess singular claw-like projections, which are developed on the outer lip of the shells.
[Sidenote: Cases 53–54.]
The “Trumpet-shells” (_Lotoriidæ_) have varices or strengthening ribs at intervals, like the Murices; the largest species, _Lotorium variegatum_, is used by South-Sea Islanders as a horn or trumpet. A hole is made in the upper part of the spire to blow through, and the sound produced can be modulated or varied by inserting the hand in the aperture or mouth of the shell.
[Sidenote: Cases 55–56.]
The “Helmet-shells” (_Cassididæ_) are used for cameo-carving; they consist of differently coloured layers, so that the ground-colour of the carving is of a different tint from the subject engraved. The most artistic shell-cameos are produced in Italy, whence the art has been introduced into France and England. The _Cassis madagascariensis_ (Fig. 12 on p. 19) is in special request by shell-carvers on account of the strong contrast of the white upper layer with the dark ground beneath. Extinct forms of _Cassis_ are found fossil in Tertiary formations, but none of them equal in size the largest living species.
[Sidenote: Case 57.]
The “Tun-shells” (_Doliidæ_) are remarkable for the globoseness of the shells, which are covered with very regular revolving ribs.
[Sidenote: Cases 58–60.]
The _Fasciolariidæ_ contains two of the largest living Gastropods: _Megalatractus aruanus_, from North and West Australia, and _Fasciolaria gigantea_, which is found off the coast of South Carolina, and attains at times a length of two feet.
[Sidenote: Cases 61–64.]
The Mitras (_Mitridæ_) are great favourites with shell-collectors, on account of their beautiful colours and varied sculpture. There are about 600 living species already known, and between one and two hundred have been found in a fossil state. Shells of this group, like the _Fasciolariæ_, are distinguished by a few plaits or folds on the inner side of the aperture (the columella). Mitras are almost exclusively found in tropical or subtropical regions, the majority being met with either at low-water mark or in comparatively shallow water.
[Sidenote: Cases 64–66.]
The family of _Buccinidæ_ also contains a very large and various assemblage of forms. Among them may be mentioned the Whelks (_Buccinum_). (See Fig. 13 on p. 19.)
[Illustration:
Fig. 12.
_Cassis madagascariensis_, with cameo engraved upon it.
Side table-case B. ]
[Illustration:
Fig. 13.
The Common Whelk (_Buccinum undatum_).
_a_, siphon; _b_, foot; _c_, tentacles; _d_, eyes; _e_, operculum. ]
[Sidenote: Cases 70–76.]
The family of _Muricidæ_, or “Rock-shells,” is another extensive group, containing many very handsome and peculiar forms. The animals of this family have a long proboscis, at the end of which is the spiny tongue (radula), and which is retractile within the body. The true Murices produce at intervals ribs or varices, which in some species are ornamented with long spines or foliations, and which indicate periods of growth, but of what duration we do not know. They are all carnivorous, feeding chiefly on other Mollusca, boring through the shells of bivalves with their spiny tongue, and slowly devouring the unfortunate inhabitant piecemeal. From certain species of _Murex_ (_M. brandaris_, &c.) found in the Mediterranean, the ancients manufactured the celebrated Tyrian purple dye.
[Sidenote: Cases 74–75.]
The “Purples” (_Purpura_) are found between tide-marks all over the world. _Magilus_, belonging to the family _Coralliophilidæ_ (Case 77), is found among coral-reefs in tropical seas, and has the remarkable habit of lengthening the aperture of its shell into an elongate tube, in order to keep pace with the growth of the coral, and to prevent its being overgrown and killed.
[Sidenote: Cases 78–81.]
The “Volutes” (_Volutidæ_) are a group of shells also much sought after by shell-collectors. Some of these attain to a very large size, the animals inhabiting them being enormous. The Boat-shells (_Cymba_) and Melons (_Cymbium_) are ovo-viviparous, the young being carried about by the parent until they are an inch in length. Volutes are found chiefly in the warmer parts of the Atlantic and Indo-Pacific Oceans, and occur in the greatest variety on the coasts of Australia.
[Sidenote: Cases 81–83.]
The Olives (_Olividæ_) are common in most tropical seas, and are remarkable for their beautiful polish and various patterns of colouring. In structure and form they are very similar to each other. They burrow in sand in quest of bivalves for food, and some species are said to have the power of swimming by expanding the lobes of the foot.
[Sidenote: Case 84.]
The Harps (_Harpidæ_) form a small well-marked group, of which probably nearly all the existing species have been discovered. The animals inhabiting these beautiful shells are also brightly coloured. They have the remarkable power of casting off a portion of the foot when disturbed. The species are known from the Indo-Pacific Ocean, the west coast of Central America, and West Africa.
[Sidenote: Cases 85–87.]
The next family, the “Slit-lips” (_Pleurotomatidæ_), consists of very numerous species, over a thousand living forms having been discovered, and almost as many fossil species from Cretaceous and Tertiary strata have been described. The typical forms are characterized by a slit in the outer side (lip) of the aperture. Species of _Pleurotoma_ are found in every sea, although most abundant in the tropics, and, although so numerous in species, the number of specimens is small in comparison with some other genera.
[Sidenote: Cases 87–89.]
The “Auger-shells” (_Terebridæ_), like the Cones, present a great similarity in form, but, unlike them, have a great diversity of “_sculpture_” or external ornamentation. They are all elongate shells, with a deep notch at the base of the aperture. Owing to the length and comparative solidity of the shells, the animals of many of the species do not carry their shelly structures on their backs, like most other species, but drag them along the sandy sea-bottom.
[Illustration:
Fig. 14.
The “Glory-of-the-Sea” Cone (_Conus gloria-maris_). Case 94.
(From the Philippine Islands.) ]
[Sidenote: Cases 89–94.]
The _Conidæ_, or Cones, form one of the most beautiful portions of the collection of Shells. This family, of which between 400 and 500 distinct kinds are known, is a great favourite with collectors on account of the brilliant colours and various patterns of the shells. Some, owing to their beauty and rarity, have been sold at very high prices, as much as £50 having been paid for a single shell. The Cones are found in all tropical seas, but are rare in cold or temperate latitudes. None are met with on our own shores, one species alone being known from the Mediterranean. They occur fossil in the Chalk and Tertiary strata. These animals are all carnivorous, and live usually in shallow water among rocks and coral-reefs. Some of them are said to bite when handled, and to be dangerously poisonous, the bite in some instances having been all but fatal.
[Sidenote: Case 94.]
The _Atlantidæ_, _Pterotracheidæ_, and _Carinariidæ_, at various times recognized as forming a distinct sub-class or order of Gastropoda, under the name of Heteropoda or Nucleobranchiata, are now regarded as families of aberrant Gastropods organised for swimming in the open sea. The _Atlantas_ are found in great numbers in warm latitudes, and are provided with a glassy, thin, flat, spiral shell, not unlike a keeled Ammonite. The glassy shell of the _Carinaria_ is one of the most beautiful structures of any mollusc, and at one time was such a rarity that £100 are said to have been given for a single specimen, which at the present time is perhaps worth only from five to ten pounds. Species of _Carinaria_ are found in the Mediterranean and warmer parts of the Atlantic and Indian Oceans. The animal is large, semitransparent, and elongate, with a compressed fin-like foot which projects from the body, and is used in swimming. The gills are placed towards the hinder part of the back and covered by the shell. They feed on jelly-fish of various kinds, and probably on other soft animals.
[Illustration:
Fig. 15.
Glassy Nautilus (_Carinaria lamarcki_).
_a_, proboscis; _b_, tentacles; _c_, shell; _d_, gills; _e_, foot; _f_, sucker. ]
Section EUTHYNEURA.
[Sidenote: Cases 94–135.]
The Gastropods belonging to this sub-class have the visceral nerve-loop straight and not twisted as in the STREPTONEURA. All the EUTHYNEURA are hermaphrodite, and their radula is generally composed of numerous similar denticles on each side of a median tooth. Scarcely any of these forms are provided with an operculum in the adult state. The EUTHYNEURA may be divided into two orders, _Opisthobranchia_ and _Pulmonata_.
Order 1.—OPISTHOBRANCHIA.
[Sidenote: Cases 94–97.]
All the Molluscs of this order are marine, some (_Tectibranchia_) breathing by means of the ordinary Gastropod ctenidium, which is generally behind the heart, whereas others (_Nudibranchia_) have developed a different type of respiratory organs.
The _Opisthobranchia_ include the “Pteropods” formerly considered as a distinct class, the “Bubble-shells” (_Bullidæ_), the “Sea-Hares” (_Aplysiidæ_), the “Umbrella-shells” (_Umbraculidæ_), the _Nudibranchs_ and some others.
[Illustration:
Fig. 16.
Shell-bearing Pteropod (_Cavolina tridentata_). Case 96.
_a._ Shell and animal. _b._ Side view of shell. _c._ Dorsal view of shell. ]
The Pteropods[4] are sometimes called Sea-butterflies, and are organized for swimming freely in the ocean. They have a pair of fins developed from the sides of the mouth or neck, which perform a flapping movement during progression. Some Pteropods (Thecosomata) are provided with small glassy shells; others (Gymnosomota) are naked. They exist in countless millions in some parts of the ocean, discolouring the water for miles. They constitute the principal food of the Baleen Whales.
[Illustration:
Fig. 17.
Shell-less Pteropod (_Clione limacina_).
_a._ Dorsal view. _b._ Ventral aspect. ]
About a hundred species are known.
[Sidenote: Case 96.]
The Sea-Hares, so called on account of a slight resemblance to a crouching hare and not for their nimbleness of foot, are found in most parts of the world, in pools at low water. At the hinder part of the back two flaps of the mantle partly conceal a thin horny shell which serves as a protection to the gills and vital organs beneath. When molested, these animals discharge a large quantity of a purple fluid, discolouring the surrounding water for a distance of more than a yard.
[Illustration:
Fig. 18.
Sea-Hare (_Tethys (Aplysia) punctata_). British.
_a_, labial tentacles; _b_, upper tentacles or rhinophores; _c_, siphonal fold of the mantle near the shell; _d_, eye. ]
[Sidenote: Case 97.]
The shell of _Umbraculum_ is shaped very like that useful article, an umbrella, of the Chinese pattern. The animal is very large, having its breathing-organs on the right side below the shell.
[Sidenote: Case 97.]
The _Nudibranchs_ or Naked-gilled Molluscs comprise some of the most beautiful and strange forms. They are unprovided with shells except in the earliest stages of their existence, when they dwell in a minute nautiloid shell, furnished with an operculum, both of which are subsequently cast off. Unfortunately the colours of these beautiful creatures cannot be preserved after death, and therefore a small series of glass models is exhibited, which will give some idea of their great variety in form and colouring. They are found in most parts of the world, chiefly in shallow water, but a few species live upon floating seaweed in the open sea. Over a hundred species exist on the British coast, the majority of which are, however, very small. They are chiefly carnivorous, feeding on other molluscs, sea-anemones, &c.
[Illustration:
Fig. 19.
The Umbrella-shell (_Umbraculum mediterraneum_).
_a_, shell; _b_, gills; _c_, tentacles; _d_, mouth; _e_, foot. ]
[Illustration:
Fig. 20.
Naked-gilled Mollusc, or Nudibranch (_Doto coronata_).
_a_, head; _b_, foot; _c_, gills; _d_, tentacle-sheath; _e_, tentacle. ]
Order 2.—PULMONATA.
[Sidenote: Cases 97–135.]
The Pulmonata are furnished with a lung cavity in place of the ordinary gill of other Gastropods, and may be termed true air-breathers. Most of them are provided with shells, and, with the exception of the _Amphibolidæ_ never possess an operculum. They are divisible into two groups or sub-orders, _Basommatophora_ and _Stylommatophora_, characterized by a difference in the position of the eyes. The Basommatophora, including the _Auriculidæ_, _Amphibolidæ_, _Siphonariidæ_, and _Limnæidæ_, have a single pair of non-retractile tentacles, at the base of which are situated the eyes. The Stylommatophora (Land-snails, Slugs, &c.), are provided with two pairs of retractile tentacles, with the eyes at the summit of the upper pair. Over ten thousand species of Pulmonata are known.
(_Basommatophora._)
[Sidenote: Cases 97–98.]
The first group of the aquatic air-breathers, the _Auriculidæ_, chiefly inhabit salt or brackish water. The largest forms are tropical and found at the mouths of rivers, among the roots and stems of mangrove-trees, or in damp woods near the sea.
[Sidenote: Cases 98–99.]
The “Limpet-Snails” (_Siphonariidæ_) seem at first sight to be out of place among the Snails and Slugs, and more nearly allied to the Rock-Limpets; but the character of the tongue (radula) and the closed respiratory cavity indicate a close relationship with the present group.
[Illustration:
Fig. 21.
Three Rows of Teeth of the Radula of _Siphonaria_.
_c_, central; _l_, lateral teeth. ]