Part 3

The shells of _Siphonaria_ may be known from Limpets by a slight bulging on one side, caused by a radiating groove which interrupts the muscle of attachment. They are marine, and are found on rocks between tide-marks, chiefly in tropical countries.

[Sidenote: Cases 99–101.]

The _Limnæidæ_ are only found in fresh water. Most of them occasionally rise to the surface to breathe, where they glide along foot uppermost, at times suspending themselves by a glutinous thread, after the fashion of a spider. All countries appear to have their peculiar species.

The freshwater Limpets (_Ancylus_) live attached to stones and leaves of plants, and have not the habit of floating, but, like the rest of the _Limnæidæ_, feed on freshwater algæ, confervæ, and decayed vegetable matter.

[Illustration:

Fig. 22.

British Pond-Snail (_Limnæa stagnalis_).

1. Upper view: _a_, foot; _b_, tentacles; _c_, eye; _d_, muzzle.

2. Lower view: letters _a_, _b_, _c_ as above; _e_, mouth; _f_, respiratory orifice. ]

(_Stylommatophora._)

[Sidenote: Cases 102–135.]

True Snails (_Helicidæ_, etc.) have a distinct head furnished with eyes, tentacles, cutting upper jaws, and rasping teeth, and all are protected by a spiral shell. They are almost exclusively vegetable-feeders, subsisting chiefly on leaves. The sexes are not distinct. Many of the species are beautiful objects on account of the brilliancy of their colouration, and some are remarkable for the variation they exhibit in this respect. Species of _Helicidæ_ are found in nearly every part of the world and in all situations, from sea-level to an altitude of 12,000 feet. They are fond of moisture, and in hot and dry weather retire within their shells, remaining torpid until the return of dew and rain. _Helix pomatia_ (Case 119), which is found on the chalk in the south of England and on the Continent, is commonly eaten in Austria, France, and Belgium.

The eggs of Land-Snails vary in texture, size, and in numbers: they are usually white, but in some instances yellow and pale green. Those of some of the large South-American forms are as hard as that of a hen, and more than an inch in length (Case 120).

[Illustration:

Fig. 23.

British Land-Snail (_Helix pomatia_).

_a_, eye-bearing tentacles (“horns”); _b_, lower or smaller tentacles. ]

Slugs (Cases 106,107) are very like Snails without external shells; most of them, however, possess a small internal shelly plate, or a few calcareous granules hidden beneath the skin of the back. Some have a large slime-pore at the end of the foot, and others are slightly phosphorescent. Like the Snails, they are fond of damp localities, and at times become great pests to farmers in devouring the young shoots of the growing corn. _Testacella_, which is found in this country, differs from the Slugs in having an external shell at the tail-end of the foot. It is not slimy, and lives under ground, feeding upon earthworms.

Class III.—SCAPHOPODA.[5]

[Illustration:

Fig. 24.

British Tooth-shell. (_Dentalium tarentinum_).

_a._ The shell. _b._ The animal, removed from its shell; _f._ the foot. ]

[Sidenote: Case 136.]

The “Tooth-shells” (_Dentaliidæ_) form a distinct group, the shells of which are very unlike those of any other mollusc, but closely resembling the shelly tubes constructed by certain kinds of marine worms. The _Dentalia_ have neither eyes nor tentacles, or a distinct head like Gastropods; their organs of circulation and respiration are of a rudimentary kind, and they have no heart. The sexes are separate. Their foot is adapted for burrowing in sand, in which they live and obtain their food, which consists of _Foraminifera_ and minute Bivalves. One species, _Dentalium pretiosum_, found on the shores of North-West America, was until recently used as money by the Indians.

Class IV.—LAMELLIBRANCHIA.[6]

[Sidenote: Cases 137–204.]

The Molluscs belonging to this Class have neither head, nor cephalic eyes, nor jaws or tongue like those of the other Classes, and are enclosed in a shell which consists of two plates or valves held together on one side of the margin by a horny, elastic substance, called the “_ligament_.” Bivalves do not creep about in search of food, but find their means of existence in the shape of minute particles, both animal and vegetable, which happen to be contained in the water which they breathe. Some, however, are capable of locomotion by means of a well-developed foot, and a few swim through the water by alternately opening and shutting their valves. The body is enclosed within two lobes of the mantle which line the interior of the valves, and which at their base are firmly attached to the shell, producing on the shell a scar or impression called the “pallial line.” The gills are lamellar or leaf-like, and placed on each side of the body. Each gill is called a ctenidium, and consists of an axis which is partly attached to the body of the Mollusc. This axis generally gives off two plates consisting of hollow filaments which are parallel with one another, directed downwards towards the ventral side, and in most cases long and refolded upon themselves, so that each plate becomes in reality a double lamella. In a few instances, however, the filaments are simple and not reflected. They are connected with one another by microscopic cilia, sometimes by vascular junctions, and the dependent and reflected portions (lamellæ) of each filament may be connected by “interlamellar vascular junctions.” The mouth is merely an oval aperture at the anterior end of the body, and generally furnished on each side with soft thin flaps, or labial palps, which have the function of conveying the food to the mouth. The mantle secretes the substance out of which the shell is formed. The two valves are always in contact at the _hinge_, which is generally formed by small interlocking projections or hinge-teeth, and they are closed by large adductor muscles, which are attached to impressions in the interior of the shell. When these muscles cease to act, as after death, the valves of the shell open in consequence of the elasticity of the ligament on the dorsal margin. The majority of species have two principal adductors, one at each end, like the Venus-shells, Cockles, Razor-shells, &c.; but in Oysters, Scallops, and a few others, there is but a single central muscle. All Bivalves are aquatic, and the majority marine. They are found burrowing in sand or attached to rocks. Some perforate stones and corals, others wood, and a few construct a sort of nest of fragments of shells, stones, &c.

Many schemes of classification have from time to time been propounded, based upon the presence or absence of respiratory siphons, the number and position of the adductor shell-muscles, the character of the shell-hinge, &c. The most recent arrangement is founded principally upon the structure of the gills. The value of such a classification has yet to be fully tested. Mr. Paul Pilseneer has suggested five orders of Lamellibranchs: Protobranchia, Filibranchia, Pseudolamellibranchia, Eulamellibranchia, Septibranchia.

[Illustration:

Fig. 25.

(From the ‘Cambridge Natural History.’ Messrs. Macmillan & Co.)

A. Protobranchia. B. Filibranchia. C. Eulamellibranchia. D. Septibranchia.

_m._ Mantle, _v._ Body. _f′._ Foot. _e._ Outer gill-lamella; _i._ Inner gill-lamella; _e′._ Reflected portion of outer lamella; _i′._ Reflected portion of inner lamella; _s._ Septum-like gill. ]

[Illustration:

Fig. 26.

Gill of _Mytilus edulis_.[7]

A. Part of four filaments showing ciliated interfilamentar junctions (cj).

B. Diagram of a single filament showing the two lamellæ connected at intervals by interlamellar junctions (ilj) and the position of the interfilamentar ciliated junctions (ep). ]

PROTOBRANCHIA. (Fig. 25, A.)

[Sidenote: Case 137.]

In this order the filaments of the gills are not reflected, but arranged in two divergent rows, the foot being expanded and flattened beneath with crenulated margins and with the byssal gland very slightly developed. The _Nuculidæ_ and _Solenomyidæ_ are the only families belonging to this order. The shells of the former are remarkable for the numerous fine interlocking hinge-teeth, and those of the latter on account of the strong fringed periostracum.

FILIBRANCHIA. (Fig. 25, B.)

[Sidenote: Cases 137–145.]

In this group the gills are smooth, with the filaments directed downwards, reflected, and connected one with another by interfilamentar ciliated junctions, but the lamellæ are not connected. The foot is usually provided with a well-developed byssal gland. _Anomia_, _Arca_, _Trigonia_, and _Mytilus_ belong to this order.

[Sidenote: Cases 137–138.]

The family of _Anomiidæ_ contains a number of more or less pearly shells remarkable for a deep notch or hole in the lower or flat valve through which a shelly plug passes, by means of which the animal attaches itself to other shells, stones, &c. _Anomia ænigmatica_ is found adhering to leaves in mangrove-swamps.

[Sidenote: Case 138.]

The _Placunidæ_, sometimes called Window-shells and Saddle-Oysters, are very flat pearly shells with a remarkable hinge, which consists of two long divergent teeth, like a =⋀=, to which the ligament is attached. The species are few in number, and inhabit sandy shores of India, China, and North Australia.

[Sidenote: Cases 139–141.]

The _Arcidæ_ are a family of strong ponderous shells varying much in form and sculpture. The animals have a longish pointed foot, deeply grooved along the bottom, no labial palpi, and free margins to the mantle, which are not prolonged into breathing-siphons. Many of the Arks often anchor themselves by means of a strong byssus. The shells of this family are usually radiately ridged; and the hinge is composed of a number of teeth arranged along the hinge-line, which is generally straight. _Arca tortuosa_, from China, has the valves curiously twisted. The section _Barbatia_ is remarkable for the coarse fibrous character of the epidermis; _Scapharca_ for its unequal valves; and _Cucullæa_, from the Indian Ocean, for the elevated ridge bounding the posterior muscular impression. _Glycymeris_ (better known as _Pectunculus_) has the hinge-teeth arranged in an arched series, and the shells are more regular in growth than in many other forms of _Arcidæ_.

[Sidenote: Case 141.]

The _Trigoniidæ_ are one of those families which have all but disappeared during our period. Only three or four living species are known, whilst more than a hundred fossil forms have been described from the Jurassic and Cretaceous formations. Australia, where some of the oldest types of animal life persist, furnishes also the existing species of _Trigonia_ (Fig. 27). The animals have a long, sharply-bent, pointed foot like the Cockles, with which they make surprising leaps. The shells are beautifully pearly within, and ribbed and noduled exteriorly.

[Illustration:

Fig. 27.[8]

_Trigonia margaritacea._

Case 141. ]

[Sidenote: Cases 142–145.]

The _Mytilidæ_, or Mussels, are too well known to need description. The small foot, which is brown in the common species, is not much used in creeping about, but has the power of spinning a byssus or bundle of tough threads, by means of which the animals attach themselves to rocks and one another, forming colonies of vast numbers. Mussels have always been much eaten in this and other maritime countries, and large quantities are brought to the London market from the Dutch coast. At times they are unwholesome; but all the exact causes of this are not known. Mussels seem to be found on every shore, and some of the species are very widely distributed—the common edible Mussel, _M. edulis_, being found on every European coast, on the shores of North and South America, in the Arctic and Antarctic Oceans, and probably on the coasts of Australia.

One group of Mussels (_Lithodomus_, Case 144) burrow in rocks and other shells, forming holes just large enough to contain their shells. _L. dactylus_ is sold as an article of diet on the shores of the Mediterranean.

PSEUDOLAMELLIBRANCHIA.

[Sidenote: Cases 145–160.]

The gills in this order are plicate, and the two lamellæ of each plate are furnished with conjunctive or vascular interlamellar junctions, and the filaments are connected by interfilamentar ciliated discs in some cases, in others by vascular concrescence. The mantle-margins are separated all round, and the foot is either small or wanting. Only a single adductor muscle is generally present. The Pearl-oysters, true Oysters, and Scallops are the forms which constitute this order.

The large family of _Aviculidæ_ includes the “Wing-shells” (_Avicula_), the “Pearl” and “Hammer Oysters” (_Meleagrina_ and _Malleus_), and _Pinna_ (Fig. 28). Some species of _Pinna_ attain to a length of two feet. They are found imbedded in the sand with the narrow pointed end downwards. They form a large silky byssus, which can be woven or knitted into gloves, socks, etc. (see side table—case B). The “Hammer-Oyster” (_Malleus_) is so called from its rude resemblance to a hammer. The “Pearl-Oysters” (_Meleagrina margaritifera_, Fig. 29) possess rather heavy strong shells, lined with very thick layers of “mother-o’-pearl.” Hundreds of tons of these shells are annually collected at the great pearl-fisheries of North and West Australia, and imported into Europe. The pearl-oyster of Ceylon (_M. fucata_, Case 146) is a smaller species, and collected more for the pearls than the shells. The round pearls, which are valued so highly, are either excrescences of the pearly layer or are found loose in the fleshy parts of the animal. Some small foreign body which has accidentally penetrated under the mantle and irritates the animal is covered with successive concentric layers of nacre, thus attaining sometimes, but rarely, the size of a small filbert. The nacre is generally of the well-known pearly-white colour, very rarely dark, and occasionally almost black. The action of the animal in secreting successive layers of nacre over any foreign body which intrudes between the mantle-folds, and thus converting it into a pearl, is strikingly illustrated by two specimens in which, in the one case, an entire fish, and in the other a small crab, have been so enclosed (see side table-case E).

[Illustration:

Fig. 28.

British “Fan-Mussel” (_Pinna pectinata_): _a_, the byssus. Case 150. ]

[Illustration:

Fig. 29.

Pearl-Oyster (_Meleagrina margaritifera_). Case 146. ]

The most ancient and, even at the present day, one of the most important of the pearl-fisheries is that carried on on the western shores of Ceylon. “The Banks,” or spots on which the oysters grow, are at an average depth of 30 to 60 feet, and extend several miles along the coast. The oysters, which should be six or seven years old when collected, are gathered in baskets by native divers and hauled up by ropes into hundreds of small boats. The shells are then brought to land and placed upon the ground to die and putrefy, and then minutely examined for the pearls, which are either found loose in the shells or imbedded in the fleshy parts of the oysters. As many as two million oysters have been brought ashore on one day; but the number obtained varies very much according to the state of the banks. A small proportion of the oysters contain pearls; in some only very small ones (seed or dust-pearls as they are called) are found, and very few contain pearls larger than a pea, which are so highly valued. In his account of the pearl-fishery of Ceylon the Rev. James Cordiner says that he saw the operation of sorting the pearls performed; the produce of 17,000 oysters weighed only ¾ lb. and was contained in a vessel smaller than a common soup-plate. Out of that quantity there were not found two fine perfect pearls; all of the largest were slightly deformed, rugged and uneven, but of the smaller sizes many were round and perfect. The chief qualities which regulate the value of pearls are size, roundness, and brilliancy of lustre. Of the smallest kind several may be bought for a shilling, whilst many thousand pounds have been given for a single fine pearl of surpassing beauty.

Other important pearl-fisheries besides that of Ceylon are carried on in the Persian Gulf, on the west coast of Central America, and especially North-west Australia, where diving-dresses are now employed in collecting the shells.

The Chinese obtain pearls artificially from a species of freshwater Mussel (_Dipsas plicata_). In order to do this they keep them in tanks and insert between the shell and the animal either small shot or small round pieces of mother-of-pearl, which soon receive regular coatings of nacre and assume the look of ordinary pearls. They also insert small metal images of Buddha, which also soon become covered with pearl and firmly cemented to the shell, the production being to the uninitiated a supernatural testimony to the truth of Buddhism. (A shell treated in this way is exhibited in one of the small cases (E) at the side of the room.)

[Sidenote: Cases 152–154.]

The _Ostreidæ_, or Oysters, undoubtedly take the first rank among molluscs as regards usefulness to mankind as an article of food. They have no foot; the mantle is entirely open, with double edges, each being bordered by a short fringe, and the labial palps are large and somewhat triangular. There are on each side a pair of simple gills, which appear closely striated; the single adductor muscle is large and nearly central (see Fig. 30). The Oyster is, except in the very young state, entirely incapable of locomotion, and always attached by the deeper valve to other shells, rocks, or other substances. The common British species is not full-grown until it is about five or seven years old. A series of different ages, from the “spat” to the adult form, is exhibited in Case 152. During the months of May, June, and July the eggs are discharged into the gills, where they remain until hatched; and it is during this period that oysters are “out of season.” In the American Oyster (_O. virginica_), on the contrary, the eggs are said to be hatched outside the parent shell. Oysters of different kinds are found on nearly every shore. The gigantic _O. gigas_ is said to grow to the length of three feet in the Bay of Taichou, Japan, where it is commonly eaten. About two hundred fossil species have already been described.

[Sidenote: Cases 154–156.]

The _Spondylidæ_, or Thorny Oysters, closely resemble the Scallops, but the shells are more spiny, heavier, united by interlocking teeth, and one of the valves is attached to rocks, corals, etc. Many of the species are very brightly coloured; and from the fact that small quantities of water are sometimes enclosed in cavities in the inner layer of the shell, they have been called “Water-Clams” or “Water Spondyli.”

[Illustration:

Fig. 30.

Common Edible Oyster (_Ostrea edulis_).

_a_, labial palpi; _b_, gills; _c_, mantle; _d_, junction of the two folds of the mantle; _e_, large adductor muscle; _f_, the shell. ]

[Sidenote: Case 156.]

The Limas (_Limidæ_) are very like the Pectens, but the inner edge of their mantle is fringed with very long thread-like filaments. The shells are always white, generally more or less oblique, and radiately ridged. They appear to be found in most seas, and either swim about freely like the young Scallops by flapping their valves or attach themselves by a byssus, sometimes forming a sort of nest, consisting of pieces of coral and shell or small stones, in which they are completely concealed.

[Sidenote: Cases 156–160.]

The Scallops or Fan-shells (_Pectinidæ_) are well known for their beautiful colours, sculpture, and excellent flavour. The animal has a distinct foot, which is not, however, used as a locomotive organ, but employed in spinning a byssus of attachment when required. The young Pectens dart through the water by opening and suddenly closing their valves. The species are very numerous, world-wide in their distribution, and may be found at depths from a few to _three thousand_ fathoms.

EULAMELLIBRANCHIA. (Fig. 25, C.)

[Sidenote: Cases 160–204.]

In this order the gills have interfilamentar and interlamellar _vascular_ junctions, and there are generally two adductor muscles. The order is very extensive, comprising nearly sixty families, of which only the more important can be referred to.

The _Carditidæ_ and _Astartidæ_ have strong solid shells, frequently ornamented with radiating or concentric ribbing, and usually are coated with a dark epidermis. They have the general appearance of certain _Veneridæ_; but the animal has no prolonged siphons, but merely a fringed opening in the mantle. One very remarkable species, _Thecalia concamerata_ (Case 161), has an internal cup-like process within the valves, which serves as a nursing-pouch for the young.

[Sidenote: Cases 162–163.]

The _Lucinidæ_ are almost invariably white shells, and may generally be recognized by the very long muscular scar in front on the inner surface of the valves. They occur in all parts of the world; and the fossil forms, which are still more numerous than those now living, have existed at every epoch from the Silurian.

[Sidenote: Cases 166–177.]

Of the freshwater Mussels or _Unionidæ_ more than 1200 species have been already discovered; they are found in most parts of the world, the greatest number having been described from North America. In _Unio_ the edges of the mantle are not united along the bottom and not prolonged into siphonal tubes; at the posterior end there are two openings, of which the upper or excretal orifice is simple, and the lower or branchial fringed at the edge. Two “cephalic eyes” have recently been noticed by Mr. P. Pelseneer. The foot is very large and adapted for crawling and burrowing. The sexes are distinct; and the shells of the females are somewhat more tumid than those of the males. _Margaritana margaritifera_, (Case 168) which is found in this country and in Europe, sometimes produces handsome pearls, but not equal to those obtained from the pearl-oyster of tropical seas. The hinge in this family is extremely variable, being in some instances delicate and toothless (_Anodonta_, Fig. 31, B), whilst in others it is enormously thickened and furnished with strong interlocking teeth (see Fig. 31, A).

[Illustration:

Fig. 31. ]

[Sidenote: Case 178.]

The family _Ætheriidæ_, or freshwater Oysters, consists of but three genera: _Ætheria_ contains African, and _Mülleria_ and _Barttettia_ Indian and South-American forms. When young the shells of _Ætheria_ (which are common in the Nile) are free and not unlike an _Anodonta_, but when adult they become attached and irregular and look like an olive-green Oyster; they are, however, provided with two muscular impressions instead of one, as in ordinary marine Oysters. Still more remarkable is _Mülleria lobata_ of Colombia, which, when young, freely moves about and has two adductor muscles, but in time becomes attached and stationary, and then possesses but a single adductor.

[Sidenote: Cases 179–181.]

The _Tellinas_ have usually thin shells, and their two siphons are longer and more completely separated from each other than in the _Veneridæ_. The pallial line is widely and deeply sinuated, and the ligament generally external. In the genus _Semele_ of the family _Scrobiculariidæ_, it is placed within the hinge-margin. They live in great numbers beneath the sand in shallow water, and are occasionally used as food.

[Sidenote: Cases 183–185.]

The _Mactridæ_ have an internal ligament to the hinge, the siphons are joined together and fringed at the ends, and the pallial line is more or less sinuated. _Spisula solidissima_ (Case 184), the largest species found on the coast of the United States, is a common article of diet.

[Illustration:

Fig. 32.

Common British Cockle

(_Cardium edule_).

_a_, foot; _b_, exhalant siphon; _c_, branchial or inhalant siphon; _d_, edge of mantle; _e_, ligament; _f_, umbones or beaks of the shell. ]