Part 4

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).

[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.

The next family, _Veneridæ_, have long respiratory siphons and a sinuated pallial line. Many of this tribe are very beautiful in form and colouring, and most of them have very hard strong shells. The valves are united above by an external ligament, and the hinge-plate is toothed. Nearly all of them live buried an inch or two beneath the sand or mud, but a few are found burrowing in rocks. Probably the majority of the species of this family might be used as food. _Venus verrucosa_, of our own southern shores, is frequently eaten both in this country and abroad; and _Venus mercenaria_ (Case 189) is commonly sold in the markets of Philadelphia and New York. _Cytherea lusoria_ (Case 185) also forms a favourite article of diet among the poorer classes in Japan, and several kinds are eaten by the natives of New Zealand and other countries.

[Sidenote: Cases 192–194.]

Some of the “Cockles” (_Cardiidæ_) from warm latitudes are highly coloured and adorned with most beautiful sculpture. Probably the majority are eatable, as the common cockle (_Cardium edule_) of the British coast. The foot of these molluscs is very large, bent, and used for leaping. The siphons are short and fringed at the margins.

[Sidenote: Cases 195–196.]

The _Tridacnidæ_, or true Clams, differ from other Bivalves with united mantle-margins in having but a central adductor muscle. In the typical species the animal is attached to the rocks by a “byssus,” a strong fibrous structure which passes through an aperture at the upper part of the shell. A species found in the Red Sea, _T. elongata_, is eaten by the natives, and the shell employed for the manufacture of lime. _Tridacna gigas_, the largest known bivalved mollusc, sometimes weighs over 500 lb., that exhibited in the upright cases at the entrance of the Gallery being 310 lb. in weight. A large pair bordered with gilt copper are used as _bénitiers_ or holy-water vessels in the church of St. Sulpice in Paris. _Tridacnæ_ are found associated in large numbers in lagoons, among coral-reefs in the Eastern and Pacific Seas. The animals are described as presenting a beautiful iridescent glare of blue, violet, and yellow variegated with fantastic markings.

[Sidenote: Case 196.]

The genus _Chama_ consists of tropical species, which are found fixed to corals, rocks, etc. Nevertheless, they have a small bent foot, but what purpose it serves is difficult to conceive.

In this place attention should be called to the _Hippuritidæ_ and _Radiolitidæ_, very remarkable extinct families of bivalved molluscs which occur abundantly in the cretaceous strata of southern and eastern Europe, Egypt, etc. They are remarkable for the solidity of the shells, the relatively small space occupied by the animal, and the complicated character of the hinge and processes bearing the adductor muscles. They are usually classed near to the _Chamidæ_, but their true position as regards living Mollusca is very problematical. A fine series of these shells is exhibited in Gallery VIII., wall-case 5, in the Geological Department.

[Sidenote: Cases 198–199.]

The _Myidæ_, popularly known as “_Gapers_,” on account of their valves being open at one or both ends, have the mantle united all round, except where the small foot is protruded. The siphons are very long, united almost to the ends, and covered with a coarse wrinkled outer skin. They bury themselves in mud and sand at low-water mark or in shallow water. The species are few in number, and chiefly from the shores of northern countries. _Mya arenaria_ of our own coasts is largely eaten in some parts of Europe and North America.

[Sidenote: Case 198.]

The _Corbulæ_ (Case 198) have one valve larger than the other and are like little _Myæ_, but the valves _are almost closed_ and their siphons are very short.

[Sidenote: Cases 199–201.]

Many of the _Solenidæ_, or Razor-shells, possess very elongated shells, and are remarkable for the great development of the foot, which can be pointed or contracted as may be required for boring into sand. By means of this powerful foot the animals, when disturbed, bore with such rapidity and to such a depth that their capture is a matter of great difficulty; and even when seized they hold on so tightly that at times they suffer their foot to be torn off rather than be captured. They not only burrow in sand, but also have the power of darting through the water, like the Scallops. Solens were considered a dainty dish by the ancient Greeks, and numbers are still eaten by the poorer coast-population of this country and abroad.

[Sidenote: Cases 201–202.]

The _Pholadidæ_, or Piddocks, are very remarkable shells, of an unusually complicated structure, some having the power of boring into rocks, wood, mud, sand, etc. Their shells are white, adorned with prickly sculpture, and, although thin, are strong. The foot is believed to be the principal excavating instrument, but the shell no doubt is used as a file to enlarge the hole as the creature grows. These animals are brightly phosphorescent; and certain species are eaten at many places on the shores of the Mediterranean. They appear to be indifferent as regards the material they bore into; for the common _Pholas dactylus_ (Fig. 36) of our own shores has been found in slate-rocks, mica-schist, coal-shale, new red sandstone, chalk, marl, peat, and submarine wood. The siphons are long in the Piddocks, united except near the end, and enclosed in tough skin. The species are world-wide in their distribution, and several are found fossil in some of the Tertiary formations.

[Sidenote: Case 202.]

The _Teredinidæ_, or Ship-worms, are also borers, like the Pholads, but do not perforate rocks. They are principally wood-borers; the large _Kuphus arenaria_, which is an exception, living buried in the sand. The ship-worm has a long worm-like body, from 6 to 12 inches in length, which is more or less enclosed in a thin shelly tube or sheath. The true bivalved shell is at the thicker end, and protects the mouth, labial palps, the liver, and other internal organs. At the opposite, or more slender, end of the animal, the mantle is produced into two small tubes, one of which conveys the water to the gills, whilst through the other the water is expelled, charged with the woody pulp excavated by the foot. At the end there is a pair of pallets, or paddles as they are sometimes termed, which are probably used as a means of defence, in closing the shelly tube after the contraction of the siphons.

These animals are most destructive to ships, piers, etc.; and wood, which is not protected by metal, when once attacked, is soon riddled through and through. They work either with or across the grain, and although the holes may be all but touching, they seldom appear to run into one another.

The “Watering-pot shell” (_Brechites_), of the family _Clavagellidæ_, is a very remarkable structure, and unlike the shell of an ordinary bivalved mollusc. On looking carefully, however, near the perforated end (the rose), two small valves will be seen imbedded in the surface. They are found with the rose downwards buried in mud or sand at low water on the shores of the Indian and Pacific Oceans.

SEPTIBRANCHIA. (Fig. 25, D.)

[Sidenote: Case 204.]

The members of this order differ from other Lamellibranchs in having the gill-plates represented by a muscular _septum_. They are provided with two respiratory siphons and two adductor muscles, and the edges of the mantle-lobes are connected at three points. The families _Poromyidæ_, and _Cuspidariidæ_, constitute this order. The species are all small, without colour-markings, are world-wide in their distribution and occur at all depths.

Class V.—CEPHALOPODA.[9]

[Sidenote: Cases 205–208.]

This Class includes the Octopus or Polypus, Cuttlefish, Squid, Spirula, the Paper and Pearly Nautilus. The body of the animal consists of a muscular sac, in the cavity of which the viscera are placed. In front of the body projects the head, which, in species belonging to the two-gilled section of the Class, is surrounded by eight or ten fleshy arms. A wide aperture below the head admits the water to the gills or branchiæ, which are situated in the interior of the sac, whilst a short tube, the so-called funnel or siphuncle, projects from the opening of the mantle—the water and various excretions being expelled through this tube, especially also an ink-like fluid, which is discharged by all Cephalopods (except _Nautilus_) when disturbed, in order to darken the water and thus escape their enemies. The centre of the head, between the base of the arms, is occupied by the mouth, which is armed with two horny or calcareous jaws, similar to the beak of a parrot (Fig. 39). The two large eyes are placed on the sides of the head. The arms or feet are more or less elongate, capable of movement in any direction, and, except in _Nautilus_, furnished on one side with numerous suckers, by means of which the animal attaches itself most securely to anything it may seize; they are employed in capturing food and in walking. Cephalopods walk in any direction head downwards, but can swim backwards only, being propelled in that direction by the water which they discharge with force through the funnel out of their branchial cavity. They are divided, according to the number of their gills (which is either two or four), into _Dibranchia_ and _Tetrabranchia_. Of the latter but one representative now exists, viz., the Pearly Nautilus, all other living Cephalopods being provided with but two gills, placed one on each side of the body within the mantle, as may be seen in the wax model of _Sepia officinalis_ (Case 207). The two-gilled section comprises forms with eight arms, as _Argonauta_ and _Octopus_, and others with ten arms, viz., the Cuttlefishes (_Sepia_) (Fig. 43), the Squids (_Loligo_, _Ommatostrephes_, _Sepiola_, _Chiroteuthis_, etc.), and _Spirula_. The “shell” of the Paper-Nautilus, or _Argonauta_, is too well known to require any description. Unlike the shells of other Mollusca, it is not attached to the animal by a special muscle, but is held on to the body by two of the arms, which are dilated and specially adapted for this purpose. Only the female Argonaut is provided with a shell, the male being shellless and a much smaller creature. The Argonaut-shell is therefore not a true shell, but simply a receptacle for the ova, serving at the same time for the protection of the parent.

_Chiroteuthis Veranyi_ is remarkable on account of the great length of the tentacular arms. These are non-retractile and are employed to seize their prey when at a distance.

The species of Octopus are found on the shores of almost all temperate and tropical seas; they do not attain to a large size, and are without the internal shell or “bone” which is found in the mantle of many Cephalopods. That of the Cuttlefish or _Sepia_ (Fig. 43 _a_) is found in abundance on our coasts; it is composed of numberless layers of a friable calcareous substance. That of the Squid tribe is of quite another character, consisting of an elongate thin horny plate, and strengthened by one or more thickened ribs, in some species somewhat resembling a quill-pen. Some species of this pen-bearing class related to the Common Squid attain an immense size. One was captured off the Irish coast in June, 1875 (probably _Architeuthis harveyi_), with the shorter arms 8 feet in length and 15 inches in circumference at the base, the two tentacular arms having a total length of 30 feet. The powerful beak measured about 4 inches across. Thus from the tip of the tail to the end of the tentacular arms this wonderful monster must have measured something like 40 feet in length. Other very large specimens of _Architeuthis_ have been captured on the coasts of Newfoundland and Labrador. Two specimens stranded on the south coast of Newfoundland, in the winter of 1870–1871, measured respectively 40 and 47 feet. Another, cast ashore at Bonavista Bay in December, 1873, had a very stout body 14 feet long, arms 10 feet, and tentacles 24 feet in length. These are only a few of the many instances of the capture of gigantic Cephalopods, which occur not only in the North-Atlantic Ocean, but also in tropical seas. Their appearance in mid-ocean may, in some instances, have given rise to the tales of “Sea-serpents.” Specimens much smaller than those mentioned above have attacked men, and pearl-fishers are in constant fear of them. One of the arms of a large Squid (_Architeuthis harveyi?_), which is supposed to have been found off the coast of South America, is exhibited in the black upright Case A at the side of the room.

The shells of _Spirula_ (Fig. 44) have been long known, and are scattered in thousands on the shores of New Zealand and other islands in the Pacific Ocean, and they are also found in the Indian and Atlantic Oceans, occasionally drifting on the coast of Devon and Cornwall. Notwithstanding the abundance of the shells, very few specimens of the perfect animal have been captured. The loosely-coiled shell resembles a ram’s horn, and is divided into a number of segments by fine concave partitions, like the shell of _Nautilus_, each one pierced by a slender tube or siphon. It is placed at the hinder end of the body, and is covered with so thin a skin, that a small portion of it appears to be exposed both in front and behind. Absolutely nothing is known of the habits of this very interesting creature, although probably they are somewhat similar to those of other Cephalopods.

The _Nautilus_ (Fig. 45), of which several shells (Case 208) and a perfect animal in spirit (black upright case A) are exhibited, is an inhabitant of the Indo-Pacific Ocean, and differs from all other living Cephalopods in being provided with four instead of two gills, and, instead of eight or ten arms with suckers and hooks, has a number of small retractile feelers. The Nautilus occasionally swims, like other members of its class, at the surface of the sea, but mostly crawls about leisurely on its feet at the bottom in search of food, which consists chiefly of small crabs or Mollusca, which it crushes with its strong calcareous parrot-like mandibles.

The chambered shell is pearly within, and covered with an external calcareous layer. The chambers are connected by a slender tube or siphon, the function of which is not at present thoroughly understood. The septa, or partitions across the shell, indicate periods of growth. When the Nautilus outgrows the capacity of the outer chamber, in which it resides, it constructs a new one of larger size, separating the additional chamber from the preceding one by a transverse partition.

A series of Cephalopods preserved in spirit is exhibited in the black upright case at the side of the room.

ALPHABETICAL INDEX OF THE FAMILIES AND PRINCIPAL GENERA OF MOLLUSCA EXHIBITED IN THE SHELL GALLERY.

This Index has been compiled to assist the numerous visitors, who wish to examine and determine specimens of shells, in finding, without trouble or loss of time, the Cases in which the genera are placed. Subgeneric terms are omitted, as they do not fall within the scope of this “Guide.”

Acanthina, 75

Achatina, 130–131

Achatinella, 134, 135

Acmæa, 4

Actæon, 94

Ætheria, 178

Amphibola, 98

Amphiperas, 35

Ampullaria, 25, 26

Amussium, 159

Anatina, 203

Ancylus, 99

Anodonta, 166, 167

Anomia, 137

Anostoma, 128

Aplacophora, 3

Aplustrum, 96

Aplysia = Tethys, 96

Aporrhais, 49

Arca, 139

Arctica, 162

Argonauta, 205, 206

Arion, 107

Aspergillum (= Brechites), 204

Astarte, 161

Atlanta, 94

Auriculidæ, 97, 98

Avicula = Pteria, 145

Batissa, 165

Brechites, 204

Bryopa, 204

Buccinum, 65

Buliminus, 128

Bulimulus, 122

Bulimus = Strophocheilus, 120

Bullidæ, 95

Calyptræidæ, 31, 32

Cancellaria, 77

Capulus, 31

Cardita, 160

Cardium, 192–194

Carinaria, 94

Cassis, 55–56

Cerion, 129

Cerithiidæ, 42–44

Chætoderma, 3

Chama, 196

Chitonidæ, 1–3

Chrysodomus, 64

Circe, 187

Clausilia, 129–130

Clavagella = Bryopa, 204

Columbella, 69

Conus, 89–94

Coralliophila, 76

Corbicula, 164

Corbis, 163

Corbula, 198

Crassatella, 161

Crenatula, 147

Crenella, 145

Cucullæa, 140

Cuma, 75

Cuspidaria, 204

Cyclophoridæ, 23, 25

Cyclostomatidæ = Pomatiidæ, 28, 29

Cylindrella, 127

Cypræa, 32–35

Cyprina = Arctica, 162

Cyrena, 164

Cythera = Meretrix, 185

Delphinula, 12

Dentalium, 136

Despoena, 22

Diplodonta, 163

Dolium, 56, 57

Donax, 182

Dosinia, 188

Dreissensia, 179

Eburna, 67

Emarginula, 7

Ennea, 103

Eucalodium, 129

Eulima, 46

Fasciolaria, 59

Ficula = Pirula, 57

Fissurella, 8

Fulgur, 61

Fusus, 57, 58

Gadinia, 99

Galatea, 165

Galeomma, 163

Gastrochæna, 201

Gena, 11

Glandina = Oleacina, 102

Glauconome, 192

Glycymeris, 141

Haliotis, 9–11

Haminea, 95

Harpa, 84

Helicarion, 104

Helicidæ, 107–121

Helicina, 21

Hemifusus, 61

Heteropoda, 24

Hinnites, 159

Hydatina, 96

Ianthina, 38

Isocardia, 162

Isognomon = Melina, 147

Kellia, 163

Latiaxis, 74

Latirus, 59

Leda = Nuculana, 137

Lepeta, 4

Lepton, 163

Lima, 156

Limax, 106

Limnæidæ, 99–102

Limopsis, 141

Lithodomus, 144

Littorina, 27

Loligo, 207

Lotorium, 53, 54

Lucinidæ, 162, 163

Lutraria, 199

Lyonsia, 203

Mactridæ, 183–185

Magilus, 77

Malletia, 137

Malleus, 146

Marginella, 83, 84

Melaniidæ, 38–41

Meleagrina, 146

Melina, 147

Melongena, 61

Meretrix, 185

Mesodesma, 182

Mitridæ, 61–64

Modiola, 143, 144

Modiolarca, 145

Modiolaria, 145

Monoceros = Acanthina, 75

Montacuta, 163

Murex, 70–73

Mutela, 177

Mya, 198

Myadora, 203

Mycetopus, 176

Myochama, 203

Mytilus, 142, 143

Nassa, 67, 68

Naticidæ, 36, 37

Nautilus, 208

Navicella = Septaria, 20, 21

Neæra = Cuspidaria, 204

Neomenia, 3

Nerita, 18, 19

Neritina, 19–20

Nucleobranchiata = Heteropoda, 84

Nucula, 137