Chapter II: Preface (2)

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.

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.
]

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.

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).

Fig. 28.

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

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.”

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).

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.

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.
]

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.

Fig. 33.

Left valve of the Giant Clam (_Tridacna gigas_).

Length, 36 inches. Weight, 154 lb.; weight of the two valves, 310 lb.
]

[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.

Fig. 34.

British Gaper (_Mya truncata_).

_a_, foot; _b_, siphon-sheath; _c_, exhalant siphon; _d_, inhalant
siphon; _e_, umbones or beaks; _f_, anterior, _g_, posterior end of
shell.
]

[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.

Fig. 35.

British Razor-shell (_Solen siliqua_).

_a_, foot; _b_, mantle; _c_, inhalant siphon; _d_, exhalant siphon;
_e_, shell.
]

[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.

Fig. 36.

Piddock, or Borer (_Pholas dactylus_). (From the British coast.)

1. Animal in the shell: _a_, foot; _b_, siphons; _c_, inhalant
orifice; _d_, exhalant orifice.

2. Shell: _e_, accessory valves or plates.
]

[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.

Fig. 37.

Ship-worm
(_Teredo norvegica_).
Case 202.

_a_, animal, removed from its shelly tube: _p_, _p_, pallets; _s_,
exhalant siphon; _s′_, inhalant siphon.

_b_, _c_, different aspects of the shell.
]

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.

Fig. 38.

Watering-pot Shell
(_Brechites vaginifer_).

Case 204.

_a_, bivalve shell of the very young animal.
]

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]

Fig. 39.

A, the upper, B, the lower beak of _Architeuthis monachus_; one-third
natural size.
]

[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.

Fig 40.

The Common Octopus (_Polypus vulgaris_), resting.
]

Fig. 41.

_Sepiola scandica_
(Natural size). British.
]

_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.

Fig. 42.

_Chiroteuthis Veranyi_ (much reduced).

_a_, general view of animal; _b_, magnified view of pedunculated
sucker of the terminal club of the tentacular arms; _c_, internal
shell or gladius.
]

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.

Fig. 43.

The Common Cuttlefish (_Sepia officinalis_), and its shell or bone
(_a_).
]

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.

Fig. 44.

The Spirula (_Spirula peronii_). (From the Indian and Pacific Oceans.)

1. Animal: _a_, portions of the shell exposed in front and behind;
_b_, the funnel or siphuncle. 2. Side view of shell. 3. Shell in
section, to show partitions or septa.
]

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.

Fig. 45.

The Pearly Nautilus (_Nautilus pompilius_).

_a_, body; _b_, siphuncle; _c_, eye; _d_, hood; _e_, tentacles; _f_,
muscle of attachment to the shell; _g_, siphon.
]

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

Nuculana, 137

Nudibranchiata, 97

Octopus, 205

Oleacina, 102

Olividæ, 81–83

Ostrea, 152–154

Ovulum = Amphiperas, 35

Paludina = Vivipara, 22

Paludomus, 41

Pandora, 202

Panopea, 201

Partula, 126, 127

Patella, 4–7

Pectinidæ, 156–160

Pectunculus = Glycymeris, 141

Pedum, 156

Periploma, 203

Petricola, 192

Philine, 96

Pholadidæ, 201, 202

Pholadomya, 204

Pholas, 201, 202

Phorus (= Xenophora), 49

Physa, 101

Pinna, 148–152

Pirula, 57

Placuna, 138

Planaxis, 44

Planorbis, 100

Pleurotomaria, 9

Pleurotomidæ, 85–87

Plicatula, 154

Pomatiidæ, 28, 29

Proserpina = Despoena, 22

Psammobia, 197

Pteria, 145

Pterocera, 51, 52

Pteropoda, 96

Puncturella, 7

Pupa, 128

Purpura, 74, 75

Pyramidellidæ, 45

Ranella, 54

Ricinula = Sistrum, 76

Ringicula, 95

Rissoiidæ, 30

Rocellaria, 201

Rostellaria, 52

Rotella, 15, 16

Saxicava, 201

Scala, 44, 45

Scalaria = Scala, 44, 45

Scaphander, 95

Scaphopoda, 136

Scintilla, 163

Scutum, 7

Semele, 181

Sepia, 206, 207

Septaria, 20, 21

Septifer, 143

Siliquaria, 47

Siphonaria, 98, 99

Sistrum, 76

Solarium, 45

Solenidæ, 199–201

Solenomya, 137

Sphærium, 165

Spirula, 207

Spondylus, 154–156 Stenogyra, 133

Stilifer, 46

Stomatella, 11

Streptaxis, 102

Strombus, 49–51

Strophia = Cerion, 129

Strophocheilus, 120

Struthiolaria, 49

Succinea, 135

Sunetta, 187

Sycotypus, 61

Tapes, 190,191

Tellinidæ, 179–181

Terebellum, 52

Terebridæ, 87–89

Teredo, 202

Testacella, 102

Tethys, 96

Thracia, 203

Trichotropis, 37

Tridacna, 195

Trigonia, 141

Triton = Lotorium, 53, 54

Trochidæ, 12–15

Trophon, 70

Truncatella, 30

Tugonia, 198

Turbinellidæ, 60,61

Turbinidæ, 16–18

Turritella, 48

Typhis, 70

Umbraculum, 97

Umbrella = Umbraculum, 97

Ungulina, 163

Unionidæ, 166–178

Valvata, 30

Vanicoro, 49

Velutina, 37

Veneridæ, 185–191

Venerupis, 192

Venus, 188

Vermetidæ, 46, 47

Vitrina, 106

Vivipara, 22

Volutidæ, 78–81

Vulsella, 147

Xenophora, 49

Yoldia, 137

POLYZOA.

(*) An asterisk against names of species denotes that specimens of these
species are in the upright part of Case A and preserved in spirit.

[Sidenote: Upright Table-Cases A and B, at south end of Shell Gallery.]

From a casual glance at the contents of these cases, it might be supposed that many of the specimens exhibited therein were seaweeds; but a closer inspection, especially with a lens, will reveal structure of a kind not to be found in any plant.

Let us select for examination _Flustra foliacea_, the Broad-leaved Hornwrack or Sea-Mat (Fig. 1), (Case A 1), commonly to be found among heaps of seaweed cast up on sandy shores round our coasts.

Fig. 1.

_Flustra foliacea._ A, natural size; B′, portion magnified in B; B,
magnified 30 diameters.

_a_, avicularium; _o_, ovicell.

[‘The Cambridge Natural History.’]
]

The brown horny fronds, which vary in width, branch upwards from a narrow flat stem attached at its base to stones and shells. Both surfaces of the fronds show a fine network pattern formed by the edges of little oblong boxes or cells termed zoœcia,[10] arranged in longitudinal parallel rows and forming a double layer back to back. The cells are broad and rounded above, narrow and truncate below, and each is roofed in by a transparent membrane with a semicircular lid or operculum situated near the upper end; four short stout spines spring from the margin in this neighbourhood. When the surface of a living frond is examined in sea-water, here and there a bundle of tentacles may be observed pushing up a lid, slowly emerging and expanding into a bell-shaped coronet; on the least alarm the tentacles are rapidly withdrawn into the cell and the lid shut. The flexible protrusible region of the cell is termed the tentacle sheath. The relation of the cell to the tentacle sheath (Figs. 2, 3) may be roughly compared to a glove finger, stiff below, but flexible at the end, and surmounted by a crown of bristles; on pulling down the glove-finger tip, the tentacles will also be drawn in, and will lie in a sheath formed by the invaginated portion of glove finger. The lid which closes over the tentacle sheath is only found in the Sub-order Chilostomata to which _Flustra_ belongs. The area of the tentacle sheath whence the tentacles arise is termed the lophophore.[11]

Figs. 2, 3, diagrams representing polypide in cell. Fig. 2,
tentacle-sheath protruded. Fig. 3, ditto, retracted; _a_, tentacles;
_b_, tentacle-sheath; _c_, mouth; _d_, gullet; _e_, stomach; _f_,
vent; _g_, retractor muscle; _h_, funiculus; _l_, ovary; _k_,
testis; _l_, lid or operculum; nerve ganglion is between mouth and
vent. Fig. 4, polypide extracted from cell; _d_, pharynx; _e_,
stomach; _f_, anus (after Van Beneden). Fig. 5, section (partly
diagrammatic) of frond of _Flustra_, showing cells back to back.
]

The mouth is situated in the centre of the lophophore, surrounded by the circle of tentacles; and the latter, by the action of their cilia, set up currents which convey food to the mouth.

The mouth leads into a pharynx and gullet, the latter opening into a stomach, whence the intestine ascends to terminate in the vent opening below and outside the circle of tentacles; the intestines, in fact, form a U-shaped tube (Figs. 4, 5) suspended in the body-cavity in the interior of the cell. A cord, the funiculus, passes from the stomach to the base of the body-cavity. A small nerve ganglion is situated within the upper part of the loop of intestine.

The tentacles, intestines, and other organs constitute the “polypide,” the cell being simply the protective house formed by the latter.

The body-cavity, which contains fluid, is in direct communication with the interior of the tentacles, which are hollow, and which act as respiratory organs by bringing the fluids of the body-cavity in proximity to the water. In _Flustra_ the body-cavities of the cells are shut off from each other, but pores and sieves in the partition walls allow of the junction of the inner linings of these cavities. The male and female reproductive elements are formed in the body-cavity. The egg develops in a helmet-shaped brood-pouch, the ovicell, situated at the summit of the cell and almost immersed in the cell above. The ciliated embryo swims about for a few hours and settles down to form the first polypide and cell; from the latter there arise buds which remain attached, and produce other buds, till a colony like that of _Flustra_ results.

Among the ordinary cells are certain smaller cells (Fig. 1, _a_) slightly raised above the general level, different in shape from the ordinary kind and with thicker lids. These peculiar cells are termed avicularia, and chiefly contain muscles for opening and shutting the lid. They arise by modification of the ordinary cells, whereby all the organs of the polypide have become atrophied except the muscles. The Polyzoa[12] were so named by Vaughan Thompson, who, in 1820, discovered that certain plant-like animals, which had previously been classed with the zoophytes, possessed a much higher organisation, in that the intestine was separate from the body-cavity and not continuous with it as in Sea-Firs, Sea-Anemones, and Corals. In 1834, Ehrenberg named the group Bryozoa[13] or Moss Animals.

With the exception of one genus (_Loxosoma_), all Polyzoa form colonies, which arise by the continual budding of the cells, the buds remaining attached to the parent cells. The colonies vary endlessly in form and habit, occurring as crusts on rocks, etc., masses, broad fronds, branching tree-like growths, bushy tufts, etc.

The texture and consistency may be gelatinous, cartilaginous, horny and flexible, or stony.

The great majority of species are marine, but a considerable number inhabit fresh water. The Polyzoa are classified as follows:—

{Sub-order 1. Chilostomata.[15]
{Order I. Gymnolæmata.[16] { Orifice of cell with a
{ Lophophore and tentacular { horny lid.
{ crown circular. {
{ Without a lobe over {Sub-order 2. Ctenostomata.[17]
{ the mouth. { Orifice of cell closed
{ { by a membranous
Group I. Ectoprocta.[14] { { comb-like frill.
Vent opens outside {Order II. Phylactolæmata.[18] { Always fleshy or
the circle of tentacles.{ Lophophore and tentacular { horny.
{ crown horse-shoe {
{ shaped. With {Sub-order 3. Cyclostomata.[19]
{ lobe over the mouth. { Without lid or frill;
{ Fresh water forms. { orifice of cell usually
{ circular; cells
{ always calcareous.

Group II. Entoprocta.[20] Vent opens inside circle of tentacles.

_Sub-order 1._—CHILOSTOMATA.

[Sidenote: Cases A and B 1.]

The Chilostomata, which contain many more species than all the other groups put together, are divided into three sections:—_A._ Cellularina, in which the cells are more or less boat-shaped or cornucopia-shaped, and joined together to form flexible branching colonies; _B._ Flustrina, in which the cells are typically shaped like oblong boxes with membranous front walls; and _C._ Escharina, in which the whole front wall is calcified.

[Sidenote: Case A 1.]

Fig. 6.

A, _Bugula turbinata_, natural size, B, portion × 50.

_a_, avicularia; _m_, mouth; _o_, ovicell.

[‘The Cambridge Natural History.’]
]

_Section A._ CELLULARINA.—_Bugula turbinata_, or the Bird’s-head Coralline (Fig. 6) grows attached to rocks near low water mark in the form of spiral tufts about two inches in height, composed of narrow flat branches in which the cells are arranged from two to six abreast and all facing upwards. Each cell is boat-shaped and with nearly the whole front surface membranous; the globular bodies at the head of certain cells are the ovicells. Attached to the outer edge of each cell is a remarkable object resembling a bird’s head, and hence termed avicularium, seated on a short stalk. The head and beak contain powerful muscles for opening and shutting a horny lid or mandible hinged on below. In life, the avicularium sways to and fro on its stalk, with the lower “jaw” continually snapping up and down in the most ludicrous fashion. The beak is capable of seizing and holding quite large objects.

The function of these curious appendages is partly to warn off trespassers and partly to capture and retain small animals till decomposition has set in; in the latter case, the currents set up by the tentacles draw in the particles to the mouths of the polypides. The avicularia have arisen by modification of the ordinary cells, in which the muscles have developed at the expense of the degenerated polypides, the cells have become much smaller, of different shape, and separated out from the rest; the mandible represents the lid or operculum of the ordinary cell. The avicularia vary greatly in size and shape in the different genera; in _Flustra_, for instance, these organs closely resemble the ordinary cells.

Fig. 7.

_Bugula bicornis._ Cells magnified. (After Busk.)
]

[Sidenote: Case A. Upright part.]

In _Bugula bicornis_(*) (Fig. 7), from 1950 fathoms in the Southern Indian Ocean, each cell is provided with two avicularia with remarkably long stalks. The graceful vase-shaped _Kinetoskias cyathus_(*) (Fig. 8), one of the treasures of the “Challenger” Expedition, was dredged from 1525 fathoms off Cape St. Vincent. The stem, which tapers gradually upwards, rises from a tuft of root fibres. The cup is formed of slender branches supported at the base by a delicate membrane. The branches are composed of biserial rows of cells (Fig. 9) opening towards the interior of the cup. The avicularia are pear-shaped and pedunclate. Probably, in life, the cup is capable of being opened out to a considerable extent. Specimens of this species were also obtained from 2160 fathoms in the South Atlantic.

Fig. 9.

_Kinetoskias cyathus._ A branch magnified.

_a_, an avicularium. (After Busk.)
]

[Sidenote: Case A 1.]

_Scrupocellaria reptans_, or the Creeping Coralline (Fig. 10 A, B) forms branching colonies, creeping over rocks and seaweeds, and attached by horny fibres often provided with curved hooks. The branches are composed of cells arranged in a double row. Each cell has the membranous area of its front surface protected by a branched flattened spine or operculum, and is produced and narrowed below; at the upper outer margin is a minute triangular avicularium. At the base of the back surface is a small sack-shaped cell with a cleft at the upper end, in which a horny bristle is articulated. The little cell is termed a vibracular cell, and the bristle a vibraculum.[21] This organ has arisen by a further modification of an avicularium, whereby the horny lid of the latter has become a long bristle. The bristles by their motion keep off intruders, and possibly act as scavengers by sweeping the surface of the cells.

Fig. 8.

_Kinetoskias cyathus._ (From Voy. Challenger, Atlantic: Wyv. Thomson.)
]

Fig. 10.

_Scrupocellaria reptans._ A. Creeping over seaweed, natural size; B.
Front surface, magnified.

_a_, branched spine covering front of membranous area; _b_,
avicularium; _c_, vibraculum.

C. Back surface; _a_, vibracular cell; _b_, vibraculum.
]

In _Caberia ellisii_ the vibracular cells are very large. The vibracula, which are long and serrated, have been observed to move in unison like a double row of oars.

[Sidenote: Case A 1, 2.]

_Section B._ FLUSTRINA.—In this group the colonies form leafy lamellæ, crusts, etc., in which the individual cells are typically in the form of oblong boxes with their front walls wholly or partly membranous. _Flustra foliacea_ has already been described. [Sidenote: Case A 1.] In _Flustra carbasea_ the fronds are formed of only one layer of cells, and not of two layers back to back as in _F. foliacea_. The fine specimen of _Flustra nobilis_ from S. Africa is so called from the large size of its long hexagonal cells which form a honeycomb pattern clearly visible to the naked eye.

[Sidenote: Case A 1.]

In _Flustra cribriformis_(*) (Fig. 11), from Torres Straits, the fenestrated frond forms a beautiful spiral. _Flustra florea_, from S. Australia, grows in the form of branching tufts of narrow spiral fronds. _Electra pilosa_ [dry and spirit specimens exhibited] (Fig. 12) forms a delicate silvery lace-work, encrusting shells and seaweeds (especially red algæ) on almost every shore. The long horny spine at the base of the membranous area of each cell gives the crust a pilose appearance. In _Electra verticillata_ from West Africa, the cells form an elegant branched colony, the branches being composed of regular verticils of cells.

Fig. 11.

_Flustra cribriformis._
]

Fig. 12.

_Electra pilosa._ A, incrusting a seaweed, natural size; B, cells
magnified; _a_, lid or operculum.
]

[Sidenote: Case A 1.]

_Membranipora membranacea_ occurs in the form of horny incrustations on bladder-wrack, which, owing to their flexibility, are able to adapt themselves to the swaying of the fronds of the Fucus.

The _Selenariidae_ (Case B 2) form free colonies, usually orbicular in shape, convex above and concave below. In _Lunulites capulus_ alternating rows of cells and vibracula radiate from the centre of the colony.

_Section C._ ESCHARINA.—In this group, the front walls of the cells are wholly calcareous. Many species form patches or crusts on shells etc., and hence the name of the section; other species, again, form stony tree-like growths, or thick plates. Frequently one and the same species occurs in the form of crusts or of erect lamellæ, the identity being recognised by the characters of the individual cells.

Often a large number of species may be found on one shell. Two good examples of this are exhibited in Case A 2.

[Sidenote: Case A 2.]

_Lepralia pallasiana_ (Fig. 13) forms sub-circular vitreous patches on stones and shells; the cells are rather large, broadly oval, and with the front wall punctured with pores; the aperture is squarish and with a slight indentation on each side.

Fig. 13.

_Lepralia pallasiana_, incrusting a shell. A, natural size; B, cells
magnified.
]

_Lepralia foliacea_ forms a massive coral-like growth composed of thin contorted plates which fuse to form labyrinthine cavities, the plates being constructed of a double layer of cells back to back. A large specimen from the English Channel is exhibited in Case B, upright part. In _Lepralia_ the orifice and lid of the cell have a straight lower margin, but one large group, _Myriozoidæ_, is characterised by having a notch in the lower margin of the orifice, (Fig. 14, _Schizoporella unicornis_).

[Sidenote: Case B 1.]

In many of the Escharina, the front wall of the cell is produced into a stout process or mucro at the lower margin of the orifice (genus _Mucronella_), or, again, a collar or tube grows up round the primary orifice, thus giving rise to a secondary orifice (_Smittia_, _Porella_, etc., Case B 1).

Fig. 14.

_Schizoporella unicornis_, magnified.
]

Fig. 15.

_Retepora beaniana._
]

[Sidenote: Case B 1.]

In the _Celleporidæ_, (Case B 1) the cells are typically pitcher-shaped and arranged vertically, and tend to be heaped up from the overcrowding.

_Cellepora pumicosa_ forms thick pumice-like masses composed of succeeding layers of cells. The _Reteporidæ_ (Case B 1) form delicate stony networks. The reticulate fronds may be expanded out, or may form tubular or contorted growths (Fig. 15, _Retepora beaniana_). The beautiful _Retepora phœnicea_ from Torres Straits is of a rich purple colour.

The _Adeonidæ_ form thick fenestrated plates which unite to form cavernous masses usually attached to rocks by a thick jointed stem. Several very fine examples from Port Phillip, Victoria, are exhibited in the upright part of Case B.

[Sidenote: Case A 2.]

The _Catenicellidæ_ are represented by a fine series of specimens from Australia. The colonies form dense clusters of finely beaded branches. The cells are arranged in single series, each cell being united to those above and below by a horny joint. The cells are usually urn-shaped with a triangular avicularium at each upper angle, and with the front surface variously sculptured with pores or bands (Fig. 16, _Catenicella ventricosa_).

Fig. 16.

_Catenicella ventricosa._ A, natural size; B, magnified. (After Busk.)
]

Sub-order 2.—CTENOSTOMATA.

[Sidenote: Case B 2, and A upright part.]

The Ctenostomata are fleshy, horny, or membranous; never calcareous. When the tentacles of a polypide are retracted into the cell, they are protected above by a membranous comb-like frill.

The cells either bud off from each other or arise as buds on a stolon or stem.

[Sidenote: Case A, upright part.]