[Illustration:
BRITISH MUSEUM (NATURAL HISTORY)
Ground Floor.
]
A GUIDE
TO THE
SHELL AND STARFISH
GALLERIES
(MOLLUSCA, POLYZOA, BRACHIOPODA, TUNICATA, ECHINODERMA, AND WORMS).
DEPARTMENT OF ZOOLOGY.
BRITISH MUSEUM (NATURAL HISTORY),
CROMWELL ROAD, LONDON, S.W.
_WITH NUMEROUS ILLUSTRATIONS._
PRINTED BY ORDER OF THE TRUSTEES.
1901
_All rights reserved._
LONDON:
PRINTED BY WILLIAM CLOWES AND SONS, LIMITED,
STAMFORD STREET AND CHARING CROSS.
PREFACE.
One of the large north galleries approached from the Bird Gallery is
devoted to the exhibition of the extensive Class of Mollusca. Specimens
of the types of all the principal divisions of this Class are exhibited,
either entire and preserved in spirit, or as models. However, as not the
animals, but their shells have always been a favourite object of study,
and a popular source of pleasure to collectors, the exhibition of the
species of shells has been made as complete as the space of this Gallery
admitted. It has thus proved adequate for the requirements of the
majority of visitors and students who consult this Collection. A
separate series of British Shells is exhibited in some small table-cases
along the west wall.
This Gallery also contains the exhibited series of Polyzoa, Brachiopoda,
and Tunicata.
The Starfish Gallery, so called from one of the best-known types of the
Echinoderma, contains an exhibition of the animals of this Class, as
well as of the somewhat heterogeneous assemblage of creatures which are
comprised under the popular name of Worms (_Vermes_). These animals
possess greater attraction to students of Natural History than to the
general public, and many, from their small size or the soft nature of
their body, are not suitable for exhibition. Therefore no attempt has
been made to show more than a carefully selected number of the types of
the larger groups. But the exhibition of very complete series,
supplemented by models or figures, to illustrate the remarkable life
history of some of these animals, also of specimens of the Worms which
possess a special interest from their relation to man, render this
Gallery particularly instructive to the student.
This guide has been prepared by Mr. E. A. Smith, Mr. F. J. Bell, and Mr.
R. Kirkpatrick, who have special charge of the collections described.
In conclusion, thanks are due to Messrs. A. & C. Black, Messrs. F. Warne
& Co., Messrs. Macmillan & Co., Crosby Lockwood & Son, and the Linnean
Society for kindly allowing the use of _clichés_ from illustrations in
various works published by them.
E. RAY LANKESTER.
TABLE OF CONTENTS.
THE SHELL GALLERY.
GENERAL NOTES ON MOLLUSCA 1
SYSTEMATIC ARRANGEMENT 7
AMPHINEURA (Chitons, etc.) 7
GASTROPODA (Marine Univalves, Land-Snails, etc.) 9
LAMELLIBRANCHIA (Bivalves, Oysters, Cockles, etc.) 29
CEPHALOPODA (Octopus, Cuttlefish, etc.) 45
INDEX TO PRINCIPAL GENERA OF MOLLUSCA 51
GENERAL NOTES ON POLYZOA 54
│CLASSIFICATION 57
AN ACCOUNT OF THE BRACHIOPODA 74
│CLASSIFICATION 77
AN ACCOUNT OF THE TUNICATA 83
│ARRANGEMENT 90
THE STARFISH GALLERY.
GENERAL ACCOUNT OF THE ECHINODERMA 106
CRINOIDEA (Lily Stars, etc.) 112
ASTEROIDEA (Starfishes) 113
OPHIUROIDEA (Brittle-Stars) 114
ECHINOIDEA (Sea-Urchins) 114
HOLOTHURIOIDEA (Sea-Cucumbers) 115
──────┬────────────────────────────────────────────────────────────────
VERMES│PLATYHELMINTHES (Tapeworms, etc.) 116
„ │NEMATODES (Round-worms, etc.) 118
„ │ANNULATA (Marine Worms, Earthworms, Leeches) 122
THE SHELL GALLERY.
GENERAL NOTES ON MOLLUSCA.
The Mollusca constitute one of the principal divisions of the Animal
Kingdom, and include such animals as the Octopus, Cuttlefish, Snail,
Slug, Whelk, Cockle, and Oyster.
[Sidenote: Definition.]
They may be characterized as soft, cold-blooded animals, without
distinctly marked external division into segments (as in Worms); their
cerebral ganglia (the centre of the nervous system) lie above the
commencement of the gullet, and are connected with the inferior ganglia
by nerve-chords. Their heart consists of two or more chambers, and is
situated on the dorsal side of the animal; it drives the blood into
spaces between the various organs of the body. Only the Cephalopods
possess internal cartilages, but all are without a bony internal
skeleton; in the majority this is compensated by an external hardened
shell which is formed (secreted) by the outer covering of the animal
termed the _mantle_. [Sidenote: The shell.] The shell may consist of two
parts (valves), as in the Oyster, or may be single, as in the Whelk and
Limpet, or composed of a series of plates, as in the “Coat-of-mail”
shells or _Chitons_: when well developed it is hardened by a rich
deposit of carbonate of lime; but it may be gelatinous, as in
_Cymbulia_, or altogether absent, as in _Octopus_; it may cover and
protect the body, as in the Oyster, lie within the folds of the mantle,
as in the Sea-hares (_Aplysiidæ_), or it may be quite internal, as in
the horny “pen” of the Squid. It may be elongated, as in the Elephant
Tooth-shell (_Dentalium_), cup-shaped, as in the Limpet, or spirally
coiled, as in the Snail.
[Sidenote: Description of the animal.]
The mantle may form a free fold on either side of the body, as in the
Bivalves, or it may become largely attached to the body-wall, as in the
Snail or the Slug, and so give rise to an air-chamber, which, when its
walls are richly supplied with blood, serves as a lung. The ventral
surface of Molluscs is produced into the so-called “foot,” which may be
very variously modified. The foot may be more or less hatchet-shaped, or
curved and capable of serving as a leaping-organ, or sole-shaped and
adapted for creeping; its margins may be produced into elongated
processes, as the so-called arms of the Octopus, eight in number and
provided with suckers, or of the Nautilus, where the arms are much more
numerous, but shorter and without suckers. In the Cephalopods, also,
another part of the foot may fold over from either side and form a
median funnel, through which the water of respiration is driven
outwards, causing the animal to move in the opposite direction—this part
of the foot having, therefore, still the function of an organ of
locomotion. By means of their muscular foot the _Solenidæ_, or
Razor-shells, burrow in the sand, the Pond-Snails (_Limnæidæ_) crawl on
aquatic plants and swim reversed on the surface of the water, the Limpet
clings to the rock, and the Cockles and _Trigonias_ take surprising
leaps.
[Sidenote: The operculum.]
Upon the upper surface of the foot, in many Gastropods, a flat hard
structure termed the _operculum_ is situated, which, when the animal is
retracted, partly or entirely closes the aperture of the shell. In some
cases, as in the _Turbos_, it is very strong and of a stony nature, but
in most instances it is horny. It is differently constructed in distinct
families: it may be annular and multispiral, annular and paucispiral,
subannular and ovate, or subannular and unguiculate. In the _Nerites_ it
is shelly, somewhat semicircular, closes the aperture of the shell, and
is furnished with a stout projection on the straight edge, fitting like
a hinge under the inner lip of the shell. A series of _opercula_ is
exhibited in side table-case C.
[Sidenote: The breathing-organs.]
Thread-like processes on either side of the body, the so-called
gill-filaments, often unite with those in front of and behind them, and
so give rise to plates; these, when well developed, are best seen in the
division to which the Oyster and the Mussel belong, and which,
therefore, has been called the division of the plate-gilled Molluscs, or
_Lamellibranchia_. Where the body is coiled or twisted on itself, as so
often happens, the gills of one side may be altogether lost. Sometimes,
as in _Phyllirhoë_, when the body is small and its wall thin, the gills
(_ctenidia_) disappear altogether, and there is no special
breathing-organ; in others the loss of the gill is compensated by the
formation by the mantle of a lung; this is most often seen in the forms
that live on land.
But these so-called gills may have other functions: in the
Lamellibranchs, where there is no head and no special means by which the
creature can obtain food, the delicate waving filaments or _cilia_ with
which they are covered cause currents in the surrounding water, by means
of which minute organisms are brought to the mouth.
[Sidenote: The radula.]
All Molluscs, except the Lamellibranchs, have a very remarkable
structure developed in the floor of their mouth-cavities; on a basis of
cartilage, which may be moved backwards and forwards by muscles, there
is developed a horny plate, which may be of considerable length, and
which has its upper surface covered with a number of more or less fine,
flattened, or spiny outgrowths, which are known as teeth. This is the
_odontophore_, _tongue_, _radula_, or lingual ribbon (see fig. 3).[1]
[Sidenote: The eyes.]
Eyes may be absent, as in nearly all the headless Lamellibranchs; but in
other Molluscs they are generally present, and may be more or less well
developed. An instructive series of stages is exhibited by the
Cephalopoda. In _Nautilus_ the eye remains an open pit; in
_Ommatostrephes_ two chambers appear, the anterior of which is bounded
posteriorly by the lens, and is open to the exterior, so that sea-water
enters it; in _Sepia_, finally, the anterior chamber becomes closed in
front. We may observe that the eyes of all Cephalopods are at first
pit-like, or pass through a stage which is permanent in _Nautilus_, one
of the geologically oldest types.
Cephalic eyes have recently been noticed in _Mytilus_ and _Avicula_.
Eyes of a more complicated structure, which are modified tentacles, are
sometimes found on the edges of the mantle in Lamellibranchs (e.g.
_Pecten_); these eyes resemble those of Vertebrates, and differ from
those of most invertebrate animals in having the fibres of the optic
nerve entering the distal and not the proximal ends of the retinal
cells. Eyes of a similar construction are to be found on the back of the
shellless _Oncidium_, and may be about one hundred in number.
Eyes of a remarkable character on the shells of some of the Chitons
appear to be modified from tactile organs, and are innervated like the
ordinary molluscan eye; they sometimes occur in enormous numbers, more
than ten thousand being present on one animal (see wax model, Case 2).
[Sidenote: Organ of hearing.]
In Cephalopods the ear, like the eye, is known to make its first
appearance in the form of an open pit, the mouth of which gradually
closes up, leaving only a narrow slit in communication with the
exterior. It is probable that in many forms the so-called ear is an
organ by means of which the mollusc becomes acquainted with changes in
the surface over which it is passing; it is often found deeply imbedded
in the substance of the foot, where it forms a closed vesicle.
[Sidenote: Sense of smell.]
There is no doubt that the carnivorous Gastropoda are gifted with a
sense of smell, and throughout the series we observe patches of modified
cells of the body-wall (the _osphradium_) which serve either as
olfactory organs or as an apparatus for testing the nature of the water
of respiration.
[Sidenote: The sexes and reproduction.]
The sexes are distinct in the most highly organized Mollusca, but are
united in the same individual in some of the lower forms, such as
Land-Snails, the _Opisthobranchia_ (including the Bubble-Shells,
Sea-Slugs, &c.), and in some Bivalves. The reproduction of Mollusca is
in all cases effected by means of eggs. In some instances the young are
actually hatched within the oviduct of the parent, as in the Freshwater
Snails (_Vivipara_); and apparently in most Bivalves the eggs are also
retained within the valves until hatched.
The ova of many molluscs are deposited in masses enclosed in capsules.
Some of them are very wonderful and complicated structures. Those of the
Cuttles and their allies are clustered like grapes, each capsule
containing but a single embryo; but in the Calamaries or Squids they
form a radiating mass of elongated sacks, each containing from thirty to
two hundred eggs, and it has been estimated that one of the
spawn-clusters of the Common Squid (_Loligo vulgaris_) contains as many
as 40,000 ova. Everybody knows the spawn-cases of the Common Whelk,
found so abundantly on the sea-beach, consisting of a large number of
yellowish capsules, heaped one upon another and forming an irregularly
rounded mass. As many as five or six hundred capsules may be piled
together in a single heap, each capsule containing several hundred eggs,
of which perhaps only thirty or forty are hatched.
In other genera, as _Tethys_, _Doris_, _Eolis_, &c., the eggs are
contained in a spirally rolled ribbon or strap-like structure; and some
of the _Naticæ_ build a somewhat similar capsule, composed of the eggs
cemented together by sand and a gelatinous material, the whole forming
two-thirds of a circle narrowed at the upper part.
Terrestrial Molluscs deposit, in comparison with their marine relations,
but very few eggs. They are sometimes covered by a thin soft skin, but
in certain groups, such as the large South-American _Strophochili_ and
the African _Achatinæ_, which include the largest of known
land-molluscs, they are protected by a hardened calcareous shell, in
some instances fully an inch in diameter. The freshwater forms (_Limnæa_
and _Physa_) deposit from thirty to a hundred eggs enveloped in a
gelatinous mass.
The number of eggs produced by some Bivalves is enormous. The Common
Oyster is said to produce a million or more, and the American variety
ten, or even sixty, times as many. Some of the River-Mussels are also
very prolific, as many as two millions being sometimes the product of a
single individual. A small series of the eggs of Land-Snails and of the
egg-capsules of some marine Gastropods is exhibited in side table-case C
at the side of the Gallery.
The ova of Mollusca may be gradually developed into the form of the
parent, or there may be a free-swimming larva, which has a circlet of
cilia near the anterior pole of its body (so-called “Veliger” larvæ), or
there may be special larvæ, as in the case of the Freshwater Mussel, the
“Glochidium,” as it is called, which has a toothed bivalve shell by
which it can fix itself to fishes.
[Sidenote: Duration of life.]
The limits of age of molluscs has been definitely ascertained in a few
instances only. Most Land-Snails probably live about two years, although
in confinement some have been kept alive for a much longer time. Some of
the marine forms live for a considerable period, the Common Oyster not
attaining full growth until about five years old, after which it may
continue to live for many years. The Giant Clam, a specimen of which is
placed in the upright cases near the entrance to the Gallery, must, one
would think, have a very long existence, judging from the size and
thickness of the shell. [Sidenote: Hibernation and torpidity.] All
terrestrial molluscs hibernate in cold climates, hiding themselves away
in the ground between roots and similar sheltered places. In tropical
countries some assume a state of torpidity (æstivate) during the hottest
and driest season of the year, closing up the aperture of their shells
with a temporary lid or door (epiphragm), in order to resist the dryness
of the atmosphere. Some of these “summer-sleepers” are endowed with a
remarkable tenacity of life. An Australian Pond-Mussel has been known to
live a year after being removed from the water; several Land-Snails have
revived after a captivity of from two to five years, without any food
whatever. One of the most remarkable instances of this kind occurred in
the British Museum. A specimen of _Helix desertorum_, a common
Desert-Snail from Egypt, was fixed to a tablet in March 1846, and in the
same month of the year 1850 it was discovered to be alive. It must have
come out of its shell in the interval, and finding it was unable to
crawl away, had again retired within it, closing the aperture with a new
epiphragm, but leaving traces of slime upon the tablet, which led to its
immersion in water and subsequent revival, having passed a period of
four years in a dry museum without the smallest particle of food. The
actual specimen is here figured, Fig. 1.
[Illustration:
Fig. 1.[2]
_Helix desertorum._
(See black table-case 1.)
]
[Sidenote: Economic uses.]
The _economic_ uses of molluscs to man are manifold, and will be
mentioned in the course of the description of the several families; but
here may be the place to direct the attention of visitors to side
table-cases B and D at the side of the room, containing some specimens
of articles manufactured from shells, such as cameos, flowers,
bracelets, brooches, &c.
[Sidenote: Geological history.]
Mollusca made their appearance on the globe at a very early epoch in the
history of the development of animal life, a large number of fossil
forms, such as _Nautilus_, _Lituites_, _Orthoceras_, &c., being found in
the oldest Palæozoic formations. Probably all these belonged to the
_Tetrabranchia_, of which one descendant only, the Pearly Nautilus, has
survived to our period. Some Gastropods and Bivalves coexisted with
those ancient Tetrabranchs; but these types abounded more in the later
geological epochs, many Tertiary forms being undistinguishable from
species which now exist.
[Sidenote: General distribution.]
The greater number of Mollusca are inhabitants of the sea, some passing
their whole life at the surface hundreds or thousands of miles away from
land; others at the bottom of the ocean at all depths, some having been
dredged at five miles from the surface. Many are found in much shallower
water, and a large number between tide-marks. Rivers and lakes furnish
an immense variety of forms, and vast numbers live on land in all
situations—on mountains, in valleys, forests, and deserts.
Molluscs are either animal or vegetable-feeders, the former preying
principally upon other members of their own class.
[Sidenote: Systemic arrangement.]
The following Table shows the systematic arrangement of the Mollusca
adopted in the Shell Gallery:—
Class I.—Amphineura.
Order 1.— Polyplacophora. Chitons.
„ 2.— Aplacophora. Neomenia, Chætoderma, etc.
Class II.—Gastropoda.
(Section Streptoneura.)
Order 1.— Scutibranchia. _Nerites_, Top-shells, Ear-shells, Limpets.
„ 2.— Pectinibranchia. Rock-snails, Whelks, Olive-shells,
Harp-shells, Cones, Strombs or Wing-shells, Periwinkles,
Carrier-shells, etc.
(Section Euthyneura.)
Order 1.— Opisthobranchia. Bubble-shells, Sea-hares, Umbrella-shells.
„ 2.— Pulmonata. Land and freshwater Snails, False Limpets.
Class III.—Scaphopoda. Tooth-shells (_Dentalium_).
Class IV.—Lamellibranchia. Bivalved Molluscs.
Order 1.— Protobranchia. Nucula, etc.
„ 2.— Filibranchia. Anomia, Common Mussel, Ark-shells, etc.
„ 3.— Pseudolamellibranchia. Pearl-Oyster, Hammer Oyster,
Wing-shells, etc.
„ 4.— Eulamellibranchia. Freshwater Mussels, Cockles,
Razor-shells, Ship-worms, etc.
„ 5.— Septibranchia. Poromya, etc.
Class V.—Cephalopoda.
Order 1.— Tetrabranchia. Pearly Nautilus.
„ 2.— Dibranchia. Octopus, or Poulp, Argonaut, Squids, and Cuttle
fishes.
Class I.—AMPHINEURA.
[Sidenote: Cases 1–3.]
The Molluscs of this class are characterized by bilateral symmetry. The
head and arms are situated at the opposite extremities of the elongated
body, the gills, genital ducts and circulatory organs being paired and
similar on both sides. The first order belonging to this division, the
Polyplacophora, includes the “Coat-of-mail shells,” or “Sea-woodlice”
(_Chitonidæ_). They have their back armed with eight shelly plates which
overlap one another like tiles, and, like woodlice, have the power of
rolling themselves into a ball. These plates are imbedded at the sides
into the fleshy mantle, beneath which, on each side of the foot, are
arranged the gills. A Chiton differs in many respects from other
Mollusca. It has a shell like an Isopod Crustacean, a heart down the
back like a sea-worm, symmetrical organs of reproduction on each side
like the bivalves, a head and crawling foot like a true Limpet, and a
posterior anal orifice. These several anatomical peculiarities at one
time induced certain eminent authorities to hesitate in considering them
molluscs; but now that the development from the egg has been
investigated, their association with the Mollusca may be considered
definitely settled.
[Illustration:
Fig. 2.
Coat-of-mail Shells, or Chitons.
1. _Chiton squamosus_ (upper surface).
2. _Chiton elegans_ (lower surface): _a_, mouth; _b_, foot; _c_,
mantle; _d_, gills.
]
Chitons are found in all parts of the world, the finest inhabiting
tropical countries. They live chiefly on rocks and under stones at
low-water or at moderate depths; but a few forms have been discovered by
the ‘Challenger’ Expedition at depths exceeding 2000 fathoms. The
numerous sections of the group are principally distinguished by
differences in the edges of the plates or valves which are inserted in
the mantle, and in the different kinds of ornamentation upon the upper
surface of the mantle-border. This, in some species, is quite smooth, in
others covered with a dense mass of minute grains or scales, and in
others armed with short prickly spines. In the giant _Cryptochiton_ of
Kamtschatka the plates are entirely covered over by the thick leathery
granular mantle, and in another set, _Cryptoplax_, which consists of
long slug-like animals, the plates are very small, and placed at
intervals along the back.
About five hundred living species are known, and about one-fourth that
number has been found fossil from the Silurian age downwards.
[Sidenote: Case 3.]
The second order of Amphineura, namely, the Aplacophora, comprises a few
somewhat worm-like Molluscs which are devoid of a shell, but have
instead the dorsal surface more or less studded with numerous minute
calcareous spines or spicules. _Neomenia_, _Proneomenia_, _Chætoderma_
are genera belonging to this order.
Class II.—GASTROPODA.
[Sidenote: Cases 4–135.]
In contradistinction to the preceding class these Molluscs are
asymmetrical, especially in respect of the gills and the spiral coiling
of the viscera and most of the shells. They may be divided into two
sections, _Streptoneura_ and _Euthyneura_, distinguished by differences
in the arrangement of the visceral nerve-loop.
Section STREPTONEURA.
[Sidenote: Cases 4–94.]
The Molluscs of this section are bisexual and furnished with a shell,
and generally with an operculum. The gills are in front of the heart and
the visceral nerve-loop is twisted into a figure of 8. The section
contains two groups or orders, Scutibranchia and Pectinibranchia.
Order 1.—SCUTIBRANCHIA.
The Scutibranchia have a free bipectinate gill, or the gill may be
absent (_Lepeta_, _Helicina_), and generally exhibit traces of bilateral
symmetry.
[Sidenote: Case 4.]
The _Acmæidæ_ are called False Limpets, because, although the shells are
identical with the true Limpets, the animals differ in having only a
small gill on the left side of the neck, whilst the _Patellæ_ have the
gills greatly developed all round the sides of the foot. Both the true
and the false Limpets are littoral and found on rocks between
tide-marks. They have the power of excavating the surface to which they
attach themselves, and adhere so firmly that it is easier to break the
shell than detach the animal. The largest known Limpet (_Patella
(Ancistromesus) mexicana_, case 7) inhabits the west coast of Central
America, its shell having sometimes a length of 12 inches. The Limpets
are vegetable-feeders and fond of seaweeds of various kinds, which they
rasp with their remarkable spiny tongues. That of the common English
Limpet (_P. vulgata_, Fig. 3) is longer than the shell itself, and armed
with as many as 1920 glassy hooks in 160 rows of twelve teeth each. The
Limpet is commonly used for bait in the sea-fishing off the Scottish
coast, and vast quantities are consumed as food in some parts of
Ireland. Some Limpets, such as _P. compressa_, _P. mytilina_, etc., are
found on the stems of floating seaweeds, and have the shells usually
thinner and smoother than the Rock-Limpets, which have to resist the
fury of the breaking waves.
[Illustration:
Fig. 3.
1. Radula of the Common British Rock-Limpet (_Patella vulgata_),
natural size.
2. Two transverse series of teeth: _a_, median teeth; _b_, laterals;
_c_, uncini or marginals.
]
[Illustration:
Fig. 4.
The Common Rock-Limpet (_Patella vulgata_). British.
1. Animal: _a_, foot; _b_, fringed mantle; _c_, tentacles; _d_, mouth;
_e_, eyes; _f_, gills.
2. Side view of shell, showing the impression or scar of the
attachment-muscle, _g_.
3. Upper surface of the shell.
]
[Sidenote: Case 8.]
The “Keyhole Limpets” and “Slit Limpets” (_Fissurellidæ_) resemble in
external shape ordinary Limpets, but are perforated at or near the apex,
or more or less slit at the front margin. The hole or slit gives passage
to a tubular fold of the mantle, through which the water apparently
flows to the gills. The largest species are from California and South
America, and others are found, but not abundantly, on most shores. The
animal of the large _Lucupina crenulata_ from California is eight or ten
inches in length, and almost conceals the shell, and the shell of the
South-African _Pupillæa aperta_ is also all but hidden beneath the
mantle of the animal.
[Illustration:
Fig. 5.[3]
_Pleurotomaria adansoniana._ Case 9.
⅓ natural size.
]
[Sidenote: Case 9.]
The _Pleurotomariæ_ are extremely rare in recent times, only five
species being known, whereas over a thousand fossil forms have been
described. The specimens of _P. adansoniana_ and _P. beyrichi_ exhibited
in case 9, are among the finest acquisitions to the shell collection of
recent years.
[Sidenote: Cases 9–11.]
The “Ear-shells” or “Ormers” (_Haliotidæ_) are found adhering to rocks
in most parts of the world, with the exception of South America. They
are lined with _pearl_, and many exhibit splendid colours and sculpture
externally. Like the Limpets they hold on to the rocks with such
tenacity that it is absolutely impossible to remove some of the larger
species by force without injuring the shell. Boiling water or mustard
and water poured over them will, however, soon compel them to relinquish
their hold. The shell of _Haliotis_ is pierced by a series of holes
parallel with the left margin. Through such of them as are open the
animal protrudes a slender filament or feeler, and the water also finds
its way through them to the gills beneath.
The Single British species (_H. tuberculata_) is not actually found on
the English coast, but common on rocks and stones at low-water in the
Channel Islands. It is frequently eaten by the poor of those islands and
the north of France; other species in New Zealand, China, Japan, West
Africa, and elsewhere, constitute a common article of diet among the
natives. _Haliotis_-shells are largely used in the manufacture of pearl
ornaments, and in all kinds of inlaid work.
[Illustration:
Fig. 6.
Top-shell (_Turbo petholatus_). (From the Indo-Pacific Ocean.)
_a._ Inner surface of operculum. _b._ Exterior of ditto.
]
[Sidenote: Cases 12–18.]
The _Trochidæ_, and _Turbinidæ_ are two extensive families, the animals
of which are very much alike, and mainly distinguished by the operculum,
which in the former is horny, and shelly in the latter. The shells of
these families are beautifully pearly within, and the external shelly
coat is generally brightly coloured and highly ornamented. Several very
pretty species are found on our own shores. The opercula of _Turbo
petholatus_ (Fig. 6), from the Indian and Pacific Oceans, are frequently
mounted in gold and silver as scarf-pins, ear-rings, &c.
[Sidenote: Cases 18–20.]
The _Nerites_ are mostly found in tropical countries, and, like the
Winkles, are very strongly made, to resist the force of the breaking
waves. The _Neritinas_ are partly found in the sea, and partly in fresh
water, and are less solid shells. The third section of _Neritidæ_, the
_Septariæ_, are shaped very much like Limpets, except that the apex is
at one end instead of central. They are, however, very different
animals, and furnished with a shelly operculum imbedded in the foot.
Order 2.—PECTINIBRANCHIA.
[Sidenote: Cases 22–94.]
In most cases the molluscs of this order have an attached monopectinate
gill and a single osphradium. A few are fresh or brackish water forms,
but the majority are marine.
[Sidenote: Case 22.]
The “River-Snails” (_Viviparidæ_) might be termed freshwater
Periwinkles, as the animals of both are very similar. The true
_Viviparæ_ are viviparous. They are rather sluggish, and found at the
bottom of ponds and rivers feeding on decaying animal and vegetable
matter.
[Illustration:
Fig. 7.
The Common British River-Snail (_Vivipara vivipara_).
_a_, head; _b_, tentacles; _c_, eyes; _d_, foot; _e_, operculum.
]
[Sidenote: Cases 23–25.]
The _Cyclophoridæ_ are land-shells, which, however, cannot properly be
considered true lung-breathers like ordinary snails. They have not the
closed lung-chamber of the Pulmonates, their eyes are placed at the base
of the tentacles instead of at their tips, they have a long proboscis
armed with a different rasping tongue (_radula_), a spiral operculum,
and the sexes are distinct, whereas the true Snails are hermaphrodite.
The operculated air-breathers have been divided into many sections,
chiefly on account of differences in the apertures of the shells and in
the opercula. They most abound in hot countries, but a few species are
met with in temperate regions.
[Sidenote: Cases 25–26.]
The “Apple-Snails” (_Ampullariidæ_) live in the rivers and marshes of
tropical regions, and, although represented by a large number of
species, exhibit comparatively slight variations in form and colour. The
animal has both a pectinated gill and a lung cavity, being thus enabled
to breathe either water or air.
[Illustration:
Fig. 8.
_Ampullaria canaliculata._
]
[Sidenote: Cases 27–28.]
The “Periwinkles” (_Littorinidæ_) are found almost on every known shore;
they feed upon all kinds of marine vegetation. Some species are met with
at low-water mark, others on rocks almost beyond the reach of the sea,
and some have been discovered inland nearly half a mile away from the
shore. It is calculated that 1900 tons of the “Common Periwinkle”
(_Littorina littorea_), of the value of £15,000, are annually consumed
in London alone.
[Sidenote: Cases 31–32.]
The family of _Calyptræidæ_ includes the “Slipper-Limpets” (_Crepidula_)
and the “Cup-and-saucer Limpets” (_Crucibulum_). Although furnished with
a foot, they rarely crawl about, but remain attached to rocks, stones,
or other shells, sometimes forming a shelly plate under the foot by
which they become fixed to the spot where they have taken up their
abode.
[Sidenote: Cases 32–35.]
The “Cowry-shells” (_Cypræidæ_) are remarkable for their varied markings
and splendid polish, which is produced and preserved by two flaps of the
mantle, one on each side, which fold over the back, a line down the
centre of which usually marks where the flaps meet. The animals are even
more brilliantly coloured than the shells. They have no operculum, but a
large foot, which they can withdraw entirely within their shell,
although the aperture is usually very narrow. Cowries, as is well known,
are sold as ornaments; and a small yellow species, “the money-cowry”
(_C. moneta_), which is very common in the Indian and Pacific Oceans,
passes current as coin among the negro tribes of certain parts of
Africa. The specimen of _Cypræa leucodon_ figured on p. 16 is extremely
valuable and supposed to be the only one hitherto discovered. The
“orange cowry” (_Cypræa aurora_) is worn by chiefs in the Friendly
Islands, and is considered the highest order of dignity. Only one small
species, _Trivia europæa_, is found on the British coast, and about 100
fossil forms have been discovered in the Chalk.
[Illustration:
Fig. 9.
The Tiger Cowry (_Cypræa tigris_). (From the Indo-Pacific Ocean.)
_a_, the shell; _b_, the mantle; _c_, foot; _d_, siphon; _e_,
proboscis; _f_, tentacles; _g_, eyes.
]
[Sidenote: Case 35.]
Of the _Ovulidæ_, the most curious is the “Weaver’s-shuttle” (_Radius
volva_), in which the shell is peculiarly beaked at both ends. It is
found living on barked corals (_Gorgoniidæ_), and some of the smaller
species exhibit differences of coloration, resembling the tints of the
Gorgonias upon which they are found.
[Sidenote: Cases 36–37.]
The Naticas are mostly blind, and have a very large foot, suitable for
burrowing in the sand when in quest of bivalves. They are very
voracious. This is one of the groups of shells that have continued to
exist from Palæozoic times.
[Sidenote: Case 38.]
The “Violet Snails” (_Ianthinidæ_) are found floating about in every
ocean, excepting in cold regions, with the spire of the shell downwards,
and the bottom, being more exposed to the action of light, is more
deeply tinted than the upper part. They feed upon Jelly-fish, and
construct a gelatinous raft, filled with air-bubbles, beneath which the
females attach their eggs.
[Illustration:
Fig. 10.
_Cypræa leucodon_. Case 32.
]
[Sidenote: Cases 38–41.]
The _Melaniidæ_ are freshwater Snails which abound in most tropical and
subtropical countries; about 1000 species are known. They are mostly of
dark colours, and are fond of muddy places.
[Sidenote: Cases 42–43.]
The _Cerithiidæ_ are chiefly marine forms, some, however, entering
brackish water. About five hundred fossil species have been described,
some of them gigantic in comparison with any now living, of which more
than two hundred are known.
[Sidenote: Case 44.]
The _Scala scalaris_ was formerly considered a great rarity, as much as
£40 having been given for a single specimen, which might now be
purchased for as many pence.
[Illustration:
Fig. 11.
_Scala scalaris._
Case 44.
]
[Sidenote: Cases 46–47.]
The “Worm-shells” (_Vermetidæ_) are a very peculiar family. Their shells
can scarcely be distinguished from the shelly tubes which are formed by
certain species of marine worms, _Serpula_, &c. They are free and spiral
in early life, but afterwards become distorted and generally attached to
rocks, stones, &c. A foot for walking purposes therefore would be of no
use; consequently it is more or less obsolete, serving only as a support
to the operculum.
[Sidenote: Case 48.]
The “Screw-shells” (_Turritellidæ_) have elongate tapering shells; about
100 recent and 200 fossil species are known. One species only
(_Turritella communis_) is now found living on the British coasts.
[Sidenote: Case 49.]
The _Xenophoridæ_ have the singular habit of cementing to the exterior
of their shell, stones, pieces of coral, and fragments of other shells;
hence they have been called “Carrier-shells,” and, according to the kind
of material chosen, have been named “Conchologists” and “Mineralogists.”
Beyond acting as a disguise, and consequently as a protection, there
does not appear to be any special utility in thus adding to the weight
of their own shells. The animals do not glide like most other molluscs,
but scramble along like the Strombs, the form of their foot being small,
divided into a front, expanded, and a hind, tapering portion admirably
adapted to the nature of the ground on which they live, which usually
consists of broken and dead shells.
[Sidenote: Cases 49–52.]
The “Wing-shells” (_Strombidæ_) are the largest of the Gastropods with a
proboscis or non-retractile snout. They do not crawl like most other
Gastropods, but progress by a sort of hopping movement. They act as
scavengers, feeding on decomposing animal matter.
The _Strombus gigas_, or “Fountain-shell,” occurs in great numbers in
the West Indies, and is a very heavy solid shell. It is a favourite
ornament for rockwork and fountains in gardens, and, like the
Helmet-shells, is used for cameo-carving. It is also employed in the
manufacture of porcelain, as many as 300,000 having been imported into
Liverpool in one year for that purpose.
[Sidenote: Cases 51–52.]
The Scorpion-shells, or “Spider-claws,” as they are sometimes called
(_Pterocera_), possess singular claw-like projections, which are
developed on the outer lip of the shells.
[Sidenote: Cases 53–54.]
The “Trumpet-shells” (_Lotoriidæ_) have varices or strengthening ribs at
intervals, like the Murices; the largest species, _Lotorium variegatum_,
is used by South-Sea Islanders as a horn or trumpet. A hole is made in
the upper part of the spire to blow through, and the sound produced can
be modulated or varied by inserting the hand in the aperture or mouth of
the shell.
[Sidenote: Cases 55–56.]
The “Helmet-shells” (_Cassididæ_) are used for cameo-carving; they
consist of differently coloured layers, so that the ground-colour of the
carving is of a different tint from the subject engraved. The most
artistic shell-cameos are produced in Italy, whence the art has been
introduced into France and England. The _Cassis madagascariensis_ (Fig.
12 on p. 19) is in special request by shell-carvers on account of the
strong contrast of the white upper layer with the dark ground beneath.
Extinct forms of _Cassis_ are found fossil in Tertiary formations, but
none of them equal in size the largest living species.
[Sidenote: Case 57.]
The “Tun-shells” (_Doliidæ_) are remarkable for the globoseness of the
shells, which are covered with very regular revolving ribs.
[Sidenote: Cases 58–60.]
The _Fasciolariidæ_ contains two of the largest living Gastropods:
_Megalatractus aruanus_, from North and West Australia, and _Fasciolaria
gigantea_, which is found off the coast of South Carolina, and attains
at times a length of two feet.
[Sidenote: Cases 61–64.]
The Mitras (_Mitridæ_) are great favourites with shell-collectors, on
account of their beautiful colours and varied sculpture. There are about
600 living species already known, and between one and two hundred have
been found in a fossil state. Shells of this group, like the
_Fasciolariæ_, are distinguished by a few plaits or folds on the inner
side of the aperture (the columella). Mitras are almost exclusively
found in tropical or subtropical regions, the majority being met with
either at low-water mark or in comparatively shallow water.
[Sidenote: Cases 64–66.]
The family of _Buccinidæ_ also contains a very large and various
assemblage of forms. Among them may be mentioned the Whelks
(_Buccinum_). (See Fig. 13 on p. 19.)
[Illustration:
Fig. 12.
_Cassis madagascariensis_, with cameo engraved upon it.
Side table-case B.
]
[Illustration:
Fig. 13.
The Common Whelk (_Buccinum undatum_).
_a_, siphon; _b_, foot; _c_, tentacles; _d_, eyes; _e_, operculum.
]
[Sidenote: Cases 70–76.]
The family of _Muricidæ_, or “Rock-shells,” is another extensive group,
containing many very handsome and peculiar forms. The animals of this
family have a long proboscis, at the end of which is the spiny tongue
(radula), and which is retractile within the body. The true Murices
produce at intervals ribs or varices, which in some species are
ornamented with long spines or foliations, and which indicate periods of
growth, but of what duration we do not know. They are all carnivorous,
feeding chiefly on other Mollusca, boring through the shells of bivalves
with their spiny tongue, and slowly devouring the unfortunate inhabitant
piecemeal. From certain species of _Murex_ (_M. brandaris_, &c.) found
in the Mediterranean, the ancients manufactured the celebrated Tyrian
purple dye.
[Sidenote: Cases 74–75.]
The “Purples” (_Purpura_) are found between tide-marks all over the
world. _Magilus_, belonging to the family _Coralliophilidæ_ (Case 77),
is found among coral-reefs in tropical seas, and has the remarkable
habit of lengthening the aperture of its shell into an elongate tube, in
order to keep pace with the growth of the coral, and to prevent its
being overgrown and killed.
[Sidenote: Cases 78–81.]
The “Volutes” (_Volutidæ_) are a group of shells also much sought after
by shell-collectors. Some of these attain to a very large size, the
animals inhabiting them being enormous. The Boat-shells (_Cymba_) and
Melons (_Cymbium_) are ovo-viviparous, the young being carried about by
the parent until they are an inch in length. Volutes are found chiefly
in the warmer parts of the Atlantic and Indo-Pacific Oceans, and occur
in the greatest variety on the coasts of Australia.
[Sidenote: Cases 81–83.]
The Olives (_Olividæ_) are common in most tropical seas, and are
remarkable for their beautiful polish and various patterns of colouring.
In structure and form they are very similar to each other. They burrow
in sand in quest of bivalves for food, and some species are said to have
the power of swimming by expanding the lobes of the foot.
[Sidenote: Case 84.]
The Harps (_Harpidæ_) form a small well-marked group, of which probably
nearly all the existing species have been discovered. The animals
inhabiting these beautiful shells are also brightly coloured. They have
the remarkable power of casting off a portion of the foot when
disturbed. The species are known from the Indo-Pacific Ocean, the west
coast of Central America, and West Africa.
[Sidenote: Cases 85–87.]
The next family, the “Slit-lips” (_Pleurotomatidæ_), consists of very
numerous species, over a thousand living forms having been discovered,
and almost as many fossil species from Cretaceous and Tertiary strata
have been described. The typical forms are characterized by a slit in
the outer side (lip) of the aperture. Species of _Pleurotoma_ are found
in every sea, although most abundant in the tropics, and, although so
numerous in species, the number of specimens is small in comparison with
some other genera.
[Sidenote: Cases 87–89.]
The “Auger-shells” (_Terebridæ_), like the Cones, present a great
similarity in form, but, unlike them, have a great diversity of
“_sculpture_” or external ornamentation. They are all elongate shells,
with a deep notch at the base of the aperture. Owing to the length and
comparative solidity of the shells, the animals of many of the species
do not carry their shelly structures on their backs, like most other
species, but drag them along the sandy sea-bottom.
[Illustration:
Fig. 14.
The “Glory-of-the-Sea” Cone (_Conus gloria-maris_). Case 94.
(From the Philippine Islands.)
]
[Sidenote: Cases 89–94.]
The _Conidæ_, or Cones, form one of the most beautiful portions of the
collection of Shells. This family, of which between 400 and 500 distinct
kinds are known, is a great favourite with collectors on account of the
brilliant colours and various patterns of the shells. Some, owing to
their beauty and rarity, have been sold at very high prices, as much as
£50 having been paid for a single shell. The Cones are found in all
tropical seas, but are rare in cold or temperate latitudes. None are met
with on our own shores, one species alone being known from the
Mediterranean. They occur fossil in the Chalk and Tertiary strata. These
animals are all carnivorous, and live usually in shallow water among
rocks and coral-reefs. Some of them are said to bite when handled, and
to be dangerously poisonous, the bite in some instances having been all
but fatal.
[Sidenote: Case 94.]
The _Atlantidæ_, _Pterotracheidæ_, and _Carinariidæ_, at various times
recognized as forming a distinct sub-class or order of Gastropoda, under
the name of Heteropoda or Nucleobranchiata, are now regarded as families
of aberrant Gastropods organised for swimming in the open sea. The
_Atlantas_ are found in great numbers in warm latitudes, and are
provided with a glassy, thin, flat, spiral shell, not unlike a keeled
Ammonite. The glassy shell of the _Carinaria_ is one of the most
beautiful structures of any mollusc, and at one time was such a rarity
that £100 are said to have been given for a single specimen, which at
the present time is perhaps worth only from five to ten pounds. Species
of _Carinaria_ are found in the Mediterranean and warmer parts of the
Atlantic and Indian Oceans. The animal is large, semitransparent, and
elongate, with a compressed fin-like foot which projects from the body,
and is used in swimming. The gills are placed towards the hinder part of
the back and covered by the shell. They feed on jelly-fish of various
kinds, and probably on other soft animals.
[Illustration:
Fig. 15.
Glassy Nautilus (_Carinaria lamarcki_).
_a_, proboscis; _b_, tentacles; _c_, shell; _d_, gills; _e_, foot;
_f_, sucker.
]
Section EUTHYNEURA.
[Sidenote: Cases 94–135.]
The Gastropods belonging to this sub-class have the visceral nerve-loop
straight and not twisted as in the STREPTONEURA. All the EUTHYNEURA are
hermaphrodite, and their radula is generally composed of numerous
similar denticles on each side of a median tooth. Scarcely any of these
forms are provided with an operculum in the adult state. The EUTHYNEURA
may be divided into two orders, _Opisthobranchia_ and _Pulmonata_.
Order 1.—OPISTHOBRANCHIA.
[Sidenote: Cases 94–97.]
All the Molluscs of this order are marine, some (_Tectibranchia_)
breathing by means of the ordinary Gastropod ctenidium, which is
generally behind the heart, whereas others (_Nudibranchia_) have
developed a different type of respiratory organs.
The _Opisthobranchia_ include the “Pteropods” formerly considered as a
distinct class, the “Bubble-shells” (_Bullidæ_), the “Sea-Hares”
(_Aplysiidæ_), the “Umbrella-shells” (_Umbraculidæ_), the _Nudibranchs_
and some others.
[Illustration:
Fig. 16.
Shell-bearing Pteropod (_Cavolina tridentata_). Case 96.
_a._ Shell and animal. _b._ Side view of shell. _c._ Dorsal view of
shell.
]
The Pteropods[4] are sometimes called Sea-butterflies, and are organized
for swimming freely in the ocean. They have a pair of fins developed
from the sides of the mouth or neck, which perform a flapping movement
during progression. Some Pteropods (Thecosomata) are provided with small
glassy shells; others (Gymnosomota) are naked. They exist in countless
millions in some parts of the ocean, discolouring the water for miles.
They constitute the principal food of the Baleen Whales.
[Illustration:
Fig. 17.
Shell-less Pteropod (_Clione limacina_).
_a._ Dorsal view. _b._ Ventral aspect.
]
About a hundred species are known.
[Sidenote: Case 96.]
The Sea-Hares, so called on account of a slight resemblance to a
crouching hare and not for their nimbleness of foot, are found in most
parts of the world, in pools at low water. At the hinder part of the
back two flaps of the mantle partly conceal a thin horny shell which
serves as a protection to the gills and vital organs beneath. When
molested, these animals discharge a large quantity of a purple fluid,
discolouring the surrounding water for a distance of more than a yard.
[Illustration:
Fig. 18.
Sea-Hare (_Tethys (Aplysia) punctata_). British.
_a_, labial tentacles; _b_, upper tentacles or rhinophores; _c_,
siphonal fold of the mantle near the shell; _d_, eye.
]
[Sidenote: Case 97.]
The shell of _Umbraculum_ is shaped very like that useful article, an
umbrella, of the Chinese pattern. The animal is very large, having its
breathing-organs on the right side below the shell.
[Sidenote: Case 97.]
The _Nudibranchs_ or Naked-gilled Molluscs comprise some of the most
beautiful and strange forms. They are unprovided with shells except in
the earliest stages of their existence, when they dwell in a minute
nautiloid shell, furnished with an operculum, both of which are
subsequently cast off. Unfortunately the colours of these beautiful
creatures cannot be preserved after death, and therefore a small series
of glass models is exhibited, which will give some idea of their great
variety in form and colouring. They are found in most parts of the
world, chiefly in shallow water, but a few species live upon floating
seaweed in the open sea. Over a hundred species exist on the British
coast, the majority of which are, however, very small. They are chiefly
carnivorous, feeding on other molluscs, sea-anemones, &c.
[Illustration:
Fig. 19.
The Umbrella-shell (_Umbraculum mediterraneum_).
_a_, shell; _b_, gills; _c_, tentacles; _d_, mouth; _e_, foot.
]
[Illustration:
Fig. 20.
Naked-gilled Mollusc, or Nudibranch (_Doto coronata_).
_a_, head; _b_, foot; _c_, gills; _d_, tentacle-sheath; _e_, tentacle.
]
Order 2.—PULMONATA.
[Sidenote: Cases 97–135.]
The Pulmonata are furnished with a lung cavity in place of the ordinary
gill of other Gastropods, and may be termed true air-breathers. Most of
them are provided with shells, and, with the exception of the
_Amphibolidæ_ never possess an operculum. They are divisible into two
groups or sub-orders, _Basommatophora_ and _Stylommatophora_,
characterized by a difference in the position of the eyes. The
Basommatophora, including the _Auriculidæ_, _Amphibolidæ_,
_Siphonariidæ_, and _Limnæidæ_, have a single pair of non-retractile
tentacles, at the base of which are situated the eyes. The
Stylommatophora (Land-snails, Slugs, &c.), are provided with two pairs
of retractile tentacles, with the eyes at the summit of the upper pair.
Over ten thousand species of Pulmonata are known.
(_Basommatophora._)
[Sidenote: Cases 97–98.]
The first group of the aquatic air-breathers, the _Auriculidæ_, chiefly
inhabit salt or brackish water. The largest forms are tropical and found
at the mouths of rivers, among the roots and stems of mangrove-trees, or
in damp woods near the sea.
[Sidenote: Cases 98–99.]
The “Limpet-Snails” (_Siphonariidæ_) seem at first sight to be out of
place among the Snails and Slugs, and more nearly allied to the
Rock-Limpets; but the character of the tongue (radula) and the closed
respiratory cavity indicate a close relationship with the present group.
[Illustration:
Fig. 21.
Three Rows of Teeth of the Radula of _Siphonaria_.
_c_, central; _l_, lateral teeth.
]
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.
]
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.
[Illustration:
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.
[Illustration:
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.
[Illustration:
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.
[Illustration:
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.
[Illustration:
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.
[Illustration:
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]
[Illustration:
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.
[Illustration:
Fig 40.
The Common Octopus (_Polypus vulgaris_), resting.
]
[Illustration:
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.
[Illustration:
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.
[Illustration:
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.
[Illustration:
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.
[Illustration:
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.
[Illustration:
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]
[Illustration:
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.]
[Illustration:
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.
[Illustration:
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.
[Illustration:
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.
[Illustration:
Fig. 8.
_Kinetoskias cyathus._ (From Voy. Challenger, Atlantic: Wyv. Thomson.)
]
[Illustration:
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.
[Illustration:
Fig. 11.
_Flustra cribriformis._
]
[Illustration:
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.
[Illustration:
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).
[Illustration:
Fig. 14.
_Schizoporella unicornis_, magnified.
]
[Illustration:
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_).
[Illustration:
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.]
_Alcyonidium gelatinosum_(*) (Fig. 17), so called from its resemblance
to the zoophyte Alcyonium, forms fleshy translucent growths occurring in
the form of nodulated branched masses, or of long finger-like growths.
The species is common round our coasts where it grows attached to stones
and shells near low-water mark.
[Illustration:
Fig. 17.
_Alcyonidium gelatinosum._ A, a small piece, natural size; B, the same
magnified.
]
[Sidenote: Case A, upright part.]
_Amathia_ forms bushy growths composed of slender horny branches. The
cells, which are cylindrical or squarish, rise from the branches in
biserial rows like Pan’s pipes. In _Amathia lendigera_(*) (Fig. 18) the
groups of cells are well separated from each other, but in _A.
spiralis_(*) and _A. convoluta_(*) the cells form a nearly or entirely
continuous series winding in a spiral round the slender stems. In
_Bowerbankia imbricata_(*) the cells are clustered on the stems.
_Vesicularia spinosa_(*), or the Silk Coralline, forms delicate brown
tufts resembling a filamentous alga; the cells arise separately in a
single series from the hollow tubular stems and are contracted at their
point of attachment.
[Illustration:
Fig. 18.
_Amathia lendigera._ A, natural size; B, magnified.
]
Nearly all the Ctenostomata are marine, but a few species live in fresh
water.
Sub-order 3.—CYCLOSTOMATA.
[Sidenote: Case B 2.]
In the CYCLOSTOMATA, which are all calcareous, the usually tubular
zoœcia have plain circular orifices without a lid or frill closing over
the retracted tentacle-sheath. There are two sections in this group,
viz., Articulata, in which the cells form branching colonies, the
branches being connected by horny joints; and Inarticulata, in which the
colonies may be encrusting, or erect and branching, but are without
joints.
The first section includes the _Crisiidæ_.
[Sidenote: Case B 2.]
_Crisia denticulata_ (Fig. 19) forms delicate white tufts, in which the
flat slender branches are composed of a double row of tubular cells. The
horny joints between the branches are black. The Inarticulata occur as
crusts or branching growths. In _Tubulipora flabellaris_ (Fig. 20) the
colonies form little fan-shaped crusts on seaweeds. _Lichenopora
hispida_ forms little white disks, in which rows of tubular cells
radiate from the centre. In _Idmonea_, the colony is branched, the
tubular cells being arranged in parallel rows on each side of the middle
line of the branch.
[Illustration:
Fig. 19.
_Crisia denticulata._ A, natural size; B, branches magnified.
]
The Cyclostomata are all marine.
[Illustration:
Fig. 20.
_Tubulipora flabellaris._
_a_, half of an incrusting colony, × 8; _b_, a few cells, × 44; _c_, a
colony, natural size.
]
Order II.—PHYLACTOLÆMATA.
[Sidenote: Table Case A, upright part.]
All the forms in this group inhabit fresh water, where, in the form of
creeping or erect branching growths or masses, they grow attached to
freshwater plants, tree-trunks, old wood, etc.; two species are capable
of slow movement from place to place. The lophophore and tentacular
crown of the polypide are horseshoe-shaped. The Order owes its name to
the presence of a lobe guarding the mouth.
In addition to the sexual, there is an asexual reproduction by means of
peculiar internal buds termed statoblasts (Fig. 21). When the colony
dies in the autumn, the liberated buds, securely protected in a horny
capsule, retain their vitality till the spring; in due season the valves
of the statoblast burst open, and the contents develope into a new
colony. The statoblasts, which resemble small seeds, are usually
provided with a ring of air cells, which act as a float, and in some
species spines are present.
[Illustration:
Fig. 21.
Statoblasts of Freshwater Polyzoa. A, _Fredericella sultena_ × 38; B,
_Plumatella repens_ × 38; C, _Lophopus crystallinus_ × 28; D,
_Cristatella mucedo_ × 28.
[‘The Cambridge Natural History.’]
]
[Illustration:
Fig. 22.
A, _Plumatella repens_, partly free, partly incrusting stem of
water-weed. B, Cells magnified. (After Allman.)
]
[Sidenote: Case A, upright part.]
_Plumatella repens_(*) (Fig. 22) forms brown branching colonies, wholly
or partly adherent to the surface of leaves of water plants, old wood,
etc. The individual cells are club-shaped, and about a quarter of an
inch long, each cell being attached to the upper back part of the cell
below; the statoblasts (Fig 21, B) are simple oval bodies with a zone of
air cells.
_Plumatella_ (_Alcyonella_) _fungosa_ forms thick masses, composed of
closely packed vertical tubes. A small specimen(*) surrounding a stick
from Hampstead Ponds is exhibited.
_Lophopus crystallinus_(*) occurs in the form of translucent gelatinous
blobs, often attached to the slender stems of duck-weed. The statoblasts
(Fig. 21, C) are elliptical and pointed at each end. The polypides are
comparatively large, and can be easily observed through the transparent
surface. When its delicate plumes are fully expanded, _Lophopus_ forms a
beautiful object.
_Cristatella mucedo_(*) (Fig. 23) occurs in the form of greenish
translucent oval or worm-like colonies with the polypides on the convex
upper surface. The animal slowly creeps about on its flattened under
surface.
[Illustration:
Fig. 23.
_Cristatella mucedo_, creeping over a stem of water-weed; × 6. (After
Allman.)
_a_, polypides with horseshoe-shaped crown of tentacles; _b_,
statoblasts seen through the tissues; _c_, muscular sole by means of
which the animal creeps; _d_, stem of water-weed.
]
Freshwater Polyzoa usually prefer dark places, but _Cristatella_ creeps
along on the stones and pebbles in clear water, and in the sunlight. The
polypides form three or more concentric rows on the upper surface. The
statoblasts (Fig. 21, D) are circular, provided with a zone of air
cells, and with hooked spines, the total diameter being about ¹⁄₃₀ of an
inch.
Sub-class II.—ENTOPROCTA.
[Sidenote: Table Case A, upright part.]
In this small group, both orifices of the alimentary canal open within
the circle of tentacles, and there is no tentacular sheath.
[Illustration:
Fig. 24.
_Pedicellina cernua._ × 27.
[‘The Cambridge Natural History.’]
]
The polypides are borne on contractile stalks. In the _Pedicellinidæ_
the stalks arise from a creeping stolon. In _Pedicellina cernua_(*)
(Fig. 24) a stolon, creeping over seaweeds, etc., gives rise to stalked
cups, the movements of which are vigorous: “the polypides, when excited,
dash themselves vehemently from side to side. The heads are easily
knocked off, but the decapitated stalks develop fresh ones. In
_Ascopodaria_ the stalks are swollen at the base; _A. fruticosa_(*),
from Port Phillip, Victoria, forms beautiful tree-like colonies. The
_Loxosomidæ_ do not form colonies, owing to the buds becoming detached
from the parent. The species of _Loxosoma_ are always found associated
with some other animal, such as a worm or Tunicate. The tentacles of the
polypide are arranged obliquely to the long axis of the body, hence the
name of the family (_loxos_, oblique). _Loxosoma phascolosomatum_(*)
occurs, in the form of delicate tufts, on the caudal end of the
Sipunculid worm _Phascolosoma_. The individuals resemble pins with
little white heads, and are capable of vigorous movements to and fro;
occasionally a stalk coils itself up into a spiral.
BRACHIOPODA.
[Illustration:
Fig. 1.
British Brachiopods (_Terebratula_ and _Crania_).
]
[Sidenote: Small Table Case A against the west wall to left of main
entrance.]
The Brachiopoda, though presenting a certain outward resemblance to
bivalved Mollusca, are quite distinct from this group. They are all
marine, and all possess a bivalve shell. They grow attached to rocks
(Fig. 1), usually by a horny peduncle or stalk passing between the two
valves, or through a foramen in one of the latter; or, peduncle and
foramen may be absent, one of the valves adhering by its surface to the
rocks; some species of _Lingula_ live in tubes in the sand or mud. They
occur at all depths, from shallow water up to 2900 fathoms, but the
largest number of species live at a depth of about 350 fathoms. Though
found in all seas, the localities whence they have been obtained are
comparatively few in number; but specimens are usually congregated in
considerable numbers, in places where they do occur. The surviving
species of Brachiopods constitute only a small remnant of a group that
flourished abundantly in former epochs. There are about 150 recent, and
over 6000 fossil species.
THE SHELL.—The valves of a Brachiopod shell differ from each other in
size and shape, but each valve is in itself symmetrical, _i.e._, similar
on each side of a middle line.
The valve through which the peduncle passes is termed the _peduncle_ or
_ventral valve_ (Fig. 2, A), the other being the _brachial_ or _dorsal
valve_. The peduncle valve, which is usually the larger and uppermost,
contains the bulk of the viscera; in the higher genera, calcareous bars
or loops (Fig. 2, B) attached to the inner surface of the brachial valve
form a support for the “arms” of the animal. The inner surface of the
valves presents certain markings and depressions where the muscles have
been attached (Fig. 6).
[Illustration:
Fig. 2.
_Magellania flavescens._ Australia. Interior of valves.
A. Peduncle valve: _f_, foramen for peduncle, below which are the two
small deltidial plates; _t_, hinge-teeth; _a_, _b_, _c_, muscle
scars. B. Brachial valve, showing the reflected loop for support of
the “arms.”
]
The shell is constructed of very minute prisms of calcareous substance
imbedded in an organic matrix. In _Lingula_ the shell is formed of
alternating layers of horny and calcareous substance.
The shell-valves are either hingeless, or joined by a hinge in which
teeth in the peduncle valve fit into sockets in the brachial valve. The
Brachiopoda are primarily divided into two sections, _Inarticulata_ and
_Articulata_, based on the absence or presence of a hinge.
The division into Orders is based on the relation of the peduncle to the
valves in its passage between them or through one of them. In the most
primitive Brachiopoda (_Lingulidæ_), the peduncle simply passes out
between the valves and not through a foramen or pore in one of them;
hence the group is named _Atremata_ (_a_, not, _trema_, pore). In the
next group, including the families _Discinidæ_ and _Craniidæ_, the
peduncle passes through a fissure in the edge of the peduncle valve, the
fissure in recent forms becoming closed round to form a slit-like
foramen; this group is named NEOTREMATA (_neos_, new, _trema_, pore). In
the third group, PROTREMATA (_pro_, in front of, _trema_, pore), which
includes the _Thecidiidæ_, the peduncle lies at the apex of a triangular
fissure in the peduncle valve, and secretes a calcareous plate to fill
in the gap. In the fourth group, TELOTREMATA (_telos_, final or
complete, _trema_, pore), including the _Terebratulidæ_, etc., the
triangular fissure in the peduncle valve is filled in by two calcareous
plates termed deltidia, secreted by the edges of the mantle.
The valves are hingeless in the first two Orders (Inarticulata), and
hinged in the last two (Articulata).
THE BODY.—The body usually occupies only a comparatively small space in
the posterior or peduncle end of the shell. From each side of the body
there is given off a thin expansion, the mantle which lines the inner
surface of the shell. The space between the valves is termed the
mantle-cavity. The mouth is situated in the centre of the front wall of
the body or floor of the mantle-cavity. The front wall gives rise to a
horseshoe-shaped platform surrounding the mouth and bearing on its upper
edge ciliated tentacles, or cirri, which set up currents carrying food
towards the mouth. In many genera the platform is produced into two
coiled “arms” (Figs. 3 and 5), which fill up the mantle-cavity.
The name Brachiopoda (_brachion_, arm, _pous_, foot) was given to the
group because these “arms” were supposed to be homologous with the
Molluscan “foot.”
The mouth leads into a gullet, which opens into a stomach and intestine.
In the more primitive forms the intestine terminates in a vent, but in
the higher forms the distal end of the intestine has become atrophied,
and consequently the gut ends blindly.
The body-cavity contains fluid, and is in communication with a system of
sinuses in the lobes of the mantle (Fig. 5). Bands of muscles pass
across from valve to valve. The peduncle consists of a horny outer
sheath surrounding longitudinal and transverse bands of muscles.
[Illustration:
Fig. 3.
_Magellania flavescens._ (After Davidson.)
A. Interior of dorsal valve to show the “arms”; some of cirri removed
on right side; _v_, mouth. B. Longitudinal section, with a portion
of the animal.
]
The sexes are usually separate. The reproductive cells are formed in the
body-cavity. The embryo swims freely for a short time before settling
down and becoming fixed. The specimens exhibited in the case are
arranged according to the following classification:—
Section I. INARTICULATA. │Order 1. Atremata. │Fam. _Lingulidæ_.
─────────────────────────┼──────────────────────┼──────────────────────
„ │Order 2. Neotremata. │Fam. _Discinidæ_.
„ │ „ │Fam. _Craniidæ_.