Order 3. PROTREMATA.—This group, formerly very abundant, is now almost
extinct, the Family _Thecidiidæ_ representing the Order at the present
day. _Thecidium mediterraneum_ (Fig. 9) forms little oval boxes about a
third of an inch in length, shaped somewhat like a pear cut in half
(peduncle valve), and with a semicircular lid (brachial valve) working
on a hinge on the upper flat surface. The foramen and peduncle are
absent; but between the pointed end of the peduncle valve and the hinge
is an area filled in by a calcareous plate characteristic of the
Protremata.
[Illustration:
Fig. 9.
_Thecidium mediterraneum._ A, natural size. B, section through shell.
Magnified.
]
The brachial valve opens like the lid of a snuff-box, and shuts down on
the least alarm with the rapidity of lightning. The peduncle valve is
fixed on the rocks by its convex surface. The species is common in the
Mediterranean in from 30 to 300 fathoms, and is also found in the West
Indies.
Order 4. TELOTREMATA.—This group, which at the present day contains the
largest number of species, includes the Lamp shells, so called from
their resemblance to an ancient lamp. The valves are joined by a
well-marked hinge, the peduncle passes through the peduncle valve
through a foramen completed by two plates secreted by the mantle edges,
and the brachial valve has attached to it a calcareous scaffolding of
processes or loops for the support of the “arms.”
The shells in this group are frequently ridged. Their colour is usually
white, but sometimes red or yellow; deep-sea forms are generally
vitreous.
_Rhynchonella psittacea_ has a black shell with a pointed incurved beak;
each of the arms forms a many coiled spiral and can be protruded beyond
the shell; the brachial skeleton is comparatively small and simple,
consisting of two separate processes.
In _Terebratulina_ the brachial skeleton forms a simple loop; in
_Magellania_ the loop is reflected on itself (Fig. 2).
The beautiful and unique specimen of _Dyscolia wyvillii_, from 390
fathoms W. Indies, is remarkable for its size, being over two inches in
length. The small vitreous specimens of _Terebratula wyvillii_ were
obtained off Chili from a depth of 2160 fathoms; specimens of the same
species were obtained also from a depth of 2900 fathoms in the North
Pacific.
TUNICATA.
[Sidenote: Wall Case to left of main entrance to Shell Gallery.]
The Tunicata are marine animals, the majority of which live, in their
adult stage, a stationary life, fixed to the rocks or sea-bottom, but a
comparatively small number are free-swimming.
[Illustration:
Fig. 1.
_Ascidia mentula_ from the right side. _at_, atrial aperture; _br_,
branchial aperture; _t_, test.
[After Herdman: _Tunicata_, Encyc. Britannica.]
]
They occur in the form of cartilaginous or leathery sacs, fleshy
incrustations, solid fleshy masses, free-swimming, barrel-shaped
animals, solitary or united into chains or hollow cylinders; or, lastly,
of minute free-swimming tadpole-shaped organisms. To explain briefly the
structure of a Tunicate, _Ascidia mentula_ (Fig. 1), is selected. The
animal, which lives on a muddy bottom, in from five to twenty fathoms,
resembles a conical sac fixed by the broader end, of grayish green
colour and about 4 inches in height. At the narrower end are two
orifices, one terminal—the branchial orifice or mouth, and the other a
little lower—the atrial orifice: the former has eight lobes and the
latter six.
When the Ascidian is undisturbed, the orifices are wide open, and
currents enter by the branchial and leave by the atrial orifice. On the
least alarm, the orifices close, jets of water being at the same time
squirted out; hence the popular name “Sea-squirts” given to these
animals.
[Illustration:
Fig. 2.
Diagrammatic section of _Ascidia_ representing the three sacs, and the
branchial sac as the pharynx or throat.
_a_, branchial; and _b_, atrial orifice; _c_, tunic or test; _d_,
mantle; _e_, branchial sac; _f_, gullet; _g_, stomach; _h_, anal
orifice; _i_, dorsal lamina; dotted line indicates the endostyle.
]
The Ascidian is orientated as follows: hold the animal with the
branchial orifice pointing forwards and the atrial upwards; the
branchial orifice will be anterior and the opposite end posterior; the
atrial orifice will lie on the upper or dorsal aspect, the opposite
aspect being lower or ventral, and the sides right and left. The
aspects, in fact, correspond with those of a vertebrate animal. A
vertical section roughly shows the animal to be formed of three
concentric sacs (Figs. 2, 3). The outermost, which is tough and
membranous, is called the Test or Tunic, the whole group owing its name
to the presence of this protective covering.
[Illustration:
Fig. 3.
Diagrammatic dissection of _A. mentula_.
_at_, atrial orifice; _br_, branchial orifice; _a_, anal orifice;
_brs_, branchial sac; _dl_, dorsal lamina; _end_, endostyle; _m_,
mantle; _ng_, nerve ganglion; _oea_, orifice of gullet; _pbr_,
peribranchial cavity; _st_, stomach; _t_, test; _tn_, tentacles.
(After Herdman: _Tunicata_, Encyc. Britannica.)
]
The middle sac, termed the Mantle, which almost corresponds in shape to
the outer, is composed of connective tissue, muscle-fibres,
blood-vessels, etc.; in spirit specimens, the mantle is shrunk away from
the test except at the orifices and at a point behind, where vessels
enter the test.
The innermost or Branchial Sac is attached behind the branchial orifice
and along the ventral edge, but otherwise hangs free in the interior,
the space around and outside of the sac being termed the atrial or
peribranchial cavity.
The delicate walls of the branchial sac, which resemble fine muslin, are
perforated by innumerable vertical slits, termed stigmata, arranged in
transverse rows (Fig. 4).
[Illustration:
Fig. 4.
_Ascidia mentula._ Part of wall of branchial sac showing stigmata.
Magnified.
]
The margins of the stigmata are lined with cilia which set up currents;
and the water which enters by the branchial orifice, passes through the
stigmata into the atrial cavity, and thence out through the atrial
orifice. The walls of the branchial sac are chiefly composed of a
sieve-like meshwork of fine blood-vessels arranged in transverse and
longitudinal rows. The currents of water passing through the stigmata
aërate the blood in the vessels. Besides the stigmata, the branchial sac
has two relatively large orifices, viz., the branchial orifice or mouth,
and, at the opposite end, the opening into the gullet. The branchial sac
is, in fact, a capacious throat or pharynx (Diagram Fig. 2 and Fig. 14).
Inside the branchial orifice is a circle of fine tentacles, which guard
the entrance to the branchial sac. The food of the animal consists of
minute animal and vegetable organisms.
It may be wondered how this food is secured, seeing that the currents of
water are continually passing through the sieve-like walls of the
branchial sac to the exterior again. Within the branchial orifice and
above the branchial sac are two circular ciliated ridges with a groove
between, which is full of viscid secretion; the cilia on the ridges
direct particles into the groove where they are retained by the mucus.
Passing backwards along the ventral edge of the branchial sac is a
thick-lipped furrow, which appears like a rod in the thin-walled sac,
and hence is called the endostyle. This organ secretes the mucus which
is carried up by ciliary action to the circular groove in front of the
branchial sac, and thence to the gullet along a fold or crest, termed
the dorsal lamina, situated along the dorsal edge of the branchial sac.
The gullet opens into a large stomach situated posteriorly on the left
side of the branchial sac. The stomach opens into the intestine, which,
after forming a loop, terminates in the anal orifice or vent opening
into the atrial cavity.
The tubular heart lies below the stomach, a remarkable feature in the
circulation consisting in the periodic reversal of the blood current. An
elongated nerve ganglion is situated between the branchial and atrial
orifices.
[Illustration:
Fig. 5.
Ascidian Tadpole with part only of the tail _C_. Magnified section.
_N_, nervous system with enlarged brain in front and narrow spinal
cord behind _n_; _N′_, cavity of brain; _O_, the single cerebral eye
lying in the brain; _a_, auditory organ; _K_, pharynx; _d_,
intestines; _o_, rudiment of mouth; _ch_, notochord or primitive
backbone.
(From Gegenbaur’s ‘Elements of Comparative Anatomy.’)
]
_Ascidia mentula_ is hermaphrodite. The egg develops into a minute
tadpole-like larva which swims about by means of its tail. Water
entering by the mouth passes out through the gill-slits. A nerve-tube
extending along the back and tail is swollen in front into a
brain-vesicle; and underneath the long nerve-tube behind the brain is a
stiff skeletal rod or axis—the notochord—which constitutes the rudiment
of a backbone. Inside the brain are two unpaired sense organs, an eye
and an organ of hearing (Fig. 5). After swimming freely for a few hours,
the larva settles down head foremost and fixes itself by papillæ on the
anterior end (Figs. 6, 7). Presently the tail becomes absorbed, and the
posterior end of the nerve-tube, and the brain with its eye and hearing
organ, undergo atrophy, the nerve ganglion of the adult alone
representing the cerebrospinal axis of the larva. The branchial sac and
intestines develop greatly, and growth proceeds in such a manner that
the mouth is pushed round to a position opposite to the fixed area, and
gradually the animal becomes the adult ascidian.
[Illustration:
Fig. 6.
Degeneration of Ascidian Tadpole to form the adult. The black pieces
represent the rock or stone to which the Tadpole has fixed its head.
]
[Illustration:
Fig. 7.
Very young Ascidian with only two gill-slits.
(Figs. 6, 7, from Lankester’s ‘Degeneration.’)
]
This wonderful metamorphosis presents a striking example of DEGENERATION
resulting from the adoption of a fixed mode of life. The active
free-swimming larva with its brain, eye, hearing organ, and muscular
tail becomes transformed into a comparatively inert sac.
[Illustration:
Fig. 8.
Tadpole of Frog and Ascidian. Surface view. (Lankester’s
‘Degeneration.’)
]
[Illustration:
Fig. 9.
Tadpole of Frog and Ascidian. Diagram representing the chief internal
organs. (Lankester’s ‘Degeneration. A chapter in Darwinism.’)
]
The tadpole of an Ascidian resembles that of a frog (Figs. 8, 9), not
merely superficially, but also in its general structure and mode of
development. The Tunicata are now generally regarded as a degenerate
offshoot from the ancestral stock of the Vertebrata, in that the larva
possesses a skeletal rod (rudimentary backbone) separating the dorsally
situated nerve-tube (cerebrospinal axis) from the ventrally situated
intestinal tube, the existence of the cerebral eye in the Ascidian
tadpole further tending to confirm the truth of this theory. Apart from
a knowledge of the course of their development, Tunicata would have been
classed among the Invertebrata, but the structure of the larva clearly
reveals the affinities of the group to the backboned animals.
_Ascidia mentula_ belongs to the group of SIMPLE ASCIDIANS which are all
fixed, and are either solitary or joined into colonies in which each
individual or ascidiozooid has a distinct test of its own. In the
COMPOUND ASCIDIANS, which form colonies by budding, the ascidiozooids
are buried in a common investing mass and have no separate tests. In a
third group, the SALPA-LIKE ASCIDIANS, the ascidiozooids are united to
form free-swimming colonies shaped like hollow cylinders open at one
end. The above three groups belong to one great Order—the ASCIDIACEA. A
second Order, THALIACEA, includes the free-swimming _Salpa_ and
_Doliolum_, which exhibit alternation of generations in their life
history. A third Order LARVACEA, includes very minute free-swimming
forms which possess a tail in the adult stage. There are sixteen
families of Tunicata.
The following is a tabular view of Prof. Herdman’s classifications:—
Order I. Ascidiacea.│Sub-order 1. Ascidiæ Simplices, 4 Families.
„ │Sub-order 2. „ Compositæ, 7 Families.
„ │Sub-order 3. „ Salpiformes, 1 Family.
Order II. Thaliacea │3 Families.
Order III. Larvacea │1 Family.
Order I.—ASCIDIACEA.
The Ascidiacea include the great majority of species. With the exception
of the one genus _Pyrosoma_, they lead a fixed or stationary life.
_Sub-order 1._—ASCIDIÆ SIMPLICES.
The Simple Ascidians are mostly solitary; in a few forms, however,
colonies arise by budding from stolons, but each individual has a
distinct test. The four families into which the sub-order is divided are
chiefly characterised by the nature of the test, the number of lobes
round the branchial and atrial orifice, and the character of the
branchial sac.
In the family _Molgulidæ_ the tough membranous test is often coated with
sand; the branchial aperture is six-lobed, the atrial four-lobed, the
branchial sac has long folds or pleats, and the stigmata are curved or
arranged in spirals.
_Molgula gigantea_, which is one of the largest of the Ascidians, and
which attains a length of over thirteen inches, forms a tough conical
sac; the branchial and atrial orifices at the upper end have six and
four lobes respectively. The test is leathery, smooth above, but coated
with sand below. The exhibited specimen, which comes from the Straits of
Magellan, has several specimens of the stalked _Boltenia legumen_
attached to the lower part of the test.
The curious _Molgula oculata_ (Fig. 10) has a soft oval or rounded body
coated with sand. The branchial and atrial orifices have respectively
six and four lobes. Specimens grow attached to the rocks and also live
free in the sand. The surface of the test is provided with hairs, which
adhere to the rocks and collect particles of sand. The adhesion not
being very firm, specimens are easily detached by currents and collected
into heaps by the eddies; when living in the sand only the two dark
orifices are visible. The sand coating has been supposed to confer
protection by mimicry of the environment; but Professor Lacaze Duthiers
found, much to his chagrin, that the sandy tests of his specimens were
of no avail in securing them from being devoured by crabs who seemed to
scent their prey from afar.
[Illustration:
Fig. 10.
_Mogula oculata._
_a_, branchial; _b_, atrial orifice.
]
In the family _Cynthiidæ_ the test is usually leathery, the branchial
and atrial apertures four-lobed, and the branchial sac folded into
longitudinal pleats.
The genera _Boltenia_ and _Culeolus_ include species in which the body
is attached to a peduncle.
The large exhibited specimen of _Boltenia pachydermatina_ is 28 inches
in length, the head being 4 and the stalk 24 inches long. The two
four-lobed apertures are along one edge, the branchial being the lower;
the body is marked with long deep furrows, and the stalk with transverse
wrinkles. _Culeolus perlucidus_, from 1600 fathoms in the Southern
Ocean, is in the form of a small pear-shaped head on a slender stalk,
the total length being 4½ inches. The branchial orifice forms a
transverse slit with raised lips near the stalk, the slit-like atrial
orifice being near the rounded end of the body. _Culeolus moseleyi_,
another slender-stalked form, was obtained from 2425 fathoms in the
Central Pacific.
[Illustration:
Fig. 11.
A. _Styelopsis grossularia_ on shell. B. Tadpoles of same, × 9. _a_,
branchial; _b_, atrial orifice. (B, after Sir J. Dalyell.)
]
The little Cynthiid _Styelopsis grossularia_ (Fig. 11), popularly known
as the “Currant Squirter,” occurs in the form of bright red
hemispherical blobs on stones and shells; when undisturbed, the
branchial and atrial orifices expand and project upwards. The eggs are
brilliant red in colour. Sir John Dalyell was the first to discover the
tadpole form, which is about ⅒ inch long (Fig. 11, B), and to observe
the tadpoles become fixed and develop into fixed Ascidians. He calls the
active little swimming larvæ “Spinulæ,” from their resemblance to small
pins.
The family _Ascidiidæ_ includes forms with a gelatinous or cartilaginous
test; the branchial and atrial orifices usually have 8 and 6 lobes
respectively; the branchial sac is without folds.
_Ascidia mentula_, described above, belongs to this family.
_Chelyosoma_ is characterised by the test forming tortoise-like horny
plates on the upper surface. The exhibited specimen of _C. macleayanum_
(Fig. 12) comes from Greenland; the upper hemispherical part of the test
is divided into 8 plates; the branchial and atrial orifices are situated
in the joints between the plates.
[Illustration:
Fig. 12.
_Chelyosoma macleayanum_, slightly enlarged. _a_, branchial; _b_,
atrial orifice.
]
The fine specimen of _Phallusia mammillata_ from Naples consists of
several individuals partly fused together; the branchial and atrial
orifices are wide open, and the mantle can be seen through the thick
knobby translucent test.
In _Rhodosoma_ the test is modified so as to form stiff plates recalling
the valves of a bivalve shell. One plate is attached to the rocks, the
other closing against the first like a lid; the anterior end of the
animal with its branchial and atrial orifices is visible only when the
lid is open. The Mediterranean species _R. callense_ (Fig. 13) grows
attached to the rocks. The little exhibited specimen is on a fragment of
shell in front of a black patch. The figure shows specimens with the lid
open and closed.
[Illustration:
Fig. 13.
_Rhodosoma callense_, × 10. A, “valve” open; B, shut. _a_, branchial;
_b_, atrial orifice. (After Lacaze Duthiers.)
]
Family _Clavelinidæ_. The body is attached to a creeping stolon or mass
of stolons, from which new individuals arise by budding. The other three
families of Simple Ascidians included solitary forms, but the
Clavelinidæ are social, and form colonies wherein each individual has
its own test.
_Clavelina lepadiformis_ (Fig. 14) forms graceful crystal vases about an
inch in height. The figure shows one individual, but usually the
processes at the base extend out as stolons whence other individuals
arise.
_Diazona violacea_, from Cornwall, forms beautiful purple disk-shaped
colonies in which the ascidiozooids arise from a basal mass of stolons.
Sometimes the ascidiozooids die down, leaving only a smooth violet pad,
which in due time produces a new crop of ascidiozooids.
[Illustration:
Fig. 14.
_Clavelina lepadiformis_; diagrammatic, showing the anatomy. The oval
bodies are the eggs; at lower end lies the tubular heart; the
root-like processes at the base grow into stolons, whence other
ascidiozooids arise.
]
[Illustration:
Fig. 15.
_Perophora listeri_; A, slightly, B, further magnified. Ascidiozooids
in right, left, and lateral aspects.
_a_, branchial; _b_, atrial orifice.
]
The remarkable _Rhopalæa neapolitana_, from Naples, may be roughly
compared to an hour-glass with a very long constriction. The test is
smooth in the upper part, but knobby and encrusted with foreign bodies
below. The upper or thoracic end contains the branchial sac, and the
lower or abdominal portion the stomach, heart, and reproductive organs,
the gullet and intestine traversing the whole length of the narrow
central region. Although from its general structure _Rhopalæa_ is a
Clavelinid, it is not certainly known to produce buds.
_Perophora listeri_ (Fig. 15) occurs in the form of little jelly-like
transparent blobs rising by short stalks from a silvery thread-like
stolon. Owing to their small size and transparency, it is possible to
examine specimens alive under the microscope, the currents passing
through the stigmata in the walls of the branchial sac, and the beating
of the heart being distinctly visible. The rapid motion of the cilia
surrounding stigmata gives the appearance of dark wheels all rotating in
the same direction. The heart beats so as to drive the blood current so
many times in one direction, and then after a short pause, in the
reverse direction.
The exhibited specimen growing on an oyster shell, is from Plymouth.
_Sub-order 2._—ASCIDIÆ COMPOSITÆ.
The Compound Ascidians are fixed forms, which give rise to colonies by
budding, the individuals being immersed in a common mass and not
possessing separate tests.
Although reduced to an extremely small size each individual or
ascidiozooid of a colony possesses the same organs as a large Simple
Ascidian, excepting that the former does not possess a separate test.
Frequently the individuals of a colony are grouped into systems, in
which the atrial orifices open into a common cloaca. The little
ascidiozooids vary greatly in shape in the different families. In the
_Polyclinidæ_, for instance, they are long, the organs being so to
speak, drawn out, and being arranged in three regions, the thoracic,
abdominal and post-abdominal, the first region containing the branchial
sac, the second the stomach, and the third the heart and reproductive
organs. In the _Distomidæ_, the body exhibits two regions, thoracic and
abdominal, the heart and reproductive organs lying alongside of the
stomach. The _Botryllidæ_ comprise only one region, the stomach and the
other organs being situated by the side of the branchial sac.
The Compound Ascidians include seven families which are characterised
chiefly by the method of bud formation, and by the arrangement of the
organs into one, two, or three regions.
It is only possible, from limits of space, to refer to a few interesting
forms.
The species of _Botryllus_ are those most commonly met with. They form
richly coloured gelatinous incrustations on rocks and seaweeds. _B.
violaceus_ (Figs. 16, 17, and 18 D) is blue with white lines; _B.
smaragdus_, green; _B. marionis_, brown with white and carmine; _B.
castaneus_, purple, and so on. The individuals are arranged in circular
systems with the branchial orifices round the circumference and the
atrial orifices opening into a common central cavity (Fig. 17), the
whole colony being composed of groups of systems.
[Illustration:
Fig. 16.
_Botryllus violaceus_ on seaweed. (After H. Milne-Edwards.)
]
The exhibited specimen of _B. violaceus_ was grown in the tanks of the
Biological Station at Plymouth. The red specimen of _B. aurolineatus_,
from Naples, shows well the branchial and cloacal orifices. In
_Botrylloides_, the individuals form elliptical or elongated systems.
_Colella thomsoni_ was obtained near the Philippines at a depth of 10
fathoms. The specimen, which is about 7 inches in length, resembles an
elongated head of clover on a thickened stalk. The individuals which
compose the head are arranged in spiral lines, the atrial orifice of
each ascidiozooid opening separately and not into a common cloaca.
[Illustration:
Fig. 17.
A. _Botryllus violaceus_, magnified, showing two systems of 6 and 7
ascidiozooids. B. One ascidiozooid extracted.
_a_, branchial; _b_, atrial orifices; _c_, branchial sac; _d_,
stomach.
(After H. Milne-Edwards.)
]
_Colella quoyi_ (Fig. 18 A), from 25 fathoms off Kerguelen Island, forms
a rounded head on a short peduncle, the total height being one inch. The
ascidiozooids are arranged in vertical lines in the “head,” each line
consisting of a double zigzag series.
[Illustration:
Fig. 18.
Colonies of _Ascidiæ compositæ_, natural size. A. _Colella quoyi._ B.
_Leptoclinum neglectum._ C. _Pharyngodictyon mirabile._ D.
_Botryllus._
(After Herdman, _Challenger_ Report and Encyclopædia Britannica.)
]
_Julinia ignota_, from the Antarctic regions, forms long narrow
colonies, which attain a length of nearly three feet. One end is
attached, the rest of the colony apparently lying along the sea-bottom.
_Amaroucium roseum_ from Naples forms translucent gelatinous masses; a
slice is exhibited, showing the long slender ascidiozooids immersed in
the mass.
[Illustration:
Fig. 19.
_Pyrosoma elegans_, natural size. A. Side view of entire colony. B.
End view of open extremity.
(Herdman: _Tunicata_, Encyclopædia Britannica.)
]
_Pharyngodictyon mirabile_ (Fig. 18 C), from 1600 fathoms in the
Southern Indian Ocean, resembles a small mushroom, and is about one inch
in height. This species is one of the few deep-sea Compound Ascidians.
_Leptoclinum albidum_ is a common and widely distributed species; it
occurs in the form of thin white crusts. The glistening white appearance
is due to the common test being densely crowded with minute stellate
spicules of carbonate of lime.
The specimen of _Leptoclinum neglectum_ (Fig. 18 B) encrusts a fragment
of sponge.
_Goodsiria pedunculata_ from the Straits of Magellan, forms a rounded
cartilaginous mass attached by a short peduncle; sometimes several
masses are attached to each other. Each of the small dark oval areas on
the surface corresponds to the branchial and atrial orifices of one
ascidiozooid.
_Sub-order 3._—ASCIDÆ SALPIFORMES.
The Salpiform Ascidians comprise only one genus, _Pyrosoma_, which
occurs in the form of free-swimming colonies shaped like hollow
cylinders closed and rounded at one end and open and truncate at the
other (Fig. 19). The wall of the cylinder is formed of a single layer of
ascidiozooids (Fig. 20), so arranged that all the atrial orifices open
into the interior of the cylinder, and all the branchial orifices on the
exterior, the two kinds of orifices being at opposite ends of the body,
and not close together, as in most simple and compound Ascidians.
Specimens vary in size from a few inches to upwards of four feet in
length, and, as the name of the genus implies,[22] they are brilliantly
phosphorescent. Sometimes they occur in innumerable multitudes, giving
rise to a zone of greenish light extending for miles. Professor Moseley
records that during the voyage of the _Challenger_ in the North Atlantic
a huge specimen of _Pyrosoma spinosum_, four feet in length, was
captured. On tracing his name on its body, the word came out in letters
of fire.
[Illustration:
Fig. 20.
Section through wall of _Pyrosoma_, magnified, showing a single layer
of ascidiozooids.
_br_, branchial; _at_, atrial orifice; _tp_, process of the test; _br
s_, branchial sac.
(Herdman: _Tunicata_, Encyclopædia Britannica.)
]
In _Pyrosoma elegans_ (exhibited), from Naples, the ascidiozooids are
arranged in verticils, and the mouth of the cylinder is surrounded by a
movable diaphragm; the outer end of each ascidiozooid is provided with a
membranous spine. Six species of _Pyrosoma_ are known. _Pyrosoma
atlanticum_ is found in the tropical Atlantic and Antarctic; _P.
giganteum_ in the Atlantic, Pacific, and Antarctic; and _P. spinosum_ in
the South Atlantic.
Order II.—THALIACEA.
The Thaliacea are free-swimming Tunicates, which exhibit alternation of
generations in their life history. There are three families, _Salpidæ_,
_Octacnemidæ_, and _Doliolidæ_.
_Salpidæ._—The Salpas are transparent barrel-shaped organisms which
occur in abundance at the ocean surface. They are so transparent that
they are rarely seen, except in calm weather from the side of small
boats; yet they frequently swarm in countless multitudes. From five to
ten bands of muscles partially or entirely surround the body, like
hoops. The branchial and atrial openings are at or near the opposite
ends of the body. The branchial sac has almost disappeared, the dorsal
lamina and ventral gutter (or endostyle) alone remaining, the interval
between the two on each side representing an enormous stigma; the dorsal
lamina, or “gill” is the transversely striated band passing obliquely
across the body and forming the only barrier between the branchial and
atrial cavities. Water enters at the mouth, and, by the contraction of
the muscle-hoops, is driven out through the atrial aperture at the
opposite end, which is then closed by a sphincter muscle. The elastic
walls of the body expand, and water again enters through the mouth, the
valve-like lips of which prevent its being driven out that way. The
_Salpa_ swims along in jerks, and along with each gulp of water takes in
Radiolaria, Foraminifera, etc., which are retained by the mucus of the
endostyle and carried to the gullet. The _Salpa_, in fact, lives, as
Professor Brooks observes, in a “living broth,” so abundant is the food
supply.
The intestines usually form an oval mass termed the “nucleus,” which is
a conspicuous object at the posterior end.
[Illustration:
Fig. 21.
Posterior part of solitary form of _Salpa democratica-mucronata_,
showing a chain of embryos nearly ready to be set free.
_gem_, young chain of _Salpæ_; _st_, stolon; _t_, test; _visc_,
visceral mass.
]
The _solitary Salpa_ above described is asexual. In the ventral region
of its body it forms a stolon which becomes segmented into a series of
buds (Fig. 21). As the stolon grows the end series of buds breaks off in
the form of a chain and swims away, other chains being detached in
succession. A chain is formed of individuals arranged in two rows, the
individuals in each row being alternate (not opposite).
Each individual of a chain differs from the solitary individual in
shape, arrangement of muscle bands, etc., but especially in having
reproductive organs. The chain Salpid is hermaphrodite; the embryo
develops into a solitary asexual _Salpa_ which produces the chains by
budding. The wonderful life history of _Salpa_ was discovered by the
poet Chamisso during a voyage round the world in 1819. He observes: “A
_Salpa_ mother is not like its daughter or its own mother, but resembles
its sister, its granddaughter, and its grandmother.” Here we have an
example of “alternation of generations,” a sexual generation (chain
form) giving rise to an asexual generation (solitary form), which latter
produces the sexual generation.[23]
Most of the species of _Salpa_ have double names owing to the chain and
solitary forms having been regarded as distinct species before they were
known to be phases in the life history of one and the same species.
_Salpa runcinata-fusiformis_, solitary form (Fig. 22 B), is
barrel-shaped, truncated at each end, with terminal orifices, and with
nine muscle-bands on the dorsal surface, some of which converge towards
each other. An individual of a chain (Fig. 22 A) is fusiform, with six
muscle-bands, and with the orifices not terminal, but at each end of the
dorsal surface.
The solitary form of _S. africana-maxima_ is barrel-shaped, with
truncated ends and terminal orifices, and with nine broad parallel
muscle-bands. The chain form is conical at one end, with six bands, and
with orifices on the dorsal surface. The exhibited specimen of the chain
form, which is in an early stage of growth, contains 202 individuals.
The solitary and chain individuals of _Salpa costata-tilesii_ attain a
length of six to eight inches. The solitary form has eighteen
muscle-bands and two large spines at the posterior end. The individual
of the chain has five muscle-bands. A chain of three individuals is
exhibited.
_Salpa pinnata_ produces a circular chain; the exhibited specimen of the
solitary form shows a small chain about to be detached; a circular chain
of six individuals is also exhibited. Species of _Salpa_ abound in all
seas, but specimens from Naples have alone been exhibited on account of
their good preservation.
Family _Octacnemidæ_ includes _O. bythius_, a deep-sea Salpid, in which
the body forms a flattened disk produced into eight radiating lobes.
[Illustration:
Fig. 22.
_Salpa runcinata-fusiformis_. A. Chain form. B. Solitary form. 1–9,
muscle bands; _em_, embryo; _m_, mantle; _visc_, visceral mass or
nucleus.
(Herdman: _Tunicata_, Encyclopædia Britannica.)
]
[Illustration:
Fig. 23.
_Doliolum denticulatum_, sexual generation, from the left side.
_m^1_-_m^8_ muscle bands; _at_, atrial; _br_, branchial apertures;
_br s_, branchial sac; _sg_, stigmata; _st_, stomach; _ng_, nerve
ganglion; _so_, sense organs.
(After Herdman, Encyclopædia Britannica.)
]
Family _Doliolidæ_. The body is cask-shaped and surrounded by circular
hoops. The branchial and atrial orifices are at the opposite ends. The
branchial sac is pierced by two oblique bands of stigmata (Fig. 23
_sg_). The life history is very complicated. The egg develops into a
tailed larva, which develops into a “nurse”; the latter is asexual, and
produces three kinds of buds on a stolon, viz. (1) nutritive buds which
provide the “nurse” with food, (2) foster forms which are set free as
cask-shaped bodies with eight broad muscle-bands, and (3) sexual forms
which are attached for a time to the foster forms, but which later
become free and give rise to the egg.
Order III.—LARVACEA.
[Illustration:
Fig. 24.
_Oikopleura cophocerca_ in its “house” (after Fol); seen from right
side, × 6. Arrows indicate course of the water; _x_, lateral
reticulated parts of the “house.”
]
The Larvacea are very minute Tunicata which live at the surface and swim
by means of a tail-like appendage, resembling in this and certain other
respects the tadpole larva of other Tunicata. They are able to form a
temporary test or “house” many times larger than the body (Fig. 24). The
organism itself, which is almost lost in its large test, is the little
hammer-shaped body in the centre of the figure; the streaked areas bound
a space in which the tail lashes vigorously. The animal can leave its
test and secrete another in a few hours.
The tail is attached to the under or ventral surface of the tiny little
barrel-shaped body, and usually points forwards; a skeletal rod, the
urochord, runs along its length. The branchial sac has two ciliated
openings or gill-clefts leading directly to the exterior, and not
opening like the stigmata of the other orders into an atrial cavity.
The order contains one family, the _Appendiculariidæ_, and four genera,
and is represented in all seas.
_Oikopleura cophocerca_, one of the largest forms, is about half an inch
in length. The exhibited specimens came from St. Andrews, Fife.
Professor McIntosh reports that occasionally specimens of this species
occur in immense quantities, the tow-nets being filled with them.
THE STARFISH GALLERY.
In the STARFISH GALLERY is exhibited a series of the animals belonging
to the class _Echinoderma_; of these the Starfishes are the best known,
while others are the Sea-Lilies, Sea-Urchins, and Sea-Cucumbers or
Sea-Slugs.
A small collection of various kinds of Worms is also exhibited in this
Gallery (Wall-cases I.–III.).
ECHINODERMA.
Six table-cases contain the dried Echinoderms arranged in systematic
order. The seventh is devoted to preparations, models, and figures
illustrative of the structure and life history of various members of the
group.
An inspection of that Case and the accompanying woodcuts will make clear
the distinctive characters of the Echinoderma. Unlike that of a Crayfish
or a Mussel, the body does not appear to be divided into two equal or
symmetrical halves, though it really is; this is due to the possession
of a number of rays, of which there are ordinarily five. The skin is
strengthened by the deposition in it of carbonate of lime, which may be
in the form of continuous plates or bars, or of separate scattered
spicules. A series of tube-feet or suckers (podia) are generally
developed along each ray, and these are supplied by a system of
water-vessels peculiar to Starfish and their allies. These rays are
often called “_ambulacra_.”
[Illustration:
Fig. 1.
A. Anchor and plate of _Synapta_. B, C. Tables of _Holothuria
impatiens_; and D. _Holothuria atra_: from various aspects. E.
Spicule from sucker of _Stichopus variegatus_, magnified about 200
times.
]
[Illustration:
Fig. 2.
Diagram of Water-vessels.
_c.c._ Circular canal, with _p.v._, its Polian vesicles; from it a
radial canal (_v.c._) is given off along the lower surface of each
arm; this supplies, by side branches, the suckers, _s_; connected
with each sucker is a contractile swelling or ampulla (_a_). The
circular canal is in connection with the exterior by _s.c._, the
stone-canal, and opens to it by the madreporite (_m_).
]
[Illustration:
Fig. 3.
Figure of a Starfish (_Asterias rubens_).
In the ray marked I. the skin has been removed from the upper surface,
and the ambulacral ossicles (_ao_) and the podia (_s_) are seen _in
situ_; the blind outgrowths (_c_) from the central stomach (_sp_)
have been dissected out. In II. the gonads (_g_) are exposed; and in
the centre above the stomach the rectal glands (_rg_) are to be
seen. The anus (_a_) is seen to be subcentral in position.
]
In the body of the Starfish (Fig. 3) the arms are seen to be continuous
with the disk and to contain portions or prolongations of the chief
organs. The middle of the arm is occupied by two rows of hard pieces
(ambulacral ossicles), the fellows of which make an open angle with each
other, and so form an open ambulacral groove; along this we find the
suckers, the water-canal that supplies them, the blood-vessel of the
arm, and a nerve-cord. At the centre of the disk is the mouth. The
ossicles at the sides of the arms bear spines, which vary in different
species; the surface of the back is supported by a network of hard
pieces, and through the intervening spaces there project membranous
pouches, which are respiratory in function. The modified plate on the
upper surface opens into a tube by means of which the water-vessels
communicate with the exterior; this plate is known as the madreporite
(Fig. 2, _m_).
The organs for masticating the food are most highly developed in the
regular Echinoids, where the complex apparatus known as the “Lantern of
Aristotle” is found (Case 38) to consist of five sets of pieces; the
tooth is strong and bevelled at its free end; it is supported by
triangular jaws on either side, a pair uniting and having the form of an
inverted pyramid; these alveoli are connected with their neighbours by
oblong pieces (_falces_); above these there are elongated bars, which
are hinged on to the inner end of the falces and have their outer ends
free. The whole lantern is connected to the test by muscles which pass
from its sides to the auricles or upstanding pillars which lie round the
mouth; and, owing to this muscular apparatus, the teeth are capable of
complicated and various movements.
In the Ophiuroids the edges of the mouth-slits are provided with short
spinous processes, varying a good deal in arrangement, but never having,
apparently, any other function than that of a filtering-apparatus; in
the Starfishes the plates round the mouth have a supporting function
only; in Crinoids and Holothurians the mouth is unarmed; the latter are
often remarkable for a deposit of calcareous plates in the walls of the
gullet, and in the former the grooves on the arms are the lines along
which food comes to the mouth.
Echinoids live on seaweeds and the animals that are found on them; such
as have no teeth, like _Spatangus_ (Case 32), use their spout-like mouth
to take up the sand and débris on which they move, and from which they
extract some nutriment. Ophiuroids live on the smaller foraminifera;
Asteroids on dead fishes (as line-fishermen well know), oysters, and
other molluscs, and even on specimens of their own particular species;
Holothurians on shell or coral débris and the minute organisms it
contains; and Crinoids on small tests of foraminifera and on the adults
of small and larvæ of larger crustacea.
In a number of Echinoids and Asteroids some of the spines are specially
modified to act as seizing-organs—the free end being divided into two,
three, or rarely four pieces, which are moved on one another by special
muscles. These minute organs were regarded by earlier observers as
parasites, and were named _pedicellariæ_; they may be movable, when they
have a stalk, or the stalk may be absent and the valves sessile.
Considerable difficulty attaches to the determination of the use that
these organs may be to their possessors; but there is reason to suppose
that they may act as cleansing-organs by removing minute particles of
dirt, and as temporary organs of fixation, while M. Prouho has observed
their use as organs of defence.
Echinoderms move but little; the unstalked Crinoids, if they cannot find
stones or worm-tubes around which to attach themselves, swim by beating
the water with their delicate arms, five being raised and five depressed
alternately. The Echinoid or Asteroid is able to move by the aid of its
podia or so-called ambulacral feet, which become erected by being filled
with water, and are then contracted; by means of this contraction
movement is effected; a similar kind of locomotion obtains with the
pedate Holothurians; in the Ophiuroids the flexible arms either serve as
the organs of movement, or act as an apparatus whereby the creature
becomes coiled round the branches of corals (see Case 20).
Echinoderms are often of exceedingly bright colours, as is shown by the
pictures on the wall, and are very conspicuous objects; this may,
apparently, be associated with disagreeable tastes or odours; sometimes
they cover themselves over with seaweed, and so hide their brilliancy;
the spines of some forms are exceedingly painful to the touch, and the
stout plates of some of the _Goniasters_ must form admirable organs of
protection. The power of restoring lost or injured parts is one of the
most remarkable points in the Echinoderm organization (see Case 6).
Echinoderms are of great geological age, and were very abundant in
earlier periods of the world’s history. Two groups (the Blastoids and
Cystids) have completely disappeared, and the Stalked Crinoids
(Lily-Encrinites) are far less common than they used to be. Echinoderms
are now found in all seas, and extend to great depths of ocean; many of
the species have exceedingly wide areas of distribution, and most are
characterized by their gregarious habits, a large number of specimens of
a single species being generally obtained by the dredge. They are most
abundant in the tropical seas.
Most Echinoderms lay their eggs in the water, where the larvæ are
developed and swim about freely; but in a few (_Hemiaster_, _Ophiacantha
vivipara_, and others) the young do not pass through any metamorphosis,
for the eggs are placed in special pouches of the body of the parent, in
which they are hatched. The free-swimming larvæ of the other Echinoderms
pass through a series of remarkable changes (Figs. 4 and 5); these are
illustrated by the twelve models of various forms of larvæ exhibited in
Case 36; in Case 35 is a set of models showing in detail the changes
undergone by a single species (_Asterina gibbosa_). A portion only of
the body of the larva is converted into the substance of the perfect
animal; the rest is either absorbed by the growing animal, or shrivels
up and disappears.
[Illustration: Fig. 4. _Pluteus._ Fig. 5. _Bipinnaria._ Developing
larvæ.]
Below the twelve models in Case 36 may be seen a representation of three
stages in the history of the Feather-star (_Antedon bifida_). The larvæ
of this Echinoderm are not free, but are attached by a stalk (Fig. 6);
in the common Feather-star and other _Comatulidæ_ the stalk is found
during larval stages only; in others, such as _Pentacrinus_, it persists
throughout life.
The presence or absence of this stalk has been taken as the first
character of importance in the classification of Echinoderma which may
be divided into two groups:—
A. PELMATOZOA,[24] or Echinoderms provided with a stalk throughout life
or in the larval stages only. To this group belong the _Crinoidea_, and
the extinct _Blastoidea_, and _Cystidea_.
[Illustration:
Fig. 6.
Pentacrinoid stage of _Antedon rosacea_.
_a_, arms; _b_, basals; _r_, radials; _s_, stalk.
]
B. ECHINOZOA, or Echinoderms without stalks at any time of their
existence. To this group belong the _Asteroidea_, _Ophiuroidea_,
_Echinoidea_, and _Holothurioidea_.
CRINOIDEA.—This Order may be described as stalked, globular, or
cup-shaped Echinoderms, in which the oral surface of the calyx or disk
looks upwards, and in which five jointed and generally branched rays
arise from the central disk. Their joints have jointed pinnules at their
sides, and the sucking-feet have the form of tentacles.
The stalked representatives of this Order are placed on tables and
brackets near the south door, and are worthy of being particularly
noticed for their fine preservation, size, and beauty. The largest
specimen of _Pentacrinus decorus_ was taken on a telegraph-wire, to the
covering of which the stalk of the Crinoid is still attached.
_Metacrinus_ is a more lately discovered genus, which appears to be
confined to the eastern seas.
A few dried unstalked Crinoids are shown in Table-case 1; these show the
leading modifications of structure in the two great genera _Antedon_ and
_Actinometra_.
[Illustration:
Fig. 7.
Comet form of _Linckia_.
]
ASTEROIDEA.—This Order comprises Echinoderms with a depressed body of
pentagonal or star-like shape, to the ventral surface of which the
ambulacral feet are confined. The rays are more or less elongate movable
arms, with skeletal structures, which consist of transversely arranged,
paired, calcareous plates, articulated with each other like vertebræ,
the series extending from the mouth to the end of the arms. The groove
in which the ambulacral feet are arranged is uncovered.
Typical specimens of this Order are exhibited in Cases 2 & 3, in which
the great variety of form in the genus _Asterias_ and beautiful examples
of _Acanthaster_ are shown. Cases 6 & 7 contain specimens illustrating
the curious habit of self-mutilation possessed by so many Echinoderms;
among Starfishes, and notably in the genus _Linckia_, the single arms
separated from the disk are able to develop a fresh disk and arms, and
so to multiply the species. Cases 9–11 contain fine series of
_Oreaster_.
OPHIUROIDEA, or “Brittle-stars.”—These Echinoderms appear to resemble
the ordinary Starfish[25]; but they differ in having the organs of
digestion, respiration, and reproduction confined to the disk, the arms
having merely the function of locomotor organs. The arms therefore are
more slender and cylindrical in form, and are sharply distinct from the
disk; the separate joints consist of two central ossicles, which leave
only a narrow canal between them, and these are covered above, below,
and at the sides by specially developed investing plates; the lateral
plates bear spines, which are always comparatively short and delicate,
as compared with the spines found at the sides of the arm in starfishes.
The principal types of this Order are exhibited in Cases 17–22; the most
exquisite of them are the forms whose arms are divided and subdivided
till they end at last in the finest threads, as in _Astrophyton_, the
so-called Basket-fish or Gorgon’s heads.
ECHINOIDEA, or “Sea-Urchins,” are Echinoderms in which the rays are not
free, as in the Starfishes or Brittle-stars, but unite to form a
compact, spherical, heart- or disk-shaped test; this test is covered
with spines, which may attain to a great length, as is shown in the fine
example of _Diadema saxatile_ from the Andaman Islands; some of the
tests are flexible and very fragile. Owing to the quantity of specimens
that are sometimes dredged at one spot, the naturalist has been able to
gain a better idea of the range of variation in the species of
Echinoderms than in some other divisions of the Animal Kingdom; an
instructive series, showing the variations of _Echinometra lucunter_, is
shown in Case 28.
The genus _Hemiaster_ offers an example of an Echinoderm in which the
eggs are laid in special pouches; the hinder ambulacra are deepened to
form pits, which are guarded by specially elongated spines (see Case
34); in these pits the young pass through all the stages of their
development.
The minute structure of the spines of Sea-Urchins is illustrated by a
series of figures on the wall.
The HOLOTHURIOIDEA, or Sea-Cucumbers, form the last order of
Echinoderms. Their body, as indicated by their English name, is
elongate, subcylindrical, with a more or less flexible integument,
according to the extent of the reduction of the calcareous skeleton; the
mouth is at one end of the body and surrounded by tentacles, the vent at
the opposite end.
As these animals cannot be shown in a dried state, some of them,
preserved in spirit, are placed in Wall-Case IV. According as they have
or have not the sucking-feet of the Echinoderma, they are ordinarily
divided into the _Pedata_ and the _Apoda_; the latter are represented by
_Synapta_, which may attain to a great length, and by _Chiridota_; the
Pedata are illustrated by the genera _Cucumaria_, _Psolus_, and
_Holothuria_. Deep-sea investigations have revealed the existence of
another group of specially modified Holothurians—the _Elasipoda_; these
are remarkable for their well-marked bilateral symmetry and the
distinctness between the dorsal and ventral portions of the body; the
prominent processes on the dorsal surface are not contractile.
An exhibition of some interest is to be found in a Table-Case against
the wall, in which there are various specimens of the edible
Holothurians—_trepang_ or _bêche-de-mer_; these were all bought in the
market at Canton, and may be taken to be typical of the kinds offered
for sale in various eastern countries.
WORMS.
By the name “Worms,” people commonly indicate a number of different
forms whose relations with one another are by no means so close as those
of a Holothurian and a Crinoid, or a Mussel and an Octopus. There are
not, indeed, any common characters by the possession of which the
worm-like animals can at once be distinguished from other animals. We
take the divisions, examples of which are here represented, either by
drawings, models, or specimens preserved in spirit separately.
The groups referred to may be enumerated as follows:—
_Platyhelmia._│Turbellaria.
„ │Trematoda.
„ │Cestoda.
_Nemertinea._ │
_Nematoidea._ │
_Chætopoda._ │
PLATYHELMIA, or Flat-Worms.—These form the lowest and simplest division
of the group.
The parasitic Platyhelmia—the Tapeworms (_Cestoda_) and the Flukes
(_Trematoda_)—occupy Case I.; the life history of the common Tapeworm
(_Tænia solium_) is shown by the aid of models and figures. A model of
the anterior end of the common Tapeworm shows the four suckers and the
crown of hooks; the unjointed neck is followed by the joints
(_proglottids_), which increase in size the farther they are from the
neck. Several entire specimens of _Tænia_ follow, showing the size of
the whole worm and the form of its joints. The structure of the body is
shown in the models of two joints. The growth and development of the
_Tapeworm_ is dependent on a migration or a change of the hosts which it
inhabits in the various stages of its life; and although the different
kinds of Tapeworm differ from each other somewhat in certain details of
their migration and development, their life history exhibits, on the
whole, the same events which we find in _Tænia solium_, a common
Tapeworm of man in Northern Europe. This worm is matured in the
intestines of man; its final joints consist merely of fertilized ova
which have already passed through the earlier stages of development;
when the joints are detached and discharged, their contents escape in
the form of embryos contained in a thick chitinous shell. If these are
now swallowed by a pig, the shell is digested by the gastric juices of
the new host, and a rounded embryo, which is provided with three pairs
of hooks, is set free; by means of these hooks the guest makes its way
through the wall of the stomach or intestine, and finally settles down
in the muscles of its host. The embryo now loses its hooks, and
gradually acquires a bladder-like form, the central cavity of which is
filled with fluid. This bladder-worm (_Cysticercus_) has its outer wall
pushed inwards at the anterior end, and on this hooks and suckers become
developed. We have now a narrow head and neck with an attached bladder,
the head being at this time hollow. If during the long time that these
bladder-worms remain alive, the pig is killed for food, its flesh is
found to be “measly”; if it is afterwards insufficiently cooked and
eaten, the worms are conveyed into the human stomach. Here the
bladder-like termination becomes absorbed, and, the neck beginning to
grow, we have the commencement of the form from which we started, and
the completion of that “vicious circle” which is so curious a
characteristic of many forms of parasitic life.
[Illustration:
Fig. 8.
_Tænia solium_: showing the head (_h_) with its suckers (_s′_) and
crown of hooks (_s_), the unjointed neck (_n_), and a few of the
succeeding joints (_j_).
]
In other Tapeworms the cyst may be more complicated than that in the
pig, as, for example, the form found in the sheep’s brain or the liver
of the horse.
Of the other Cestode parasites mention should specially be made of those
of Fishes; the vulgar notion that the parasites of these animals are
dangerous to man has been shown to be entirely erroneous.
[Illustration:
Fig. 9.
_Limnæa truncatula._
]
The _Flukes_ infest animals of all kinds; that which is most dangerous
to sheep, and the cause of much pecuniary loss (_Distoma hepaticum_), is
selected here as a type; its structure is shown by a large model, and
its life history by a series of diagrams (Figs. 10–13). Here, again, we
have a creature which infests two hosts. If the larvæ which escape from
the sheep fall on wet ground in or near a pool, they make their way to a
small pond-snail (_Limnæa truncatula_, Fig. 9), into the lung-chamber of
which they bore their way. On leaving them the larva may be, and is, too
frequently, eaten by a sheep, and makes its way into the liver of that
animal, where it causes the disease known as the “liver rot.”
The damage done by the liver-fluke may be imagined from the fact that in
the winter of 1879–80 no less than three millions of sheep died of rot
in the United Kingdom; this heavy loss is no doubt largely due to the
immense number of eggs to which a single fluke may give rise. It has
been estimated that every fluke may produce, during its life, several
thousands of eggs; and in one case Prof. A. P. Thomas found as many as
7,400,000 eggs in the gall-bladder of a sheep which was suffering from
rot, and which, at that time, had in its liver about 200 flukes.
The _non-parasitic_ Flat-worms are shown, magnified, in the upper parts
of Cases I. & II. The _Turbellaria_ proper, without any or with a simple
or a branched intestine, but without a vent, are represented by
_Convoluta_ and _Thysanozoon_: the general structure is shown by a
diagram in Case II., which is here reproduced (Fig. 14). _Planaria_,
_Thysanozoon_, and _Bipalium_ serve to illustrate the forms of members
of this group.
The Nemertine Worms (_Nemertinea_), with a straight intestine, with a
vent, and with a proboscis, may attain to a very considerable length;
_Carinella_ and _Lineus_ are represented by large figures, and various
species are shown in spirit. These forms, which used to be very
unsatisfactory to exhibit, on account of the great difficulty of
preserving them complete and uninjured, are now, with improved methods,
very satisfactorily shown, as the specimens purchased from the Marine
Biological Laboratory at Plymouth prove.
[Illustration:
Stages in the life history of the Fluke.
Fig. 10. Egg of Fluke, showing the operculum and the contained
yolk-spheres. Magnified 340 diams.
Fig. 11. An embryo forcing its way by its boring-papilla (_p_) into
the wall of the lung of a Snail (_e.p._) Magnified about 340 diams.
Fig. 12. A young _Rédia_ (natural size, ½ millimetre or ¹⁄₅₀ inch):
_pl._, pharynx; _g_, contained germs; _p_, characteristic posterior
processes of the _Rédia_.
Fig. 13. Free-swimming _Cercaria_, before the commencement of the
formation of the cyst. Magnified 100 diams.
]
[Illustration:
Fig. 14. Diagram of the structure of a Turbellarian: _ng_, nerve-
(cerebral) ganglia; _nb_, nerve-branches; _yg_, yolk-glands; _t_,
testis; _o_, ova; _ov_, ovary; _c_, cirrus; _m_, mouth; _ph_,
pharynx.
Fig. 15. Diagram of a Nemertine: _b_, brain; _m_, mouth; _n_, renal
organs; _id_, diverticula of intestine; _g_, gonads; _sn_, side
nerve-trunk; _pr_, proboscis in its dorsal sheath.
Fig. 16. Diagram of the structure of a Nematoid; _m_, mouth; _ph_,
pharynx; _a_, anus; _o_, orifice of genital tube.
]
NEMATODES (Thread-Worms or Round-Worms).—These are for the most part
parasitic, and infest plants as well as animals; the common Round-Worms
living parasitically in man (_Ascaris_, _Stronaylus_, _Trichocephalus_)
belong to this Order. Sometimes they are parasitic in their early stages
and later live a free life—such are _Gordius_ and _Mermis_. A specimen
of a Mantid is exhibited from which half the body of the infesting
_Gordius_ has already protruded (Fig. 17). One of the most remarkable
_Gordii_ is the great elongated _G. fulgur_, or “Lightning Snake,” from
Celebes. Another very large Nematode is the so-called Guinea-worm, or
_Dracunculus medinensis_, which is found beneath the skin of the leg; it
is very possible that this worm was the cause of the illness which
afflicted the Israelites in their journey through the desert from Egypt
to the Promised Land.
[Illustration:
Fig. 17.
_Gordius_ escaping from a Mantid.
]
[Illustration:
Fig. 18.
Figure of _Trichina spiralis_, showing the worms encysted in muscle.
]
Of all Nematodes the most dangerous to man is the small worm which is
known as _Trichina spiralis_ (Fig. 18); a series of models are shown
which give a good idea of the structure of the female and the smaller
male. The young make their way through the walls of the stomach of their
host, and encyst themselves among its muscles: a piece of a
sternothyroid muscle is shown, taken from a man in whose body it was
calculated there were forty millions of encysted _Trichinæ_.
Other Nematodes infesting man, such as _Filaria sanguinis hominis_, are
too small for exhibition.
Plants are not free from the attacks of Nematodes, and examples are
shown, accompanied by an illustrating figure, of the Ear-cockle gall of
wheat; this gall is due to the injuries inflicted by a minute
Thread-worm—_Tylenchus tritici_. Wheat is, of course, by no means the
only cultivated plant that is attacked by these minute worms; the
history of most has, however, still to be made out.
Holding a somewhat uncertain position in relation to the Round-worms are
the parasitic _Acanthocephali_ (Thorn-headed Worms) and the
free-swimming _Chætognatha_, or Bristle-jawed Worms; examples of both of
these groups are shown, together with diagrams illustrative of their
general structure.
ANNULATA or Chætopoda.—So-called because consisting of a series of
rings, and being provided with chætæ or bristles; they are to be
associated with the Arthropoda, under the one head “Appendiculata,” a
better name than “Articulata,” since Cuvier did not include worms in his
group. The creatures that are most familiarly called worms are to be
found in Case III.; here are a few examples of the numerous kinds of
worms that are found living freely in the sea, of earth and freshwater
Worms, and of Leeches. All these worms are distinctly characterized by
the fact that they consist of a number of definite rings (somites),
whence they have been called _Annulata_. The marine Worm and the
Earthworm differ from the Leech in that these rings are provided with
chætæ or bristles, of which there are a number in each bundle in the
marine, and a few only in the terrestrial or freshwater form: hence the
marine Worms are called _Polychæta_ and the latter _Oligochæta_.
The former are divisible into two great groups. There are those that are
free-swimming and are able to forage for themselves, such as the lovely
Sea-mouse (_Aphrodite aculeata_), the large _Eunice gigantea_, the
common _Nereis pelagica_, or the exquisitely coloured _Chloeia flava_.
Others live a more retired life, dwelling in tubes, which they fashion
for themselves; they lead either a solitary or a social life. Here we
have examples of _Sabella_, _Sabellaria_, _Serpula_; a number of forms
of worm-tubes, showing their great variety and beauty (see especially
the delicate _Filograna_), are to be seen in the small Table-cases
placed against the north wall of the Gallery. Attention should be
especially directed to Mr. A. T. Watson’s beautiful preparations of
_Terebella littoralis_. We give a figure (Fig. 19) after a drawing by
that gentleman of the home of _Panthalis oerstedi_, the tube-forming
habits of which have been carefully observed by him.
[Illustration:
Fig. 19.
Home of _Panthalis oerstedi_.
]
[Illustration:
Fig. 20.
Section across the body of an earth-worm to show the disposition of
the more important organs; the body wall (_w_) consists of dermis,
circular, and longitudinal muscles; the body cavity is divided by
membranes (_c_) into a series of chambers, in each of which opens
the mouth of a coiled nephridium (_n_). The axis of the cavity is
occupied by the intestine (_i_); above and below it is a longer
blood-vessel (_v_), and below it is also the central nerve-cord
(_nc_).
]
The _Oligochæta_ are represented by the common Earthworm, the influence
of which in the formation of mould and in the general ploughing of the
soil was carefully investigated by Mr. Darwin; and by the little
_Tubifex rivulorum_ (Bloodworm), which owes both its red colour and its
ability to dwell in mud, which is so poor in oxygen as to be unfit for
respiration, to the same chemical compound as that which gives the red
colour to our blood and carries the oxygen of respiration all over the
body.
[Illustration:
_Acanthobdella_: _e_, eyes; _ch_, chætæ; _s_, sucker.
]
The _Hirudinea_, or Leeches, are often said to be distinguished from the
_Chætopoda_ by the absence of bristles, but, as a fact, _Acanthobdella_
(Figs. 21 and 22) has very well marked bristles. They always have a
sucker at the hinder end of the body by which they are attached to their
prey; they are found in fresh water (_Piscicola_), on sea-fishes (as
_Pontobdella_), or in moist places, as the Leech (_Hirudo_). The
last-named has three jaws, armed with as many as ninety denticles.
_Trochetia subviridis_ (Land-Leech) is a species which is found rarely
and sporadically in England.
The Myzostomaria form a division of Polychæta all the members of which
live parasitically on Crinoids, and otherwise present great differences
in their habits. Some move about freely on the Crinoids they infest,
others are more sluggish and rarely move, others produce galls or cysts
on their host, and yet others are internal parasites, and live in the
alimentary canal. It is of interest to note that there are corresponding
degrees of difference between the young and old specimens of the
different groups of species.
The general organisation of Myzostomaria is shown in the accompanying
figure (Fig. 23) in which the dorsal wall of the body is supposed to be
transparent so as to allow of the chief internal organs being seen.
[Illustration:
Fig. 23.
Diagram of Myzostomum to show the general form of the body and the
marginal extensile cirri (_c_); within these and on the ventral
surface are four pairs of suckers, and more internally five pairs of
appendages each bearing two hooks; the proboscis (_p_), the
digestive tract and its ramifications, and the reproductive organs
are outlined as if seen through a transparent wall; _a_, anus.
]
The last group of Worms here represented is that of the _Gephyrea_; with
the advance of our knowledge it is probable that they will be found to
be more intimately allied to the _Annulata_ than is now generally
supposed; it will be seen indeed that _Echiurus_ has bristles at its
hinder end; _Sipunculus_ is the best known representative of the unarmed
_Gephyrea_; _Bonellia_ is interesting both from the fact that it owes
its green colour to a matter closely resembling the chlorophyll of green
plants, and from the possession by the female of a proboscis, which is
protruded from the hole in the rock occupied by the worm: the male is
very much smaller than the female, and is not nearly so well developed.
Owing to the mode of lighting the Gallery, the visitor may have to shift
his position several times before gaining a good view of the whole
length of the proboscis.
INDEX.
Acanthaster, 114
Acanthobdella, 124
Acanthocephali, 122
Acmæidæ, 9
Actinometra, 113
Adeonidæ, 66
Ætheriidæ, 39
Alcyonidium, 67
Amaroucium, 99
Amathia, 67, 68
Amphineura, 7–9
Ampullariidæ, 14
Ancylus, 27
Annulata, 122
Anodonta, 39
Antedon, 111–113
Aphrodite, 122
Aplacophora, 9
Aplysiidæ, 23
Apoda, 115
Appendiculariidæ, 105
“Apple-Snails”, 14
Architeuthis, 48
Argonauta, 46
Ascaris, 120
Ascidia, 83, 93
Ascidiacea, 90
Ascidiæ Compositæ, 96
Ascidiæ Salpiformes, 99
Ascidiæ Simplices, 90
Ascopodaria, 73
Astartidæ, 38
Asterias, 108, 114
Asterina, 111
Asteroidea, 112, 113
Astrophyton, 114
Atlantidæ, 22
Auger-shells, 21
Auriculidæ, 26
Aviculidæ, 33
Bartlettia, 39
Basket-fish, 114
Basommatophora, 26
Bêche-de-mer, 115
Bipalium, 118
Bird’s head Coralline, 58
Bladder-worm, 117
Blastoidea, 112
Bloodworm, 124
Boat-shells, 20
Boltenia, 91, 92
Bonellia, 126
Borer, 43
Botryllidæ, 96, 97
Botrylloides, 97
Botryllus, 97, 98
Bowerbankia, 67
Brachiopoda, 74–82
Brechites, 44
Bristle-jawed worms, 122
Brittle Stars, 114
Bubble-shells, 23
Buccinidæ, 18, 19
Bugula, 58, 59
Bullidæ, 23
Caberia, 62
Calamaries, 4
Calyptræidæ, 14
Cardiidæ, 40
Carditidæ, 38
Carinariidæ, 22
Carinella, 118
Carrier-shells, 17
Cassididæ, 18, 19
Catenicellidæ, 66
Cavolina, 23
Cellularina, 58
Cephalopoda, 45–50
Cerithiidæ, 17
Cestoda, 116
Cestode parasites, 116
Chætoderma, 9
Chætognatha, 122
Chætopoda, 116, 122
Chama, 41
Chelyosoma, 93
Chilostomata, 57
Chirodota, 115
Chiroteuthis, 47
Chitonidæ, 7, 8
Chloeia, 122
Clams, 41
Clavagellidæ, 44
Clavelina, 94, 95
Clavelinidæ, 94
Clione, 24
Coat-of-mail shells, 7
Cockle, 40
Colella, 97, 98
Comatulidæ, 111
Conchologists, 17
Conidæ, 21
Convoluta, 118
Coralliophilidæ, 20
Corbula, 42
Cowries, 14
Crania, 74, 80
Creeping Coralline, 60
Crepidula, 14
Crinoidea, 112
Crinoids, 109
Crisia, 69
Crisiidæ 69
Cristatella, 71, 72
Cryptochiton, 8
Cryptoplax, 8
Ctenostomata, 57, 66
Cucumaria, 115
Culeolus, 92
Cup-and-saucer Limpets, 14
Currant Squirter, 92
Cuspidariidæ, 45
Cuttlefish, 48
Cyclophoridæ, 13
Cyclostomata, 57, 68
Cynthiidæ, 91
Cypræidæ, 14
Cysticercus, 117
Cystidea, 112
Dentaliidæ, 28
Desert-snail, 6
Diadema, 114
Diazona, 94
Dipsas, 36
Discinidæ, 80
Distoma, 118
Distomidæ, 96
Doliidæ, 18
Doliolidæ, 100, 103
Doliolum, 90, 103
Dracunculus, 121
Dyscolia, 82
Ear-cockle gall, 122
Ear-shells, 12
Earthworms, 124
Echinoderma, 106–115
Echinoidea, 112, 114
Echinometra, 114
Echinozoa, 112
Echiurus, 126
Ectoprocta, 57
Elasipoda, 115
Electra, 62, 63
Elephant-tooth shell, 1
Entoprocta, 57, 73
Escharina, 64
Eulamellibranchia, 38
Eunice, 122
Euthyneura, 23
False Limpets, 9
Fan-Mussel, 34
Fan-Shells, 38
Fasciolariidæ, 18
Feather-star, 111
Filaria, 122
Filibranchia, 32
Filograna, 123
Fissurellidæ, 11
Flat-worms, 116
Flukes, 118, 119
Flustra, 54, 62
Flustrina, 62
Fountain-shell, 17
Fredericella, 71
Freshwater Limpets, 27
Freshwater Mussels, 38
Freshwater Oyster, 39
Freshwater Polyzoa, 70
Freshwater Snails, 7, 26
Freshwater Worms, 122
Gapers, 42
Gastropoda, 9–28
Gephyrea, 126
Giant Clam, 41
Glassy Nautilus, 22
“Glory-of-the-Sea” Cone, 21
Glottidia, 78
Goodsiria, 99
Gordius, 120, 121
Gorgon’s head, 114
Guinea-worm, 121
Gymnolæmata, 57
Gymnosomata, 24
Haliotidæ, 12
Hammer Oyster, 33
Harpidæ, 20
Harp-shells, 20
Helicidæ, 27, 28
Helmet-shells, 18
Hemiaster, 110, 114
Heteropoda, 22
Hippuritidæ, 41
Hirudinea, 124
Hirudo, 124
Holothuria, 107, 115
Holothurioidea, 112, 115
Ianthinidæ, 16
Idmonea, 69
Julinia, 98
Keyhole Limpets, 11
Kinetoskias, 59–61
Kuphus, 43
Lamellibranchia, 29–45
Land-Snails, 26
Larvacea, 104
Leeches, 124
Lepralia, 64
Leptoclinum, 98, 99
Lichenopora, 69
“Lightning-Snake”, 121
Lily-Encrinites, 110
Limidæ, 37
Limnæa truncatula, 118
Limnæidæ, 26
Limpet, 9
Limpet Snails, 26
Linckia, 113, 114
Lineus, 118
Lingula, 74, 75, 77–79
Lithodomus, 33
Littorinidæ, 14
Liver-fluke, 118
Loligo, 46
Lophopus, 71, 72
Lotoriidæ, 18
Loxosoma, 73
Lucinidæ, 38
Lunulites, 64
Mactridæ, 40
Magellania, 75, 77, 82
Magilus, 20
Malleus, 33
Margaritana, 39
Marine worms, 122
Melaniidæ, 17
Meleagrina, 33–35
Melons, 20
Membranipora, 63
Mermis, 120
Metacrinus, 113
Mineralogists, 17
Mitridæ, 18
Molgula, 91
Molgulidæ, 90
Mollusca, 1–53
Money-Cowry, 15
Moss Animals, 56
Mucronella, 65
Mülleria, 39, 40
Muricidæ, 20
Mussels, 33
Mussels (freshwater), 38
Myidæ, 42
Mytilidæ, 33
Myzostomaria, 125
Naked-gilled Molluscs, 24
Natica, 15
Nautilus, 45, 49
Nematoidea, 116, 120
Nemertinea, 116, 118, 120
Neomenia, 9
Nereis, 122
Neritidæ, 13
Non-parasitic Worms, 118
Nucleobranchiata, 22
Nuculidæ, 31
Nudibranchia, 23, 24
Octacnemidæ, 100
Octopus, 46, 47
Oikopleura, 104
Oligochæta, 122, 124
Olividæ, 20
Ophiacantha, 111
Ophiuroidea, 112, 114
Opisthobranchia, 23
Orange Cowry, 15
Oreaster, 114
Ormers, 12
Ostreidæ, 36
Ovulidæ, 15
Oyster, 36
Panthalis, 123
Paper-Nautilus, 46
Patella, 9
Pearl-Oyster, 33
Pearly Nautilus, 50
Pectinibranchia, 13
Pectinidæ, 38
Pedata, 115
Pedicellinidæ, 73
Pelmatozoa, 111
Pentacrinus, 111, 113
Periwinkle, 14
Perophora, 95, 96
Phallusia, 93
Pharyngodictyon, 98, 99
Pholas, 43
Phylactolæmata, 70
Piddocks, 43
Pinna, 33, 34
Piscicola, 124
Planaria, 118
Platyhelmia, 116
Pleurotomaria, 11
Pleurotomatidæ, 20
Plumatella, 71, 72
Polychæta, 122
Polyclinidæ, 96
Polyplacophora, 7
Polyzoa, 54–73
Pond-Mussel, 5
Pond-Snails, 27
Pontobdella, 124
Poromyidæ, 45
Poulp, 7
Proneomenia, 7
Protobranchia, 31
Pseudolamellibranchia, 33
Psolus, 115
Pterocera, 18
Pteropoda, 23
Pterotracheidæ, 22
Pulmonata, 25–28
Purpura, 20
Pyrosoma, 90, 99
Radiolitidæ, 41
Razor-shells, 42
Retepora, 65, 66
Rhodosoma, 93, 94
Rhopalæa, 96
Rhynchonella, 82
River-Snails, 13
Rock-shells, 20
Round Worms, 118
Sabella, 122
Sabellaria, 122
Saddle-Oysters, 32
Salpa, 90, 100–103
Salpidæ, 100
Scala, 17
Scallops, 38
Scaphopoda, 28
Schizoporella, 65
Scorpion-shells, 18
Screw-shells, 17
Scrobiculariidæ, 40
Scrupocellaria, 60, 62
Scutibranchia, 9
Sea-Butterflies, 23
Sea-Cucumbers, 106, 115
Sea-Hare, 23, 24
Sea-Lilies, 106
Sea-Mat, 54
Sea-Mouse, 122
Sea-Slugs, 106
Sea-Urchins, 106, 114
Sea-Woodlice, 7
Selenariidae, 64
Semele, 40
Sepia, 46–48
Sepiola, 46
Septibranchia, 45
Serpula, 122
Ship-worm, 43
Silk Coralline, 68
Siphonariidæ, 26
Sipunculus, 126
Slipper-Limpets, 14
Slit-Limpets, 11
Slugs, 28
Snails, 27
Solenidæ, 42
Solenomyidæ, 31
Spatangus, 109
Spirula, 49
Spondylidæ, 37
Squid, 46
Stalked Crinoids, 110, 112
Starfishes, 108
Stichopus, 107
Streptoneura, 9
Strombidæ, 17
Strongylus, 120
Styelopsis, 92
Stylommatophora, 27
Synapta, 107, 115
Tænia, 116, 117
Tapeworm, 116
Tectibranchia, 23
Tellina, 40
Terebella, 123
Terebratula, 74, 82
Terebratulina, 82
Terebridæ, 21
Teredo, 43, 44
Testacella, 28
Tethys, 24
Thaliacea, 100
Thecalia, 38
Thecidium, 81
Thecosomata, 23
Thorn-headed Worms, 122
Thorny Oysters, 37
Thread-worms, 118
Thysanozoon, 118
Tooth-shells, 28
Top-shells, 12
Trematoda, 116
Trepang, 115
Trichina, 121
Trichocephalus, 120
Tridacnidæ, 41
Trochetia, 125
Trochidæ, 12
Trumpet-shells, 18
Tubifex, 124
Tubulipora, 69, 70
Tunicata, 83–105
Tun-shells, 18
Turbellaria, 116, 118, 120
Turbinidæ, 12
Turritellidæ, 17
Tylenchus, 122
Umbraculidæ, 23, 24
Umbraculum, 24, 25
Umbrella-shells, 23
Unionidæ, 38, 39
Veneridæ, 40
Venus-shells, 30
Vermetidæ, 17
Vesicularia, 67
Violet Snails, 16
Viviparidæ, 20
“Volutes”, 20
Volutidæ, 20
Water-Clams, 37
Watering pot-shells, 44
Water Spondyli, 37
Weaver’s-shuttle, 15
Whelk, 18
Window-shells, 32
Wing-shells, 33
Winkles, 14
Worms, 116–126
Worm-shells, 17
Worm-tubes, 123
Xenophoridæ, 17
LONDON: PRINTED BY WILLIAM CLOWES AND SONS, LIMITED, STAMFORD STREET
AND CHARING CROSS.
-----
Footnote 1:
A framed series of photographs, illustrating different kinds of
radulæ, is placed on the east wall of the gallery.
Footnote 2:
From Woodward’s ‘Manual of the Mollusca,’ published by Lockwood & Son.
Footnote 3:
From ‘The Cambridge Natural History,’ Messrs. Macmillan & Co.
Footnote 4:
From the Greek: _pteron_, wing, and _pous_, foot.
Footnote 5:
From the Greek: _scaphe_, a small boat, and _pous_, a foot—the foot of
some Scaphopods being somewhat pointed like the prow of a vessel.
Footnote 6:
The term Lamellibranchia is used instead of Pelecypoda in deference to
the wish of Professor Lankester.—E. A. S.
Footnote 7:
From the ‘Encyclopædia Britannica.’ Messrs. A. & C. Black.
Footnote 8:
From ‘The Cambridge Natural History.’ Messrs. Macmillan & Co.
Footnote 9:
From the Greek: _kephale_, head, and _pous_, foot.
Footnote 10:
_Zoon_, animal; _oikos_, house.
Footnote 11:
_Lophos_, plume; _pherein_, to bear.
Footnote 12:
_Polus_, many; _zoon_, animal.
Footnote 13:
_Bryon_, moss.
Footnote 14:
_Ektos_, outside; _proktos_, vent.
Footnote 15:
_Cheilos_, lip; _stoma_, mouth.
Footnote 16:
_Gumnos_, naked; _laimos_, throat.
Footnote 17:
_Ktenos_, of a comb.
Footnote 18:
_Phulassein_, to guard; _laimos_, throat.
Footnote 19:
_Kuklos_, circle.
Footnote 20:
_Entos_, inside; _proktos_, vent.
Footnote 21:
_Vibraculum_, a bristle.
Footnote 22:
_Pyrosoma_—_pur_, fire; _soma_, body.
Footnote 23:
It should be mentioned that one high authority, Prof. W. K. Brooks,
does not regard the life history of _Salpa_ as an example of
alternation of generations, but considers the solitary _Salpa_ to be,
not asexual, but a female which produces a chain of males; but it is
impossible to enter into a difficult question of controversy here.
Footnote 24:
From the Greek _pelma_ = a stalk.
Footnote 25:
The Asteroidea or Ophiuroidea may be counted under the name
Stelliformia.
------------------------------------------------------------------------
TRANSCRIBER’S NOTES
1. Typos fixed; non-standard spelling and dialect retained.
2. Used numbers for footnotes, placing them all at the end of the last
chapter.
3. Enclosed italics font in _underscores_.
4. Enclosed bold font in =equals=.
5. The caret (^) serves as a superscript indicator, applicable to
individual characters (like 2^d) and even entire phrases (like
1^{st}).