Zoophytes, Cœlenterata, Medusæ, Corals, Hydrozoa.
[Illustration: Fig. 344.--Gorgonia Nobilis.]
A study of the earliest growth of the Cœlenterata has shown that their
internal cavities are nothing more than regular radiate out-growths of
the internal structures. The result of this development is a condition
which does not occur again in the whole of the animal kingdom. There
is a system of cavities all in open communication one with another, no
closed blood vascular system, and no specialised respiratory apparatus.
Again, all the animals that constitute this large group are radiate
in structure, that is, when viewed from above they are typically
star-shaped, and if cut across, every horizontal section shows a
symmetrical arrangement of the several parts around a centre. There
are other radiate animals, as the Echinoderms, but while in these five
is the fundamental number of rays, in the Cœlenterata the rays are a
multiple of four, six and upwards. The skeleton or framework of each
differs, and when the Cœlenterata form calcareous structures, these are
quite different from the tests of the sea-urchins; and in all cases
the anterior portion of the body is crowned with one or more circles
of tentacles, which remain perfectly flexible and flower-like. The
most highly-developed of the free forms are the sea-anemones and the
jelly-fish. These have no hard or calcareous skeleton whatever, but
withal they are, in the opinion of naturalists and microscopists, the
most beautiful objects among Zoophytes.
In spite of their variety of forms, the Cœlenterata seem to be as
incapable of higher development as do the Echinoderms, and they have
failed to make headway in fresh water, but it is not improbable that
some of the simplest forms of the whole group may have given rise to
higher animal forms, while the sea anemones, corals, &c., being those
descendants of the primitive simple form, have retained the original
type of organization almost unchanged.
[Illustration: Fig. 345.--_Hydra viridis_, adhering to a stalk of
_Anacharis alsinastrum_.]
The type of the group is the Hydra, a fresh-water polyp, commonly found
attached to the leaves and stems of many aquatic plants, or floating
pieces of stick. Two species are well known to microscopists, the _H.
viridis_, or green polyp, and the _H. vulgaris_, somewhat darker in
colour, probably dependent upon the nature of its food. The third,
less common species, the _H. fasca_, is distinguished from both by the
length of its tentacles, which, when fully extended, greatly exceed
those of either of the before-mentioned. The fresh-water group measures
from one-eighth to the one-third of an inch in length, and form simple
stocks of one, two or more branches. They almost exactly resemble in
form the polyps of the Hydractinia, which are provided with a circle of
tentacles. When placed in a vessel of water and left undisturbed they
often attach themselves to the side, where they may be examined with
a moderate power at leisure. They are then seen to spread out their
tentacles like fine threads, and seize upon any small creature that may
come in their way, and by the same means convey it to a mouth capable
of great extension. All Hydra possess stinging-cells, by means of which
they paralyse their prey. Many Hydra attain to a large size, and shoot
out long poisonous filaments; they also possess smaller kinds of smooth
cells, which appear to be employed for an entirely different purpose,
but for what is not positively known. Hydra usually multiply by means
of buds, an out-growth from the body, and these remain attached to
the mother stalk for some time, often long enough to give rise to one
or two smaller buds. Single eggs are also developed in the ectoderm
beneath capsules, or wart-like prominences. The adult animal can be cut
to pieces, and from each piece a new individual will be developed. This
method of reproduction was first tried by the naturalist Trembley in
1739, whose experiments in this direction excited the greatest interest
among the naturalists of the middle of the last century. _Hydra fusca_
in various stages of development is given in outline in Fig. 346.
[Illustration: Fig. 346.
1, 2, 3. Hydra in various stages of development; 4. Group of _Stentor
polymorphus_, many-shaped Stentor; 5. Englena; 6. Monads.]
In the polyps belonging to this family the body-structure for the most
part consists of a homogeneous aggregation of vesicular granules, held
together by an intercellular sarcode, and capable of great extension
and contraction, so that these animals can assume a variety of forms
and extend their body and tentacles until the latter become almost
invisible. It was the resemblance in this respect to the fabled Hydra
that originated the name. Its organ of prehension is termed the
_hasta_; this consists of a sac or opening at the terminal end of the
tentacle, within which is seen a saucer-shaped vesicle, supporting a
minute ovate body, which carries a sharp calcareous piece termed a
_sagitta_ or arrow. Although the fresh-water Hydra may be regarded
as typical of this group of animals, marine fauna furnish a far more
extensive group in the corals, jelly-fish, and sea-anemones.
A smaller group, the Ctenophora, although members of this sub-kingdom,
have not yet found their true position; nevertheless they are
interesting glassy, transparent creatures, either shaped like apples,
melons, or Phrygian caps, or else forming bands of some considerable
length; all are wonderfully transparent, with the single exception of
the Beroë. These inhabit the open sea, and are only seen inshore when
driven in by currents or strong winds. Their position in the water is
usually more or less vertical, the mouth being turned downwards. The
portion from which this group derives its name is the ribs, which are
symmetrically arranged, and consist of rows of short transverse combs,
each forming rows of cilia, which, as they wave to and fro, constitute
a swimming or rowing plate, their activity in the water depending upon
the will of the animal. They are also provided with an oral umbrella,
and capturing filaments or tentacles with hair-like branches. These
tentacles, attached to the sides of the animal, are capable of erection
or withdrawal into pockets. Great variety is seen in these accessory
organs of locomotion; for instance, the Cydippidæ (Plate XVII.) have
only arms, but these are remarkable for their length, and serve for
the purpose of capturing food as well as for steering. The most
interesting, if not the most beautiful of the Ctenophora, are the
Beroidæ; it is this family that bear a resemblance to the Phrygian
cap (Plate XVII., _e_). The mouth is wide, but it appears to have no
capturing tentacles, and yet their habits are carnivorous; they will
even devour their own relations. Many of the genus are phosphorescent,
and in place of stinging-cells have small spherical knobs beset with
sticky globules, in which their food becomes entangled, and these are
apparently in constant use.
[Illustration: PLATE XVII.
ZOOPHYTES, ASTEROIDS, NUDIBRANCHIS, ACALEPS, ECHINOIDS, CTENOPHORA,
TUNICATA, AND CRUSTACEANS.]
_The Stinging Series_, _Cnidaria_, comprise sea-anemones, corals,
jelly-fish among marine animals, and Hydra among the fresh-water
Cœlenterata; and derive their name from a remarkably curious feature,
the so-called stinging capsules. These are not only offensive, but
also defensive weapons with all the animals belonging to this group;
the possession of which has converted the bell-like jelly-fish into
a simple Cnidarian. The principal change is in the gelatinous layer
between the outer wall and the inner digesting layer of the ectoderm.
But without entering further into their structure and relations, the
stinging-cells and batteries claim especial attention. These cells vary
considerably in size without their characteristics being essentially
changed. The protoplasm of the cell is modified into a tolerably firm
substance, enclosing an oval or cylindrical vesicle. Closely associated
with this is a pointed process, standing up far above the level of the
outer covering, known as the _cnidocil_. Within the vesicle is found,
either spirally rolled up or in an irregular tangle, a long filament
or hollow tube, a prolongation of the vesicle, but turned outside in.
This tube is more than twenty times as long as the cell, is pointed
at the tip, and beset with two rows of fine spirally-arranged barbed
hooks. When the cnidocil is touched or irritated, this filament is
violently shot out, being then turned inside-out, like the fingers of
a glove. So long as the thread remains rolled up within the vesicle
the barbed hooks remain in their tube, but when shot out, they change
to the outside. The rolled-up filament appears to be filled with
some poisonous material, which is ejected when the tube is shot out,
and where the point strikes a wound is inflicted, so that unless the
prey is stronger than the attacker it cannot escape. The greater the
struggle, the larger the number of capsules discharged in order to kill.
[Illustration: Fig. 347.--The Stinging Capsules of Cnidaria.
1 and 2. Retracted filaments; 3. Partly protruded; 4. Fully protruded.
Magnified × 600. (Warne.)]
_Polypomedusæ._--Among the higher development of the stinging
group is the jelly-fish. The Siphonophora, as represented by the
Portuguese man-of-war, are in their turn the highest development of
swimming-bells, and exhibit many modifications and combinations of
individuals. The tentacles of the Physalia, the best known, are stiff
with batteries of stinging-capsules, the sting of which is more like
the shock of the electric current. The _Challenger_ soundings brought
to light some remarkably interesting forms, and these have furnished
much work for the microscope, as all their larval forms are extremely
curious. Among the Hydromedusæ there are many different life histories.
Take the jelly-fish, the eggs of which have given up forming stocks,
and are hatched out at once as Medusæ. There are others, the eggs of
which form stocks; others, again, in which the sexual individuals do
not swim away as jelly-fish. The last were at one time described under
a new name, because of one or two curious forms being taken creeping on
the ground. This creeping Medusa (_Clavatella prolifera_) has six arms,
the tips of which are provided with true suckers, and on these it walks
as on stilts, while from each arm a short stalk arises, the swollen
end of which is beset with stinging capsules. It has an extensile
mouth-tube, and feeds upon small crustaceans found on seaweeds.
[Illustration:
Fig. 348.--_Plumularia primata._ _Doris tuberculata_ seen clinging to a
fucus.]
Among the forms that swim away as jelly-fish a very curious example
is presented in _Corymorpha mutans_. These swim about for a time, and
then firmly attach themselves by numerous thread-like appendages,
forced into the sand, and where the young prepare for their next
metamorphosis. As an example of the stocks of those representatives
which do not swim away as jelly-fish, take the beautifully-feathered,
plant-like creatures found erect along the seashore, the Sertularia
(Fig. 358, No. 12) and Plumularia. _Plumularia primata_, Fig. 348.
Other members of these groups will be found in Plate IV., Nos. 95-99.
[Illustration: Fig. 349.--Group of female stock of _Hydractinia
echinata_.
_a, a._ Nutritive individuals; _b, b._ Female individuals and groups of
eggs. Highly magnified.--(Warne.)]
In addition to the nutritive individuals, there are the egg-bearing;
these do not become free-swimming individuals. One small family is
neither branched nor feathered--the _Hydractinia echinata_, found in
the North Sea and on the Norwegian coasts, where it attaches itself to
the shells of gastropods, selecting those inhabited by hermit crabs.
The part of the stock common to all the individuals is the skin-like
portion which adheres to the surface of the shell. In some spiny
processes are produced, and the nutritive canals running down the stems
of the polyps are continued into the membrane belonging to the stock,
as seen in Fig. 349.
The nutritive individuals are distinguished by long tentacles, mouths,
and digestive canals. The females have no mouths, and are supplied with
food through the system of canals running to them from the nutritive
males. These reproductive members are furnished with stinging threads
instead of tentacles for the protection of their ova. The ciliated
larvæ, in a very short time, swim off to found new colonies.
[Illustration: Fig. 350.--Medusæ, Jelly-fish.]
The free-swimming jelly-fish (Fig. 350, and Plate XVII., _c_ and _d_)
belong to the order Scyphomedusæ. These are characterised by their
delicate colouring, and from the arrangement of their nervous system,
which can only be made out by staining. Some new and curious forms
were dredged from a depth of more than 6,000 feet off the coast of New
Zealand, varying in size from an inch to twenty inches; many having
from four to eight or ten eyes arranged along the margin.
_Anthozoa._--From the free-swimming we turn to a group of permanently
fixed polyp forms, the sea-anemones and corals. The development of
Monoxenia commences with the egg, repeatedly dividing into many parts
(Fig. 351, C, D, and E), by a process common to the animal kingdom,
termed egg-segmentation, in this particular instance proceeding from
an apparently hollow sphere, A, enclosing a single layer of cells,
G. Each cell sends out a long cilia, or whip-like process, _F_, by
means of which the larva turns about and swims in the body fluid of
the parent polyp. One half of the sphere now becomes enfolded into
the other half, H, and forms what is termed a _gastrula_, I, K. The
gastrula stage of Monoxenia is of the simplest kind, the larva forming
a sac, with walls consisting of two layers, an outer, or ectoderm,
and an inner, or endoderm. The transition from the flat dish shape,
H, to the sac with a narrow mouth is at once clear, and the knowledge
that all the Cœlenterates proceed from similar larvæ, and that all the
complications of their various systems are developed from a simple
gastrula, throws much light on their anatomy. During these transitions
the endoderm, whose cells multiply, continues as an uninterrupted
lining to the stomach and its appendages, while the ectoderm yields the
cuticular elements.
[Illustration: Fig. 351.--Stages in development of _Monoxenia
Darwinii_, × 600.--(Warne.)]
A third intermediate gelatinous layer, the _mesoglæa_, arises between
the two layers in which muscles and connective interstitial tissue
appear. In the mesoglæa of one species of coral calcification takes
place; this internal calcification has but a small share in the work of
the great rock-making corals, their most important calcification being
external. In Monoxenia, although the transition from the gastrula larva
to the adult animal has not been seen, there can be no doubt as to how
this is carried out, the transformations having been watched throughout
in other species. The larva attaches itself with the end opposite the
mouth, the cilia disappear, and after the mouth-tube has been formed
by the folding in of the anterior end along the longitudinal axis of
the body, and has thus become marked off from the stomach, eight hollow
tentacles rise round the mouth as outgrowths of the body cavity, or as
direct continuations of the stomach.
Like all other corals, Monoxenia periodically multiply by means of
eggs, which are formed either in the walls of the radiating partitions
or septa, or along the free edges. These are ejected through the oral
opening. As a rule, the polyps are either male or female; but in
stock-forming species individuals of the two sexes are often mixed.
Monoxenia may be taken as the simplest type of the regularly radiate
polyps; in all the different organs being repeated in regular rings
round a central axis; the mouth also is circular. From this interesting
account, drawn by Haeckel, of a simple polyp, it will be at once seen
what kind of radiate animal it is that builds up the coral reefs.
“No garden on earth can match the gardens of the sea that circle
the northern part of Australia. As the tide ebbs in azure sunset,
coral-reefs peer out symmetrically arranged in beds and intersected by
emerald pathways coursing through corals of all hues and tints fathoms
deep in the channels.”
In a growing polyp-stock the individuals usually remain in organic
connection; that is to say, each first provides for itself and then
shares its superfluity with others, sometimes by means of a continuous
reticulated system of canals perforating the calcareous substance which
often separates the members of one stock from another. The whole colony
may thus be physiologically one creature with many mouths. There are
others that remain single, as the inverted pyramidal-looking bodies,
Fungidæ, commonly called “Sea-mushrooms,” found in great variety. The
colour of the polypidom is white, of a flattened round shape, made
up of thin plates or scales, imbedded in a translucent jelly-like
substance, and within is concealed a polyp; the footstalk, by means of
which the animal is attached to the rock, is of a calcareous nature
(Fig. 352, No. 1).
[Illustration: Fig. 352.--Sea-Anemones.
1. _Actinia rubra_, tentacles displayed and retracted; 2. _Heticictis
bellis_; 3. _H. bellis_, seen from above.]
_Hexactinia_ (six-rayed polyps) are not limited to six rays, as the
name given them may seem to imply; they are, in fact, very numerous
in some of the largest and most gorgeous of the sea-anemones. All are
distinguished by their solitary manner of life, their size, and their
vivid and variedly beautiful colouring. The endoderm is firm, and when
the animal withdraws its tentacles and shuts its body substance in,
there is some difficulty in penetrating to the interior. It does not,
however, secrete a calcareous skeleton inside or out, as do the true
coral polyps. Among the Hexactinia the sea-anemone (Fig. 352) takes the
first place.
These beautifully coloured creatures are, for the most part, found
attached to the spot selected by the larvæ; a few species bore into the
sand with the posterior part of the body, or build a sheath, which they
inhabit. They are voracious feeders, and devour large pieces of flesh,
and even mussel and oysters, sucking them in by means of their long
grasping tentacles. Well-fed anemones change their skin frequently,
during which process they remain closely retracted; the shed skin forms
a loose girdle around the base. _Actinia bellis_ not infrequently
attach themselves to the shells of crabs and whelks, and are thus
carried to pastures new.
[Illustration: Fig. 353.--Larvæ of Sea-Anemones, _Actinia effœta_,
highly magnified.]
On account of the ease with which anemones are kept in captivity, their
mode of reproduction can be closely observed. With but few exceptions
they develop from eggs, and in the course of a few weeks are hatched
into ciliated infusorial larvæ, presenting most curious and exquisite
representations of jugs and jars, with cover lids (as seen in Fig. 353,
_Actinia effœta_). These evince the handiwork of a master hand in the
ceramic art. They are, however, of so translucent a nature as to permit
of the internal structure being seen to consist of nerves and vessels,
and which are rendered more apparent by staining. These settle down
in a week or ten days, and then shed their cilia, the first tentacle
appearing during the process of attachment.
In some species the young Actiniæ are seen to pass through their whole
development within the body cavity of the parent. Most anemones are
provided with several circles of more or less cylindrical tentacles,
and there are a few specially beautiful species which, besides
tentacles of the usual form, have, either within or without the
ordinary circle of tentacles, lobed or leaf-like tactile and seizing
organs. These belong to the family of the beautiful Crambactis of the
Red Sea. Below these grasping tentacles comes a circle of thicker arms
unlike the former, being spindle shaped. All the tentacles of the
sea-anemones are hollow with a fine aperture at the tip, through which,
on closing rapidly, it is seen to expel a jet of water.
_True Corals._--It will have been noticed in the foregoing remarks
that in the soft body-division of the Hexactinia there are both single
individuals and colonies joined together to form stocks. The same
diversity in this respect will be found among corals proper, with this
difference, that the skeleton-forming polyps, by combining, build up
substantial structures in the most secure and advantageous positions.
Now it so happens that all the corals found about our coasts are
generally small and solitary dwellers, one of the best known of which
is the scarlet crisp coral, Flabellum, and is characterised by the
slit-like form of the mouth. Viewed sideways it resembles a small fan
fastened along the edges, and just inside a row of fully developed
tentacles is seen protruding. An interesting form of budding occurs
in these corals: the buds fall off, and in this budding condition the
coral might pass, and indeed has been described as a different species
of Flabellum. The colour of the coral is a beautifully transparent
red. Remarkable as the solitary corals are, they are surpassed both in
number and in form by those which form compound stocks, that is to say,
in which the buds do not fall off, but go on building up coral islands
and barrier reefs in the warmer seas. Some very few typical forms only
are given in the group accompanying, shown in Fig. 358.
A different kind of stock is developed in a number of forms, some
producing many buds, as in the Madrepores, in which selected polyps
spring up above the rest, their sides also becoming covered with small
buds, each one of which is a living, feeding, coral animal surrounded
by a crown of tentacles. These Madrepores play a very important part in
the building up of coral reefs.
[Illustration: Fig. 354. Developmental stages of Larvæ, _Astroides
calycularis_, × 40.]
Another massive coral, the _Astroides calycularis_, has a different
mode of growth, the tubes not being fused together. When seen standing
out these yellowish-red polyps have been mistaken for small anemones.
The larvæ of this coral leave the egg while still in the large
chambered body cavity of the parent, where they swim about for a time,
till they escape through the mouth. They are worm-like in form, and
swim by means of cilia, which are thicker at the foremost end. The
mouth first appears after leaving the parent, but as they soon become
exhausted by the effort they assume a contracted form, and attach
themselves, as do anemones, by pressing the thicker end of the body
against a rock, the whole contracting into a thick round disc, while
longitudinal furrows become visible at the upper part where the mouth
sinks in. At the end of these furrows twelve tentacles appear. The
accompanying illustration shows the various stages through which the
larvæ pass in rapid succession (Fig. 354); at the same time it has
already commenced to secrete its calcareous skeleton. This is not
formed as a connected whole but from a number of separate centres
of secretion formed between the polyp and the substance to which it
has attached itself, and which become gradually fused into a perfect
skeleton. A section of the polyp at this stage forms an interesting
microscopical object.
The so-called eight-rayed corals consist of the one genus Tubipora, the
members of which are few in number and not varied in form (Fig. 358,
No. 10). In the structure, however, of skeletons they are unique among
extant corals. Each individual secretes a smooth-walled tube without
calcification of the vertical septa. These tubes, like the pipes of an
organ, stand almost parallel, and are united to form a stock by means
of transverse platforms. The formation of buds does not appear to take
place in this family.
Another of the eight-rayed corals is Gorgoniidæ. These are permanently
fixed to the spot on which they are found, and form a bush-like growth,
giving no idea of the living coral, as it rises in graceful branching
colonies, in deep water, and represents a portion of _Gorgonia nobilis_
with polyps expanded (Figs. 344 and 358, No. 9).
Other corals present numerous other departures from the types we have
been considering, but so far modified in form as that of the Sea-pen,
Veretillum (Fig. 355), the stock part of which is surrounded by polyps
continued down a portion of the cylindrical stalk. The best known of
the species is _Pennatula phosphorea_ of the Mediterranean.
[Illustration: Fig. 355.
1. _Pennatula phosphorea_; 2. _Synapta chirodata_; 3. Anchor-shaped
spiculum and plate from the ectoderm of same.]
_Pennatulidæ._--This family derives its name from _penna_, a quill.
Their spicula also resemble a penholder in appearance, shown in
Fig. 358, No. 3. The polyps are without colour, provided with eight
rather long retractile tentacula, beautifully ciliated on the inner
aspect with two series of short processes, and strengthened by these
crystalline spicula, a row being carried up the stalk, together with a
series of ciliated processes. The mouth, occupying the centre of the
tentacula, is somewhat angular. The ova lie between the membranous
part of the pinnæ; these are globular, of a yellowish colour, and by
pressure can be made to pass through the mouth. Dr. Grant wrote:--“A
more singular and beautiful spectacle could scarcely be conceived than
that of a deep purple _Pennatula phosphorea_, with all its delicate
transparent polyps expanded and emitting their usual brilliant
phosphorescent light, sailing through the still and dark abyss, by the
regular and synchronous pulsations of the minute fringed arms of the
polyps.”
The spicula are seen to be a continuous series of cones fitting into
each other.
Bryozoa, Moss-animals.
The exact position in which the Bryozoa, or moss-animals, should be
placed in the animal kingdom has not been finally determined. They were
at one time associated with corals; then with sponges; but, on further
acquaintance, it became evident that they did not belong to either.
Naturalists also claimed them as Rotifers and Ciliata, but this claim
met with no better reception. Since they appear to have no settled
classification, there can be no objection to linking them once more to
corals, as they apparently resemble these animals by always living in
colonies, the individual members of which are joined in a number of
different ways to form stocks, the individuals themselves, however,
being very much smaller than those of corals proper. The advantage is
that the structure of the Bryozoans can be more readily studied, as
many of them live in transparent chambers or cells, the walls of which,
although somewhat firmly agglutinated together, are flexible enough to
fold up, as the animals instantly withdraw their bodies and close up
the top on the slightest alarm (Fig. 356).
[Illustration: Fig. 356.--Paludicella, tentacles expanded and cell
closed.]
[Illustration: Fig. 357.--Sea-moss, Flustra, the body having been
withdrawn from its cell.]
The general structure of the Bryozoan individual, figured attached
by its footstalk to a stem of wood, consists of a mouth at the
anterior part of the body opening into a muscular pharynx in the
alimentary canal, together occupying a considerable amount of space.
The terminal portion turns upon itself towards the oral opening, its
chief attachment being a short strand of tissue termed the funiculus
(shown in Fig. 358, No. 11). In all adults two masses of cells are
found attached to the wall of the chamber; the upper yields the
eggs, within the lower the male elements are developed. Moss-animals
are hermaphrodite, fertilisation being effected by the two elements
mingling together in the body fluid. These are the essential points
in the structure of the whole seventeen hundred species. Among the
larger colonies a number of fresh-water genera are found attached to
the roots and branches of aquatic plants, most of which, however, are
inconspicuous. The beauty of these minute bodies can only be seen
under the microscope. Many consist of delicate branching growths, the
Sea-mats (Flustra), for instance; others again appear as attractive
lace corals, between the open meshes of which multitudes of minute
apertures crowned with tentacles are displayed. The several individuals
of the genus Lepralia are arranged in rows, and further distinguished
by the animals being developed only on one side of the stock. The
marvellous variety of forms presented by these small animals is in
a measure determined by the particular manner of their buddings.
The greater number of fresh-water moss-animals belong to the order
Phylactolæmata, so called because the mouth is provided with a
tongue-shaped lid. The crown of tentacles is furnished with rows of
cilia, and is horseshoe-shaped, the whole being surrounded at its base
by an integument forming a kind of cup, which is either soft or horny.
Those belonging to the wandering types (Cristatella, Plate IV., Nos.
95-98) form flattened elliptical colonies, some of which creep or move
about on a kind of foot. A nervous system pervades the mass of polyps,
while in each separate polyp a nerve ganglion is seen to be situated
between the œsophagus and the posterior part of the alimentary canal.
The colony nerve system regulates the movements of the stock.
[Illustration: Fig. 358.--Typical forms of Corals.
1. _Fungia agariciformis_; 2. Alcyonium, _Cydonium Mulleri_; 3.
Cydonium, polyps protruding and tentacles expanded, others closed; 4.
A stock viewed from above; 5. _Madrepore abrotanoide_; 6. Madrepore,
slightly magnified, showing oral opening; 7. Corallidæ; 8. Coral,
polyps protruding from cells; 9. _Gorgonia nobilis_, with polyps
expanded; 10. _Tubipora musica_; 11. Tubes of same, with polyps
expanded, one cut longitudinally to show internal structure; 12.
Sertularia, polyps protruded, and withdrawn into their polypidoms.]
[Illustration: Fig. 359.
1. _Coryne stauridia_; 2. A tentacle detached and magnified 200
diameters.]
There are many beautifully formed freshwater polyps deserving of more
than a passing notice, as the slender Coryne (_Coryne stauridia_),
found adhering to the footstalk of a _Rhodymenia_ (Fig. 359), about
which it creeps in the form of a white thread. On placing both under
the microscope, the thread-like body of the little animal appears
cylindrical and tubular, perfectly transparent, and permeated by a
central core, apparently cellular in texture, hollow, and within which
a rather slow circulation of globules is perceived. The parent Coryne
sends off numerous branches, the terminal head of which is oblong,
cylindrical, and at the extreme end there are arranged four tentacles,
long and slender, each being furnished with a nodular head. A magnified
view of one detached is shown erect (Fig. 359, No. 2). This polyp
is much infested by parasites, vorticella growing on it in immense
numbers, forming aggregated clusters here and there, individuals of the
parasitic colony adhering to each other, and projecting outwards in
every direction.
Alcyonella, another fresh-water polyp, is found in the autumn of the
year in all the London Docks adhering to pieces of floating timber. _A.
stagnorum_ partakes of the character of a sponge rather than that of a
polyp. It is usually found in gelatinous colonies, and when stood aside
for a short time these put forth a number of ciliated tentacles (shown
in Fig. 360, magnified 100 diameters).
The ova contained within the sac, and viewed by transmitted light,
appear as opaque spheres surrounded by a thin transparent margin; these
increase in thickness as the ova is developed, and such of the ova as
lie in contact seem to unite and form a statoblast. A rapid current
in the water around each animal, drawing with it loose particles and
floating animalcules, is seen moving with some velocity as in other
ciliated bodies; and a zone of very minute vibrating cilia surrounds
the transparent margin of each tentacle.
[Illustration: Fig. 360.--_Alcyonella fluviatella._]
Dr. Percival Wright discovered on the western coast of Ireland a new
genus of Alcyonidæ, which he named after the well-known naturalist
Harte, _Hartea elegans_ (Plate IV., No. 86). This polyp is solitary,
the body cylindrical, and fixed by its base to the rock; it has eight
ciliated tentacles, which are knobbed at their base and most freely
displayed. It is a very beautiful polyzoon of a clear white colour, and
when fully expanded stands three-quarters of an inch high.
_Lophopus crystallinus_ (Plate IV., No. 98) displays beautiful plumes
of tentacles arranged in a double horseshoe-shaped series. When
first observed these polyps resemble in many respects masses of the
water snail ova, for which they are often mistaken. On placing these
jelly-like masses into a glass trough with some of the clear water
taken from the stream in which they are found, delicate tubes are
seen to cautiously protrude, and the beautiful fringes of cilia are
quickly brought into play. The organisation of _L. crystallinus_ is
simple, although it is provided with organs of digestion, circulation,
respiration, and generation. The nervous[70] and muscular systems are
well developed. This polyp increases both by budding and by ova,
both of which conditions are shown in Plate IV., No. 98. The ova are
enclosed in the transparent case of the parent. In Lophopus and some
other fresh-water genera, Cristatella, Plumatella, and Alcyonella, the
neural margin of the Lophopore is extended into two triangular arms,
giving it the appearance of a deep crescent.
Another family presents a contrast: there is no lid to the mouth, and
the tentacles are arranged in a circle on a disc. An important rise in
organisation is found in the Gymnolæmata, especially in the lip-mouthed
forms; the individuals belonging to this order vary in structure and
fulfil different physiological functions. There are structures known
as zoæcia, stolons, avicularia, vibracula, and ovicells, some of which
are merely modified individuals. The zoæcia are the normal individuals
of the colony, fully developed for most of the functions of life; the
stolons have a much humbler function, but are indispensable--they
are the root-like outgrowths of the stock, and serve for attaching
the colony to foreign objects. The most remarkable are those known
as _avicularia_, so called because they resemble the head of a bird.
This process acts as a pair of forceps, the large upper blade of
which is very like the skull and upper jaw of a bird, and the smaller
lower blade (like the lower jaw) constantly opens and shuts by means
of a complicated arrangement of muscles (shown in Fig. 361). These
avicularia are movably attached by short muscles to the neck, and are
found near the entrance to a zoæcium. They turn from side to side,
snapping in all directions, catching at every particle of food that may
come near; at length the morsel is drawn into the mouth by the cilia
on the tentacles. From this very peculiar structure the Chilostomata
were originally named bird’s-head corallines, then specifically
shepherd’s-purse corallines, _Notamia bursaria_. Equally interesting,
again, are the _vibracula_, long thread-like structures, attached by
short footstalks. These keep up a constant whip-like motion, the object
of which is not quite clear. The ovicells, or egg receptacles, are
found at the lower ends of the zoæcia in the form of shields, helmets,
or vesicles. In Plate IV., Nos. 95 and 96, a front and edge view of the
statoblast is shown highly magnified.
[Illustration: Fig. 361.
1. _Notamia bursaria_, shepherd’s-purse Bryozoa; 2. Polyp magnified and
withdrawn into its cell; 3. Portion of a colony of Hydroid polyps.]
Another sub-order consists of the Cyclostomata, or round-mouthed
Bryozoans, of which the Tubulipora is the typical form. The stocks are
cup-shaped incrustations, the individuals radiating outwards, as in
Plate IV., No. 92. _Tubularia dumortierii_ is a very interesting form,
the germinal bodies, statoblasts, being formed as cell masses on the
strand, or funiculus, which also maintains the stomach in its place.
They are round or oval in shape, and brown or yellow in colour, and
consist of two valves fitted one upon the other like watch glasses, as
shown in No. 96. A number of other statoblasts are shown, Nos. 97, 98,
and 99. The edge running round No. 95 is seen to have barbed tips; the
ring itself contains small air chambers, and is termed the swimming
belt. It is, in fact, a perfect hydrostatic apparatus, giving support
to the winter buds or statoblasts on the surface of the water. The
barbed hooks apparently act as anchors, and by their means they catch
on at points suitable for their development during the coming spring.
As soon as the time comes, the two halves split apart and the germinal
mass emerges forth. Out of these winter buds and statoblasts asexually
produced individuals arise, which reproduce themselves sexually, their
descendants again yielding winter germs. In short, an alternation of
generations is a continually recurring process.
[Illustration: Fig. 362.--_Lingula pyramidata._]
_Brachiopoda._--Here again we have to do with an enigmatical class
of arm-footed animals, of which the Lamp-shells may be regarded as
typical. These have remained unaltered from the earliest geological
epochs. Brachiopods are divided into two orders: those having shells
without hinges, and those with shells hinged together. On the whole
they possess less interest for the microscopists than many other
animals, except in their earliest developmental stages of existence.
One of the most interesting of the hinge-class group, living chiefly
near the shores of the warmer seas, is the Lingulidæ. The valves
are almost exactly similar, but are not hinged together, and have
no processes for the support of the thick fleshy spiral arms of the
animals. In _L. pyramidata_, found around the Philippine Islands (Fig.
362), the stalk is nine times longer than the body. The animal does not
attach itself by this, but moves about like a worm, making tubes out
of sand, into which it can withdraw itself and disappear. The cilia at
the mantle edge form a fine sieve, thus preventing foreign particles
from entering the gills. Its internal structure possesses points
of interest, and the parasitic growths covering the cartilaginous
structure, miscalled a shell, are curious, and excite the attention of
the naturalist.
Another bivalve so unlike a crustacean, among which it has been
placed, I may venture to describe among Lamp-shells. I refer to the
barnacle (Lepas) generally met with covering the bottoms of ships.
These, as in the former genus, are more interesting to the microscopist
in the early stage of existence, and also for the curious parasites
known to infest them. The barnacle protrudes through its two valves
six pairs of slender, bristly, two-branched filamentous limbs, which
keep up a constant sweeping motion, and whereby it secures its supply
of food (Fig. 363). When first hatched the young are in the Nauplius
stage, being furnished with a median eye and three pairs of flagellated
appendages. After enjoying a free life the larva moults and passes into
a second stage, in which with its two eyes and compressed carapace
(shown in Fig. 364) it so nearly resembles a Daphnia. Before these
thoracic appendages entirely disappear they first change places, and
then each is seen to be provided with a sucker; by this means the larva
fixes itself to its permanent resting-place, while a cement gland
pours out a secretion that glues it firmly to the point of attachment
chosen. These Cirripedes are not true parasites, inasmuch as they do
not extract nourishment from the body to which they are attached.
[Illustration: Fig. 363.
1. Spat of oyster, some ciliated; 2. Barnacles attached by footstalks.]
One species, the Proteolepas, is in the adult stage a maggot-like,
limbless, shell-less animal found living within the mantle chamber
of other members of the same order, while the root-headed Cirripedes
(_Peltogaster curvatus_, as Fig. 364, No. 1) live parasitically upon
higher crustaceans.
_Echinodermata._--This sub-kingdom includes the star-fishes,
stone-lilies, sea-urchins, feather-stars, and sea-cucumbers, some
of which have been already alluded to, and are so well known that
they need no lengthy description, while of the fossil sea-urchins of
our chalk formations, the Pentremites and Crinoids, whose silicious
remains are so abundant and so familiar to naturalists and geologists,
but little remains to be said. They are chiefly interesting to the
microscopist from their calcareous and silicious appendages, known as
spicula. In the sea-urchin, brittle-star, or feather-star, the outer
body surface consists almost wholly of a deposit of calcium carbonate,
combined in the form of little plates built up into a rigid “test,”
whereas in the star-fish it usually forms a kind of scaffolding,
between the layers of which there stretches a firm leathery skin. Among
the sea-cucumbers, the living specimens of which present extraordinary
variations both in form and character, the deposit consists chiefly
of small spicules which grate when the skin is cut with a knife. If a
thin section of the skin is examined under the microscope, the spicules
are seen to be profusely distributed in the middle layer. The
same deposit takes place in the stalked column of a crinoid and in
sea-urchins (Echinodermata), which has tended to preserve them in the
fossilised state. Fig. 365 is selected as exhibiting to perfection the
Medusa-headed Pentacrinoid. This echinoderm differs in two characters:
first, its microscopic structure is that of a meshwork deposited in
the spaces of a network of soft tissue; secondly, that each element,
whether a spicule or a plate, is, despite its trellised structure,
deposited around regular lines of crystallisation (shown in Plate IV.,
Nos. 89 and 90). Owing to these characteristics the minutest portion
of an echinoderm skeleton is readily recognised, even when fossilised,
under the microscope. Even the species of the sea-cucumber can be
determined by the shape of their spicules.
[Illustration: Fig. 364.--Parasitic Barnacles.
1. _Peltogaster curvatus_; 2. Nauplius larva of Parthenopea; × 200.]
Another noticeable feature in the radiate structure is that in many
cases it gives to the animal a star-shape, to which the names of
star-fish and brittle-star are given (see Plate IV., No. 91, and
Plate XVII., _f_ and _n_). The ordinary five-rayed star-fish is found
everywhere around the English coasts. This constant arrangement of
organs holds good in the majority of the echinoderms; it can be
detected in the Holothurians, where, beside the feathery tentacles of
the head, rows of shorter sucker-like processes will be found, which in
some instances extend the whole length of the body, the fixed number of
rows being also five in their internal organs. Hence these animals were
formerly grouped under Radiata. But if a sea-cucumber or sea-urchin
be dissected, a marked distinction will be found between them, in one
portion of the organism in particular: the intestine is shut off from
the rest of the body-cavity, often coiling round inside. Examine a
star-fish or sea-urchin on the under-surface of the rays, and, passing
in five bands from top to bottom, a number of small cylindrical
processes are seen gently waving about; these lie in two rows with a
clear space between them, and are termed in consequence _ambulacrum_.
They end in sucker-like discs, which enable the animal to attach
itself, or pull itself against strong currents.
[Illustration: Fig. 365.--Medusa-headed Pentacrinoid.
_a._ Crown and part of stem; _b._ Upper surface of body, the arms
broken away, showing the food grooves passing to the central
mouth.--(Warne.)]
Just one other special feature should be noticed: radial canals pass
along under the ambulacra, and join a ring-canal around the mouth, well
supplied by nerve cells.
[Illustration: Fig. 366.
1. Transverse section of a branch of Myriapore; 2, and the others]
Section of the stem of _Virgularia mirabilis_; 3, Spiculum from
the outer surface of Sea-pen; 4, Spicula from _Isis hippuris_; 5,
from _Gorgonia elongata_; 6, from Alcyonium; 7, and from _Gorgonia
umbraculum_; 8, Calcareous remains of a Crinoid.]
_Crinoids_ (stone-lilies), on the other hand, are formed of a series
of flat rings, pierced through by a narrow canal. The ossicles, as
they are termed, are joined by ligaments passing through their solid
substance and endowed with muscular power; the central part serves for
the passage of blood-vessels, and is surrounded by a sheath of nervous
tissue that controls the movements of the stem, the latter being
encrusted by a number of fine rootlets. The stems possess a limited
power of bending. In the words of Professor Agassiz, “The stem itself
passes slowly from a rigid vertical attitude to a curved or even a
drooping position; the cirri move more rapidly than the arms, and the
animal uses them as hooks to catch hold of objects, and on account of
their sharp extremities they are well adapted to retain their hold of
prey.” The rosy-feather star-fish is often found clinging to a tube
of the Sabella worm; the food of crinoids consists of foraminifera,
diatoms, and the larvæ of crustaceans. There are so many curious
features in connection with the Echinodermata that my readers may with
advantage consult “The _Challenger_ Reports” and Warne’s “Natural
History” on other points of interest.
_Holothuroidea_ (sea-cucumbers) are elongated slug-like creatures,
the skin being in structure similar to that of the slug, with a
comparatively small amount of calcareous matter. Usually this occurs
in small spicules, which assume very definite shapes, as the anchors
of Synapta (Plate IV., No. 87, and in Fig. 355). There are also rings
of calcareous plates around the gullet, five of which have the same
relation to the radial water-vessels as the auricles round the jaws of
a sea-urchin, and which likewise serve for the attachment of muscles.
These plates are seen in Plate VIII., Nos. 171 and 172, as they appear
coloured by selenite films under polarised light. Around the mouth
in Cucumaria is a fringe of branched tentacles connected with the
water-vascular ring; these appear to be used as a net to intercept
floating organisms.
Correlated with the star-fishes is a small family based on the
character of their pincer-like organs, called pedicellariæ, on the
surface of the test (shown in Plate IV., Nos. 93 and 94, magnified ×
25). Movable spines cover the surface of these echinoderms, varying
in size from minute bristle-like structures to long rods. The
pedicellariæ are, it is believed, derived from the smaller spines,
and two of them are united at the base by muscles, slightly curved,
and made to approach each other at their extremities. There is a
gradual modification of this type through the whole series. Many uses
have been assigned to them, as the holding of food, as they have been
seen to hold to the fronds of seaweed and keep them steady until the
spines and tube feet can be brought into action. The inner surface of
the pedicellariæ are known to be the most sensitive, and the blades
close on the minutest object touching the inner surface. Beside these
peculiar bodies the surface of the skin has small tubular processes,
and tubular feet with suckers at the end. At the extremity of each arm
is a single tube-foot with an impaired tentacle, and above this again
is a small eye coloured by red pigment.
Passing by many other points of interest in the Echinoidæ, the spines
are seen to be attached to the test or shell by a ball and socket joint
and well-arranged muscles, whereby the spines can be moved in any
direction. The tubercles, however, do not cover the whole test, but
are disposed chiefly in five broad zones extending from one pole to
another. When a transverse section of a spine is examined by a medium
power it is seen to be made up of a series of concentric and radiating
layers (shown in Plate XVIII., Nos. 1 and 2), the centre being occupied
by reticulated structure and structureless spots arranged at equal
distances; these may be termed ribs or pillars. Passing towards the
margin are other rows conveying the impression of a beautiful indented
reticulated tissue. Many of the spines present no structure, while
others exhibit a series of concentric rings of successive growth,
which strongly remind one of the medullary rays of plants. When a
vertical section of a spine is submitted to examination, it is seen to
be composed of cones placed one above the other, the outer margin of
each cone being formed by the series of pillars. In certain species of
Echinus the number of cones is very considerable, while in others there
are seldom more than one or two to be found; from these, transverse
sections may when made show no concentric rings, only the external row
of pillars.
The skeleton of echinoderms contains but a small amount of organic
matter, as will be seen on dissolving out the calcareous portion in
dilute nitric or hydrochloric acids. The residuum structure will
appear to be meshes or areolæ, bounded by a substance having a fibrous
appearance, intermingled with granulous matter; in fact, it bears a
close resemblance to the areolar tissue of higher animals, and the test
may be considered as formed, not by the consolidation of the cells
of the ectoderm, as in the mollusc, but by the calcification of the
fibro-areolar tissue of the endoderm. This calcification of a simple
fibrous tissue by the deposit of a mineral substance, not in the meshes
of areolæ but in intimate union with the organic basis, is a condition
of much interest to the physiologist; it presents an example of a
process which seems to have an important share in the formation and
growth of bone, namely, in the progressive calcification of the fibrous
tissue of the periosteum membrane covering of the bone.
The development of the sea-urchin from the fertilised egg first divides
and then sub-divides, and in a short time the embryo issues forth
with a small tuft of cilia, by means of which it swims off freely.
The larvæ, in its full development, measures about one millimetre in
diameter, and is a curious and remarkable creature.
=The sub-kingdom Mollusca= comprises some fifty thousand species, and
fresh forms are being constantly discovered, the number of the aquatic
genera being more than double that of the terrestrial species, for
it matters not to what depth of ocean the dredge is let down, some
new form is certain to be gathered. The _Challenger_ expedition has
enriched our knowledge of the deep-sea fauna to an enormous extent; so
much so, that fifty volumes have already been published descriptive
of animals brought to the surface. Nevertheless, we are told that
the great coast lines of South America, Africa, Asia, and parts of
Australia have been but imperfectly explored for smaller kinds of
Mollusca.
Molluscs are soft-bodied, cold-blooded animals, without any internal
skeleton, but this is compensated for by the external hardened shell,
which at once serves the purpose of bones, and is a means of defence.
These bodies are not divided into segments like those of worms and
insects, but are enveloped in a muscular covering or skin, termed
the mantle, the special function of which in most species is the
formation and secretion of the shell. The foot, which serves the
double purpose of locomotion and burrowing in the sand or rock, is an
organ particularly characteristic of most molluscs. There are many
departures from this rule, as, for instance, in the group Chitonidæ,
where the shell takes the form of a series of eight adjacent plates;
and in another, the Pholadidæ, there are one or more accessory pieces
in addition to the two principal valves. Some are bivalved, others
univalved, and concealed beneath the skin. All shells are mainly
composed of carbonate of lime, with a small admixture of animal
matter. Their microscopic examination reveals a great diversity of
structure, as we shall presently see, and they are accordingly termed
porcellaneous, nacreous, glassy, horny, and fibrous. Most molluscs
have the power of repairing injuries to their shells; many exhibit an
outer coat of animal matter, termed the _peristracum_, the special
function of which is to preserve the shell from atmospheric and
chemical action of the carbonic acid in the water in which they dwell.
The shells of gastropods are enlarged with the growth of the mollusc
by the addition of fresh layers to the margin. In some species the
periodic formation of spines occurs; a typical case will be found among
Muricidæ. The varied colours of shells are due to glands situated on
the margin of the mantle, and beneath the peristracum; occasionally
the inner layer of porcellaneous shells is of a different colour to
the outer, as, for example, in the helmet-shells (Cassis), much used
by carvers of shell cameos. Light and warmth, as in the vegetable
kingdom, are the great factors in the production of brilliant colours.
In cold climates land snails bury themselves in winter time in the
ground or beneath decaying vegetable matter, and in hot seasons they
close up the aperture of the shells with a temporary lid, called an
_epiphragm_. These exhibit great tenacity of life, as, for instance,
in the Egyptian desert-snail, _Helix desertorum_. The reproductive
system is in all cases effected by means of eggs. The ova are usually
enclosed in capsules, and deposited in masses, and the number of eggs
contained in the squid and the whelk have been stated to be thirty or
forty thousand. The ova of molluscs may be gradually developed into
the adult, or there may be a free-swimming ciliated larval stage, or a
special larval form, as in the fresh-water mussel. Most are provided
with a more or less distinct head; both cephalopods and gastropods
are furnished with eyes. In land snails these are found placed on
projecting stalks. In most cases the utility of molluscs far outweighs
the injury occasioned by a few species, as, for instance, the Teredo,
and the burrowing habits of the Pholas and Saxicava, compact marble
having been found bored through by them.
Mr. J. Robertson wrote me in 1866:--“Having, while residing here
(Brighton), opportunities of studying the _Pholas dactylus_, I
have endeavoured during the last six months to discover how this
mollusc makes its hole or crypt in the chalk--by a chemical solvent?
by absorption? by ciliary currents? or by rotatory motions? My
observations, dissections, and experiments set at rest controversy
on this point. Between twenty and thirty of these creatures have been
at work in lumps of chalk in sea water in a finger glass and a pan,
at my window for the last three months. The _Pholas dactylus_ makes
its hole by grating the chalk with its rasp-like valves, licking it up
when pulverised with its foot, forcing it up through its principal or
branchial siphon, and squirting it out in oblong nodules. The crypt
protects the Pholas from Conferveæ, often found growing parasitically
not only outside the shell but even within the lips of the valves, thus
preventing the action of the siphons. In the foot there is a spring,
or style, which when removed is found to possess great elasticity, and
this seems to be the mainspring of the motion of the Pholas.”
[Illustration: Fig. 367.--Hexabranchus.]
I must pass by many groups and orders to more aberrant types,
represented by the naked-gilled orders, Opisthobranchiata and
Nudibranchiata. These gastropods constitute a large sub-order of
extremely beautiful molluscs, remarkable in shape, and often brilliant
in colour. The distinguishing character of these typical forms consists
in the peculiar nature and situation of their breathing organs, which
are exposed on the back of the animal or around the anterior part, and
are not protected by the mantle. But the situation is varied, and the
gills are sometimes placed on each side of the body, respiration being
effected by the ciliated surface of the whole. For these and other
reasons they have been placed in four groups. Nudibranchs are found
in all parts of the world, and are most abundant in depths where the
choicest seaweeds and corallines abound. Their fecundity is very great,
as many as sixty thousand eggs being deposited by a single female at
one time. They are eaten as a luxury where they most abound.
[Illustration: Fig. 368.--Longitudinal section of _Pleurobranchus
aurantiacus_, showing circulation and gills or branchiæ.--(Warne.)]
In the Opisthobranchs the branched veins as well as the auricle are
placed behind the ventricle of the heart. They differ from Nudibranchs
inasmuch as they are usually furnished with a pair of tentacles and
labial palpi, or an expansion of the skin like the veil of the larval
form. To clearly understand the character of the internal organisation
of these curious animals, the longitudinal section given in Fig. 368
must be consulted: _p_ is the foot; _a_ the mouth, covered above
with the veil-like expansion, over which are the tentacles, _c_; the
branchial veins, _v_, carry the blood to the gills, from which it flows
into the heart at _h_. This disposition is the opposite of that which
characterises the Prosobranchus. Another anatomical peculiarity, which
may here be referred to, is the direct communication of the system of
blood vessels with the surrounding medium; a characteristic common
to most other molluscs, and on which depends the changeable external
appearance of the animal. In the illustration of Pleurobranchus here
given, _g_ indicates the opening of the duct which conveys water direct
to the blood, and through which the blood vessels permeate the back and
foot. Like the holes in the sponges, it can be filled or emptied at the
will of the animal.
Although this, in the main, is the principle of the circulation in
most of this order, one branch possesses no special breathing organs,
respiration being carried on throughout the naked skin of the body.
With regard to the Nudibranchiata, the group having the most
symmetrical form is the extensive family Dorididæ, characterised by
differences in the branchiæ, the relative proportion of the mantle to
the foot, and variations in the radula and jaws. The general aspect of
the genus Doris, although drawn on a small scale, is represented in
Plate XVII., Fig. _b_. The whole sub-order of Nudibranchs has become
more generally known and admired since the publication of Alder and
Hancock’s monograph with its many attractive coloured illustrations.
These gastropods can be kept alive for some time in a small aquarium
if the precaution is observed of often changing the water and adding a
little fresh seaweed. Numerous curious microscopic forms of life may be
found adhering to them.
[Illustration: Fig. 369.--_Aplysia dipilans._]
_Tunicata._--The most remarkable group of animals belonging to this
sub-order are the Ascidians. They derive their name from the test or
tunic, a membranous consistence, in which they dwell, and which often
includes calcareous spicules. The test has two orifices, within which
is the mantle. Few microscopic spectacles are more interesting than the
circulation along this network of muslin-like fabric, and that of the
ciliary movement by which the fluid is kept moving. In the transparent
species, as Clavelina and Perophora, the ciliary movement is seen to
greater advantage. The animals are found adhering to the broad fronds
of fuci near low water-mark. They thrive in tanks, and multiply both
by fission and budding. Two species are figured in Plate XVII., Figs.
_i_ and _k_, the zooids of which were found arranged in clusters, as
represented.
_Aplysiidæ_ (sea-hares), so called on account of a slight resemblance
to a crouching hare. The body form is elongated with a partially
developed neck and head, oral and dorsal tentacles, and furnished
beneath the mantle with a shelly plate to protect the branchiæ. The
mouth is provided with horny jaws, and the gizzard is armed with
spines, to prepare the food for digestion. The side lobes are thin and
large, and are either folded over the back or used in swimming. Fig.
369 is a reduced drawing of _A. dipilans._
The Pectinibranchs are known as violet sea-snails, Ianthinidæ and
Scalariidæ. The radula consists of numerous rows of pointed teeth
arranged in cross series, forming an angle in the middle. There is
no central or rachidian tooth, and they have thin trochiform shells
adapted for a pelagic life. They are mostly of a violet colour, from
which they derive their name, the colour being more vivid on the
underside, which is turned up towards the light when the animal is
swimming near the surface of the sea (Fig. 370).
[Illustration: Fig. 370.--Ianthinia, Violet Sea-snail.--(Warne.)
The bubble _b_, drawn somewhat too large, is about to be joined to the
anterior end of the float; _c._ Shell; _l._ Float; _p._ Foot; _t._
Head.]
The most interesting feature in connection with these oceanic
snails is the curious float which they construct to support their
egg-capsules. It is a gelatinous raft, in fact, enclosing air-bubbles,
which is attached to the foot, the egg capsules being suspended from
its under-surface. They are unable to sink so long as they are in
connection with their floats, and are therefore often cast on shore
during storms, and furnish an endless series of microscopic specimens.
The violet snails feed on various kinds of jelly-fish, and occur in
shoals.
_Pond Snails._--The three families, Limnœidæ, Physidæ, and Chilinidæ,
form a special group of the pulminate, sessile-eyed fresh-water snails.
The larger family of these belongs to the genus Limnœa, having a
compressed and triangular head with two tentacles and eyes placed at
their inner base. They are prolific and gregarious, and their ova are
enclosed in transparent gelatinous capsules, deposited in continuous
series, and firmly glued to submerged stems and leaves of aquatic
plants. _L. stagnalis_ is common in all ponds, marshes and slow-running
rivers of Great Britain.
[Illustration: Fig. 371.--Ova and young of _Limnæus stagnalis_.]
One of the species, _L. trancatula_, is the host of the liver-fluke so
fatal to sheep. The fluke parasite passes one stage of its existence in
the intestine of the pond snail.