BRANCH CŒLENTERATA: THE POLYPS, SEA-ANEMONES,
CORALS, AND JELLYFISHES
The structure and life-history of an example of the polyps (the
Fresh-water Hydra, _Hydra_ sp.) has been studied in Chapters X and XI.
OTHER POLYPS, SEA-ANEMONES, CORALS, AND
JELLYFISHES
TECHNICAL NOTE.--The teacher should have, if possible, several
pieces of coral and a few specimens of Cœlenterates in alcohol or
formalin, which will show the external character, at least, of
these animals (see account of laboratory equipment, p. 450). If
the school is on the coast, the pupils should be shown the
sea-anemones of the tide-pools.
The animals which are included in the branch Cœlenterata are, at least
in living condition, unfamiliar to most of us. Like the sponges, they
are almost all inhabitants of the ocean; a few, like _Hydra_, live in
fresh water. Like the sponges, too, most of the members of this branch
are fixed, and in their general appearance suggest a plant rather than
an animal. The name zoophytes, or plant-animals, which is often
applied to these animals is based on this superficial resemblance. But
many of the Cœlenterates lead an active free-swimming life. This is
true of the jellyfishes which float or swim about on or near the
surface of the ocean. Many of the zoophytes spend part of their life
in an active free-swimming condition before settling down, becoming
attached and thereafter remaining fixed. In localities near the
seashore many animals belonging to this great group can be readily
found and observed. The beautiful sea-anemones with their
slowly-waving tentacles, the fine many-branched truly plant-like
hydroids with their hosts of little buds, and the soft colorless
masses of jelly, the jellyfishes, which are cast up on to the beaches
by the waves are all animals belonging to the branch Cœlenterata.
=General form and organization of body.=--The general or typical plan of
body-structure for the Cœlenterata, these animals which come next to the
sponges in degree of complexity, can best be understood by imagining the
typical cylindrical or vase-like body of the simple sponges to be
modified in the following way: The middle one of the three layers of the
body-wall not to be composed of scattered cells in a gelatinous matrix,
but to be simply a thin non-cellular membrane; the body-wall not to be
pierced by fine openings or pores, but connected with the outside only
by the single large opening at the free end, and this opening to be
surrounded by a circlet of arm-like processes or tentacles, which are
continuations of the body-wall and similarly composed. Such a
body-structure, which we saw well shown by _Hydra_, is the fundamental
one for all polyps, sea-anemones, corals, and jellyfishes. The variety
in shape of the body and the superficial modifications of this type-plan
are many and striking, but after all the type-plan is recognizable
throughout the whole of this great group of animals.
The two chief body-shapes represented in the branch are those of the
polyps on the one hand, and the jellyfishes or medusæ on the other. The
polyp-shape is that of a tube with a basal end blind or closed, attached
to some firm object in the water and with the free end with an opening,
the mouth-opening. At this mouth-end there is a circlet of movable, very
contractile tentacles. The mouth may open directly into the interior of
the body, which interior may be called the digestive cavity, or it may
lead into a simple short tube produced by the invagination or bending in
of the body-wall, which may be looked on as the simplest kind of
œsophagus. This œsophageal tube opens into the body-cavity or digestive
cavity. This cavity may be incompletely divided by longitudinal
partitions which project from the sides into the cavity.
The jellyfish or medusoid body-form corresponds in general to an
umbrella or bell. Around the edge of this umbrella are disposed
numerous threads or tentacles (corresponding to the circlet of
tentacles in the polyp). The mouth-opening is at the end of a longer
or shorter projection which hangs down from the middle of the under
side of the umbrella. The interior body-cavity or digestive cavity
extends out into the umbrella-shaped part of the body, usually in the
condition of canals radiating from the centre and a connecting canal
running around the margin of the umbrella.
=Structure.=--Although the Cœlenterata show little indication of the
complex composition of the body out of organs, as it exists among the
higher animals, yet they do show an unmistakable advance on the simple,
almost organless body of the sponges. This is chiefly shown by the
differentiation among the cells which compose the body. In the polyps
and jellyfishes some of the cells are specialized to be unmistakable
muscle-cells, some to be nerve-cells and fibres, and so on. A very
simple nervous system consisting of small groups of nerve-cells
connected by nerve-fibres exists. Some very simple special sense-organs
may occur. The digestive system, although in the simpler Cœlenterates
consisting merely of the cylindrical body-cavity enclosed by the
body-wall and opening by the single hole at the free end of the body, in
some is rather complex and is composed of different parts. Those
Cœlenterates which are not fixed but lead an active, free-swimming life,
viz., the jellyfishes or medusæ, are the most highly organized.
The tentacles which surround the mouth-opening and serve to grasp food
and carry it into the mouth, and the stinging or lasso threads with
which these tentacles are provided are special organs possessed by
most of these animals.
=Skeleton.=--Like the sponges, some of the Cœlenterata possess a hard
skeleton. This skeleton is always composed of calcium carbonate and is
called _coral_. Those polyps which form such a skeleton are called the
corals. Coral will be described in connection with the account of the
coral-polyps.
=Development and life-history.=--The polyps and jellyfishes reproduce
both asexually and sexually. The asexual mode is usually that of
budding. On a polyp a bud is formed by a hollow outgrowth of the
body-wall. The bud grows, an opening appears at its distal end, a
circlet of tentacles arises about this mouth-opening and a new polyp
individual is formed. This individual may separate from the parent or
it may remain attached to it. By the development of numerous buds, and
the remaining attached of all of the individuals developing from these
buds, a colony of polyp individuals may be formed, plant-like in
appearance. The various polyp individuals of a colony may differ
somewhat among themselves, and these differences are correlated with a
division of labor. Thus some of the individuals may devote themselves
to getting food for the colony, and these have mouth and tentacles.
Others may be devoted to the production of new individuals by budding
or by producing germ-cells, and may not have any mouth-opening or any
food-grasping tentacles.
In case of many polyps all or some of the new individuals which arise
by budding do not become polyps, but develop into medusæ or jellyfish,
which separate from the fixed polyp and swim off through the water.
These medusæ or jellyfish produce sperm-cells and egg-cells. The
sperm-cells fertilize the egg-cells and a new individual develops from
each fertilized egg. This new individual is at first an active
free-swimming larva called a _planula_, which does not resemble either
a medusa or polyp. After a while it settles down, becomes fixed and
develops into a polyp. Thus a polyp may produce a medusa or jellyfish
which, however, produces not a new jellyfish, but a polyp. This is
called an _alternation of generations_, and is not an uncommon
phenomenon among the lower animals. It results from such an
alternation of generations that a single species of animal may have
two distinct forms. This having two different forms is called
_dimorphism_. Sometimes, indeed, a species may appear in more than two
different forms; such a condition is called _polymorphism_.
Not all medusæ or jellyfish are produced by polyp individuals, nor do
jellyfish always produce polyps and not jellyfishes. There are some
jellyfishes (we might call them the true jellyfishes) which always
have the jellyfish form, producing new jellyfishes either by budding
or by eggs, and there are some polyps which always have the true polyp
form, producing new individuals, either by budding or by eggs, always
of polyp form and never of jellyfish form. That is, some species of
Cœlenterata exist only in polyp form, some species exist only in
jellyfish form, while some species (those having an alternation of
generations) exist in both polyp and jellyfish form, these two forms
appearing as alternate generations.
=Classification.=--The branch Cœlenterata is divided into four
classes: (1) the Hydrozoa, including the fresh-water polyps, numerous
marine polyps, many small jellyfishes and a few corals; (2) the
Scyphozoa, including most of the large jellyfishes; (3) the Actinozoa,
including the sea-anemones and most of the stony corals; (4) the
Ctenophora, including certain peculiar jellyfishes.
[Illustration: FIG. 13.--The Portuguese Man-of-War (_Physalia_ sp.).
(From specimen from Atlantic Coast.)]
=The polyps, colonial jellyfishes, etc. (Hydrozoa).=--To the class
Hydrozoa belongs the _Hydra_ already studied. There are a few other
fresh-water polyps and they all belong to this class. The most
interesting members of the class are the "colonial jellyfishes,"
constituting the order Siphonophora. These colonial jellyfishes are
floating or swimming colonies of polypoid and medusoid individuals in
which there is a marked division of labor among the individuals,
accompanied by marked differences in structural character. The
individuals are accordingly polymorphic, that is, appear in various
forms, although all belong to the same species. Because these various
individuals forming a colony have given up very largely their
individuality, combining together and acting together like the organs
of a complex animal, they are usually not called individuals, nor on
the other hand organs, but _zooids_, or animal-like structures. The
beautiful "Portuguese man-of-war" (fig. 13) is one of these colonial
jellyfishes. It appears as a delicate bladder-like float, brilliant
blue or orange in color, usually about six inches long, and bearing on
its upper surface which projects above the water a raised
parti-colored crest, and on its under surface a tangle of various
appendages, thread-like with grape-like clusters of little bell- or
pear-shaped bodies. Each of these parts is a peculiarly modified
polyp- or medusa-zooid produced by budding from an original central
zooid. The Portuguese man-of-war is very common in tropical oceans,
and sometimes vast numbers swimming together make the surface of the
ocean look like a splendid flower-garden.
[Illustration: FIG. 14.--A colonial jellyfish (Siphonophora). (After
Haeckel.)]
Usually the central zooid in a Siphonophore to which the other zooids
are attached is not a bladder-like float, but is an upright tube of
greater or less length. In the Siphonophore shown in figure 14, the
compound body is composed of a long central hollow stem with hundreds
or thousands of variously shaped parts, each of which is reducible to
either a polyp or medusazooid, attached around it. The upper end is
enlarged to form an air-filled chamber, a sac-like boat, by means of
which the whole colony is kept afloat. Around the upper end of the
central stem are many medusoid structures, the swimming-bells, by
means of whose opening and closing the whole colony is made to swim
through the water. Each swimming-bell is a modified medusa-zooid,
without tentacles, without digestive or reproductive organs, but
exercising the power of swimming by contracting and forcing the water
out of the hollow bell just as is done by the free medusæ. Below the
swimming-bells, at the lower end of the central stem, are grouped many
structures presenting at first sight a confusion of variety and
complexity, but on careful examination revealing themselves to be
polyp- and medusa-zooids modified to form at least five kinds of
particularly functioning structures. There are many flattened
scale-like parts whose function is simply that of affording a passive
protection, in times of danger, to the other structures. These
protecting-scales are greatly modified medusa-zooids, each consisting
of a simple cartilage-like gelatinous mass penetrated by a
food-carrying canal. Under the broad leaves of these protecting-zooids
are a number of pear-shaped bodies which have a wide octagonal
mouth-opening at their free end, and possess in their interior certain
digestive glands. Each one is provided with a very long flexible
tentacle which bears many fine stinging-threads. The tentacle waves
back and forth in the water, and on coming in contact with an enemy or
with prey its poisonous stinging-threads shoot out and paralyze or
wound the unfortunate animal. These pear-shaped bodies are the feeding
structures, each being a modified polyp-zooid. Scattered among these
dangerous structures are many somewhat similarly shaped but wholly
harmless structures, the sense-structures. Each of these has a
pear-shaped body but without mouth-opening, and also a long, very
sensitive, tentacle-like process. The sense of feeling is highly
developed in these tentacles, and they discover for the colony the
presence of any strange body. These sense-structures are modified
polyp-zooids. Finally there are two other kinds of structures, usually
arranged in groups like bunches of grapes, which are the reproductive
structures, male and female. They are modified medusa-zooids grown
together and without tentacles. This whole colony, or this compound
animal, floats or swims about at the surface of the ocean, and
performs all of the necessary functions of life as a single animal
composed of organs might. Yet the Siphonophore is more truly to be
regarded as a community in which the hundreds or thousands of animals,
representing five or six kinds of individuals, all of one species, are
fastened together. Each individual performs the particular duties
devolving upon its kind or class. Thus there are food-gathering
individuals, locomotor individuals, sense individuals, and
reproductive individuals. The modifications of the various kinds of
individuals are more extreme than in the case of the various kinds of
individuals composing a bee-community, for example, but the holding
together or fusing of all into one body or corporation is a condition
which makes this greater modification necessary and not unexpected.
And there is no difficulty in seeing that each of these parts is
really, structurally considered, a modified polyp or medusa.
[Illustration: FIG. 15.--A jellyfish or medusa, _Gonionema vertens_,
eating two small fishes. (From specimen from Atlantic Coast.)]
=The large jellyfishes, etc. (Scyphozoa).=--To the class Scyphozoa
belong most of the common large jellyfishes. When one walks along the
sea-beach soon after a storm one may find many shapeless masses of a
clear jelly-like substance scattered here and there on the sand. These
are the bodies or parts of bodies of jellyfishes which have been cast up
by the waves. Exposed to the sun and wind the jelly-like mass soon dries
or evaporates away to a small shrivelled mass. The body-substance of a
jellyfish contains a very large proportion of water; in fact there is
hardly more than 1 per cent of solid matter in it.
The jellyfishes occur in great numbers on the surface of the ocean and
are familiar to sailors under the name of "sea-bulbs." Some live in
the deeper waters; a few specimens have been dredged up from depths of
a mile below the surface. They range in size from "umbrellas" or disks
a few millimeters in diameter to disks of a diameter of two meters
(2-1/6 yards). They are all carnivorous, preying on other small ocean
animals which they catch by means of their tentacles provided with
stinging-threads. The tentacles of some of the largest jellyfishes
"reach the astonishing length of 40 meters, or about 130 feet." Many
of the jellyfishes are beautifully colored, although all are nearly
transparent. Almost all of them are phosphorescent, and when irritated
some emit a very strong light.
=The sea-anemones and corals (Actinozoa).=--Almost everywhere along
the seashore where there are rocks and tide-pools a host of various
kinds of sea-anemones can be found. When the tide is out, exposing the
dripping seaweed-covered rocks, and the little sand- or stone-floored
basins are left filled with clear sea-water, the brown and green and
purple "sea-flowers" may be found fixed to the rocks by the base with
the mouth-opening and circlet of slowly-moving tentacles hungrily
ready for food (fig. 16). Touch the fringe of tentacles with your
fingertip and feel how they cling to it and see how they close in so
as to carry what they feel into the mouth-opening. A host of
individuals there are, and scores of different kinds; some small,
some large, some with the body covered outside with tiny bits of stone
and shell so that they are hardly to be distinguished from the rock to
which they cling; some of bright and showy colors. These are the most
familiar members of the class Actinozoa.
[Illustration: FIG. 16.--Sea anemones, _Bunodes californica_, open and
closed individuals. The closed individuals in upper right-hand corner
show the external covering of small bits of rock and shell,
characteristic of most individuals of this species. (From living
specimens in a tide-pool on the Bay of Monterey, California.)]
But in other oceans, along the coasts of other lands, especially those
of the tropics and sub-tropics, there are some other members of the
class which are of unusual interest. They are the corals, or coral
polyps. We know these animals chiefly by their skeletons (fig. 17).
The specimens of corals which one sees in collections, or made into
ornaments, are the calcareous skeletons of various kinds of the coral
polyps. Some of the corals live together in enormous numbers, forming
branching colonies fixed as closely together as possible, and secrete
while living a stony skeleton of carbonate of lime. These skeletons
persist after the death of the animals, and because of their abundance
and close massing form great reefs or banks and islands. These coral
reefs and islands occur only in the warmer oceans. In the Atlantic
they are found along the coasts of Southern Florida, Brazil and the
West Indies; in the Pacific and Indian Oceans there are great coral
reefs on the coast of Australia, Madagascar and elsewhere, and certain
large groups of inhabited islands like the Fiji, Society, and Friendly
Islands are exclusively of coral formation. Coral islands have a great
variety of form, although the elongated, circular, ring-shaped and
crescent forms predominate. How such islands are first formed is
described as follows by a well-known student of corals:
[Illustration: FIG. 17.--Skeleton of a branching coral, _Madrepora
cervicornis_. (From specimen.)]
"A growing coral plantation, with its multitudinous life, oftentimes
arises from great depths of the ocean, and the sea-bed upon which it
rests is probably a submarine bank or mountain, upon which have lodged
and slowly aggregated the hard skeletons of pelagic forms of life.
When, through various sources of increase, this submarine bank
approaches the depth of from one hundred to one hundred and fifty feet
from the surface of the water, there begins on its top a most
wonderful vital activity. It is then within the bathymetric zone of
the reef-building corals. Of the many groups of marine life which then
take possession of the bank, corals are not the only animals, but
they are the most important, as far as its subsequent history goes. As
the bank slowly rises by their growth, it at last approaches the
surface of the water, and at low tide the tips of the growing branches
of coral are exposed to the air. This, however, only takes place in
sheltered localities, for long before it has reached this elevation it
has begun to be more or less changed and broken by the force of the
waves. As the submarine bank approaches the tide level, the delicate
branching forms have to meet a terrific wave-action. Fragments of the
branching corals are broken off from the bank by force of the waves,
and falling down into the midst of the growing coral below fill up the
interstices, and thus render the whole mass more compact. At the same
time larger fragments are broken and rolled about by the waves and are
eventually washed up into banks upon the coral plantation, so that the
island now appears slightly elevated above the tides. This may be
called a first stage in the development of a coral island. It is,
however, little more than a low ridge of worn fragments of coral
washed by the high tides and swept by the larger waves--a low, narrow
island resting on a large submarine bank."
When the coral island rises thus a little above the surface of the
water, the waves break up some of the coral into fine sand, which fills
in the interstices, and offers a sort of soil in which may germinate
seeds brought in the dried mud on the feet of ocean birds or carried by
the ocean currents. With the beginning of vegetable growth the soil is
more firmly held, is fertilized and ready for the seeds of plants which
need a better soil than lime sand. Flying insects find their way to the
island, especially if it be near the mainland, birds begin to nest on
it, and soon it may be the seat of a luxuriant plant and animal life.
For an account of coral islands see Darwin's "The Structure and
Distribution of Coral Reefs."
There are over 2000 kinds of coral polyp known, and their skeletons
vary much in appearance. Because of the appearance of the skeleton
certain corals have received common names, as the organ-pipe coral,
brain coral, etc. The red coral, of which jewelry is made, grows
chiefly in the Mediterranean. It is gathered especially on the western
coast of Italy, and on the coasts of Sicily and Sardinia. Most of this
coral is sent to Naples, where it is cut into ornaments.
There are other interesting members of the class Actinozoa like the
beautiful sea-pens, sea-feathers and sea-fans, delicate, branching,
tree-like forms found all over the world.
=Ctenophora.=--The members of this class are mostly small, peculiar
jellyfishes which do not form colonies, and are extremely delicate,
being usually perfectly transparent. They swim by means of cilia. They
never appear in a polyp condition, but are always medusoid in shape.