ZOOPHYTES.
Not many years ago our knowledge of the lower forms of life was very
imperfect, and it was believed that the gulf between the animal
and vegetable kingdoms was bridged over by certain creatures which
could not properly be classed in either, because they appeared
to unite the characters and organization of each. Such was the
case with the sponges, already dealt with, and with the creatures
now to be considered. These last were on that account called
Zoophytes, or animal plants, a term which we must render to-day as
plant-like animals. Some of us have again got to the notion that there
is no sharp division between animal and plant-life; but with increased
knowledge we have put back the debatable or common ground much lower in
the scale of life.
With the whole of the families included in this division of life, I
do not propose to deal in the present chapter: the Sea-Anemones and
the Sea-Jellies, for instance, being treated in succeeding chapters,
for each group deserves and demands a chapter to itself. It is
characteristic of the Zoophytes that they form a bag of jelly-like
material, with an opening at one end which may be regarded as a mouth,
though it is without tongue or teeth, and opens directly into the
stomach. Around this mouth are set a number of limb-like organs, called
tentacles, which are used for seizing the prey and conveying it within
the orifice. Their entire structure is very simple, and apart from
primitive muscular and nervous systems, and the possession of stinging
threads, which can be quickly extruded through the exterior walls of
the body, they appear to be almost innocent of organs. This form of
structure is generally referred to as a Polypite, and its appearance
has been made familiar by the descriptions and figures of the Hydra or
Polyp of our stagnant fresh-water ponds. From their general agreement
in structure with the Hydra, the creatures to which much of this
chapter will be devoted, are called Hydroid Zoophytes. There are,
however, but few species that occur solitarily, like the Hydra. In
most cases they are associated in inseparable colonies. The egg of a
zoophyte gives rise, it is true, to an organism resembling Hydra, but
this individual does not long remain solitary; it produces many buds,
which rapidly develop, and in turn produce other buds, so that before
long there is a colony that may number its thousands of polypites.
However numerous the individuals may be, we may be sure that the colony
has been the production of a single egg. One came from that egg, but
all the others were produced vegetatively by budding from the original
polypite, or as later generations from such bud-originated polyps.
A slight examination of such a colony will show that the polypites
themselves are held in association by an investing substance
(_cœnosarc_), which takes the form of a living tube of thin flesh,
which adheres to rock or shell or seaweed, acting as a support for
the community, and also reproducing the polypites. It consists of two
distinct layers, an inner and an outer, and sometimes there is a third
layer of a different kind between these two, muscular in character.
In most cases the outer wall of this tube secretes a sheath of a
substance called _chitin_, of which the external skeletons of insects
are composed. This sheath is known as the polypary, because into it
the individual polypite withdraws itself. It is this polypary that the
seaside visitor finds attached to weeds or shells, and concludes, from
its moss-like aspect, it is a seaweed, and probably adds it to his
collection as such.
[Illustration: SEA OAK CORALLINE.]
Now if we get down among the rocks near low-water, and look among
the coarse brown weeds, we shall not look long before we find one
whose stem and parts of the frond are covered with a plantation of
erect-growing “somethings,” that look like the backbones of some small
fishes. They are only about an inch in height, very slender, and
regularly notched on each side. Some of the specimens have one or two
branches, but most of them are simple erect stems. It is known as the
Sea Oak Coralline (_Sertularia pumila_), and if we examine it with
our lens, we shall find that each of the notches represents the space
between the elegant crystal vases that are arranged symmetrically along
each side of the stem. These vases are known as _calycles_, and in each
there stands a polypite, reaching out its upper portion and waving its
tentacles. In case of danger the polypite can be withdrawn into the
calycle; and certain species have an automatic contrivance for closing
the mouth of the vase when they have retreated within. All genera have
not these calycles.
[Illustration: CALYCLES OF SERTULARIA ENLARGED.]
Returning to the animal for a moment, it should be explained that
its organization is so low that there is no true circulatory system
for the renewal of the body, by the carrying of elaborated food
from the stomach to distant parts of the body; but by the activity
of innumerable eye-lash-like hairs on the surface the whole of the
particles of food digested in the stomach are carried all over the
system to be then assimilated by different parts.
Within the circle of tentacles is the mouth, which is sometimes cut
into lobes, and is generally borne upon a very mobile proboscis,
which may be withdrawn or protruded, and in some genera takes a
trumpet-shape; in others it is conical. In the winter the cœnosarc may
frequently be found with all its calycles empty; and it might then
be supposed that the zoophyte is dead and only its skeleton remains.
But this is not necessarily so, and a closer inspection may convince
us that the organism is alive. In spring it will furnish its calycles
with new polypites, and all will go merrily again. At certain seasons
buds of a peculiar structure are formed, which develop into polypites,
whose function it is to produce eggs, instead of catching and digesting
food for the colony. These are known as _gonophoræ_, and sometimes they
remain where they were produced, simply bursting to discharge their
contents. In other cases they detach themselves from the parent colony
at a certain stage in their development, and float off, having all the
appearance of minute jelly-fishes. Some of these, instead of remaining
small, attain an enormous size, so that it is difficult to credit their
origin to the so-called coralline upon which they were produced, and of
which in turn they are really the egg-bearers. The eggs they scatter
will develop into plant-like growths such as they were produced by;
from the edge of their jelly-umbrella and from its handle, buds are
given off, which open as jelly-fish like itself.
Growth proceeds rapidly among these creatures, and if a balk of timber
be immersed in the sea, it is not long ere there is a fine forest
in miniature upon its surface, and that forest will consist of some
of these corallines. The species are generally distributed along
our coasts, but a few are local. Thus the finest of all the British
species of _Sertularia_--_Diphasia pinnata_--is found only on the
coasts of Devon and Cornwall, where most other species attain their
maxima of beauty and luxuriance. Its relative _Diphasia alata_, as
well as _Calycella fastigiata_ and _Aglaophenia tubulifera_, have been
found in Britain, only in Cornwall, Shetland, the Hebrides, and on
the west coast of Scotland. On the other hand, certain species belong
to the north, and such species as _Salacia abietina_, the Sea-fir,
and _Sertularia tricuspidata_ are not found on our shores below the
north-east coast. _Sertularia fusca_ is similarly confined, so far
as our seas are concerned, to the east coast of Scotland and the
north-east of England; and _Thuiaria thuja_, found on the east coast,
is rare in Devon and Cornwall; whilst the species of _Aglaophenia_ are
plentiful on our south-west and north-west coasts, and rarely seen on
the north-east.
Although some species are distinctly deep-water forms, necessitating
the dredge for their capture, the vast majority inhabit the littoral
and laminarian zones. Among the littoral species are many of the rarer
forms, and some of these are found only on special species of seaweeds,
or on the shells of particular mollusks. Mr. Hincks, whose beautiful
work on the “Hydroid Zoophytes” you must see, gives some very good
advice as to collecting in the littoral zone. He recommends my own
favourite plan of lying flat beside the rock-pool, and bringing the eye
close to the water. “He should bring his eye to the edge of the pool,
and look _down_ the side, so as to catch the outline of any zoophytes
that may be attached to it amidst the tufts of minute _Algæ_. He must
not be content with a hasty glance, but look and look again until his
eye is familiar with the scene, and may accurately discriminate its
various elements. And let him watch for the _shadows_; for in following
them he will often secure the reality. I have frequently detected
the tiny _Campanulariæ_ and _Plumulariæ_ in this way, by means of
the images of their frail forms which the light had sketched on the
rock beneath them. For tools, the hunter must have his stout, flat,
sharp-edged, collecting knife, a long-armed and substantial forceps,
and a varied array of bottles, ranging from the homœopathic tube to
the pickle-jar. If his choice of ground be good, and his patience
proof, and his eye quick, he will have an ample reward for his labour
in the rich spoil of beauty which he will bear away, even if he should
not hit upon any novelty; but amongst the minute zoophytes there is
still, I have no doubt, much to be done in the discovery of new forms,
as there certainly is in working out thoroughly the history of those
that are known.” I hope that in the foregoing remarks I have made
it quite clear that our Sea Oak Coralline is not an individual but a
community of individuals--a community on the strictest of co-operative
principles, in which the good fortune accruing to one of the polypites
by food falling in its way, is shared by all alike; for a polypite
cannot digest it and retain it to its own selfish use, instead, it goes
to the nutriment of the commonwealth.
Some of these Hydroid Zoophytes, though sharing the communist
character, are much simpler in form, and we shall find a common example
ready to our hand on almost anything in the way of stone or shell
removed from a rock-pool. It is a minute creature, as stout as a “short
white” pin, and about a third of the length, white or pinkish; a number
of them spring in a row from a creeping stem of firmer substance, in
which are well-defined tubular openings, in which the upright bodies
stand. These answer to the calycles of _Sertularia_, just as the
upright bodies agree with the polypites of that genus. The name of this
creature is _Clava multicornis_, and it may conveniently be called
the Many-horned Club. It gets its name _Clava_ from the shape of the
polypite which thickens towards the top, and then tapers off again to
the summit, where its mouth is situated. It has a number of tentacles,
varying from ten to forty, according to age, but these do not form a
regularly disposed crown round the mouth; instead, they are placed
anyhow on the thickened part of the polypite. The name _multicornis_
refers to these many-horns or tentacles. An advance on this type is
seen in _Coryne pusilla_, a much larger but equally common inhabitant
of our rock-pools, in which the tentacles are knobbed, and are arranged
in a series of more definite whorls.
[Illustration: PLUMULARIA PINNATA.]
There is another group which is more likely to be confounded with the
Sertularians by those who are content with hasty glances at things;
but species of the one group may be readily distinguished from the
other by the aid of a simple lens. The Sertularians, as we have seen,
have the calycles arranged symmetrically on each side of the axis.
The Plumularians, as the other group are called, have their calycles
arranged along one side only of stem and branches. The Sertularians
are frequently spoken of as Sea-firs, the arrangement of the calycles
giving some species a very close resemblance to the branches of
fir-trees. In the Plumularians, the resemblance much more nearly
approaches a feather.
[Illustration: PLUMULARIAN, PORTION ENLARGED.]
Hincks, describing _Plumularia cornucopiæ_ says:--“In the present
species a conspicuous band of opaque white encircles the body, like a
girdle, a little below the tentacles, and adds much to the beauty of a
colony in full life and activity, when its many polypites are in eager
pursuit of prey, stretching themselves forward, and casting forth their
flower-like wreaths, now suddenly clasping their arms together, and
then as suddenly flinging them back; now holding them motionless, the
tips elegantly recurved, and then on some alarm shrinking into half
their size, and folding them together like flowers closing their petals
when the sun has gone.”
In addition to the calycles in which the polypites live, there are
special reproductive chambers as in the Sertularians. In this species
(_P. cornucopiæ_) “they assume the shape of an inverted horn, and are
formed of material translucent as the finest glass. Each one of them,
in fact, is a little crystal cornucopia, in which is lodged one of the
reproductive members of the commonwealth, a class totally distinct
from that which is charged with the function of alimentation. These
graceful receptacles are several times larger than the calycles, from
the base of which they spring, singly or in pairs, and within them the
ova are produced and the embryos matured which are to give rise to new
colonies.”
One of this group, the Lobster-horn or Sea-beard (_Antennularia
antennina_), shown at the back of the illustration of acorn-shells on
page 183, has the calycles arranged in whorls all around the axis,
which produces a very singular appearance, not at all unlike the
antennæ of some of the larger crustacea.
In the Creeping Bell (_Calycella syringa_) so common on seaweeds, etc.,
the calycles are more bell-shaped, and the mouth of the bell is fringed
with a series of large triangular teeth, similar to the _peristome_ of
many moss-fruits. When the polypite withdraws into his calycle, these
teeth bend inwards, and so close the opening.
Many of the forms of Jelly-fish to be described in the next chapter,
though they are described with separate names, are now known to be
merely stages in the history of some of these Hydrozoa or Hydroid
Zoophytes--the developed free-swimming gonophoræ previously mentioned.
[Illustration: HALICLYSTUS.]
A singular member of the group has the form of a jelly-fish, but does
not act as one. This was formerly named Lucernaria, but is now known as
_Haliclystus octoradiatus_. It was thought to swim like a jelly-fish,
but it really creeps. Its form is like a ladies’ sunshade that, instead
of being the ordinary umbrella shape, tapers off to the stick at the
top. What would be the ferrule of the sunshade is the footstalk of
_Haliclystus_. By this footstalk it attaches itself to a weed, say, and
hangs down its eight arms with their connecting web, and by means of a
little knob on the edge of the web alternating with its “arms,” it is
able to take hold until it has “looped” like a geometer caterpillar, by
bringing its footstalk forward and taking fresh hold. The extremities
of the eight arms (or ribs of the sunshade) are ornamented with tassels
of tentacles, and it uses these after the manner of a sea-anemone when
it wishes to secure food. It, in fact, has some of the peculiarities of
both jelly-fish and anemone, though it will not act quite consistently
with either character. I have found it on Laminaria and other weeds at
low water, and a few months since I picked one off the plumage of a
dead guillemot, that had been drowned in a storm and afterwards washed
ashore.
There is an important group of incrusting organisms that you will
find represented on almost the first specimen of _Fucus_ you pick
up, and which you may be tempted to class with these zoophytes; but
they occupy a much higher position in the scale of life. I refer to
the Sea-mats, the Sea-scurfs, the Bird’s-head Coralline, and allied
forms, whose proper designation is Marine Polyzoa. They are more
nearly allied to the mollusks, the structure approaching towards that
of the Lamp shells. They are associated in colonies (_zoaria_), but
there is no connecting _cœnosarc_ as in the Hydrozoa, although there
is communication between the chambers by wisps of animal matter. Each
chamber of the Sea-mat marks the habitation of a complete individual,
who catches, eats, and digests for himself alone, not for the colony.
These chambers are of a horny, persistent character, secreted of course
by the polypide; with a small opening through which the creature
protrudes its mouth and fringe of tentacles. Its body consists of a
thin bag filled with a clear fluid, in which can be traced the gullet
enlarging into a simple stomach, contracting again into the intestine.
There are muscles by means of which the upper part of the sac with the
mouth and tentacles are withdrawn inside the lower part. Add to this a
nerve-ganglion beside the gullet, sexual organs within the sac, and
the polypide is fully described.
The original founder of the colony was produced from an egg, and was
for a time a restless larva, swimming and creeping and whirling around
by the aid of _cilia_. Finally it settles down on weed or stone, and
becomes anchored; drops its cilia and develops its horny chamber and
its crown of tentacles. Having reached its full degree of growth,
it buds at the sides, and originates other creatures like itself.
Just as the solitary daisy root or chrysanthemum throws out what the
gardener terms suckers, and soon becomes the centre of a clump of
similar plants; so the solitary Sea-mat soon becomes only one in a
symmetrically arranged colony, containing hundreds of individuals, all
produced by budding from the original egg-produced polypide.
Some of these colonies have a number of queer adjuncts, which bear a
startling likeness to the head of a bird of prey, with moveable jaws,
that are for ever snapping. These have, of course, given rise to many
theories to account for them; but it appears now to be generally
accepted that the “bird’s-head” is a specialised member of the zoarium
who serves some purpose, probably of defence, or of scavenging, that is
of advantage to the whole colony. In some species, this differentiation
of individuals takes the form of a long whip-like process, constantly
lashing, instead of the snapping jaws. The forms of the marine polyzoa
are very varied, but we shall be unable to do more than indicate a few
of them here, leaving the reader to make wider acquaintance with a most
interesting group by studying the species in Hincks’s _British Marine
Polyzoa_.
[Illustration: SEA-MAT (FLUSTRA).]
The Sea-mat (_Flustra foliacea_) is a deep-water form, whose colonies
take the shape of fronds, resembling _Fucus serratus_ in outline; but
it is thrown up on the beach in great quantities, and it will be one
of the first things you will find on the shore, especially if you
rout about among the weeds washed up by every tide. Creeping over
these flat frond-like masses you will probably find other species that
take a more branching form, such as the common Creeping Coralline
(_Scrupocellaria reptans_), or the more bushy Bird’s-head Coralline
(_Bugula avicularia_). The Tufted Ivory Coralline (_Crisea eburnea_)
has tubular chambers of ivory whiteness; it is of branching habit,
and occurs on some of the red seaweeds. The Foliaceous Coralline
(_Membranipora pilosa_) runs in very narrow ribbons, covered with a
“pile” of bristles, up the stems of various weeds; and many another of
the nearly two hundred and fifty British species will be sure to fall
to the patient and sharp-eyed investigator.
The horny cell in which the polypide resides is really its own
cuticle or outer skin, to which it is inseparably attached. If
careful examination be made, it will be found that at the mouth of
the so-called cell the horny material suddenly changes its character
and becomes a very fine and delicate tissue, capable of the greatest
freedom of movement and folding such as is absolutely impossible
with the horny portion. This remarkable change of character in the
two portions of the same cuticle allows the anterior portion of the
polypide, with its crown of tentacles, to be suddenly and completely
withdrawn out of danger, just as easily as the tip of a glove-finger
can be withdrawn into its lower portion.
The tentacles that encircle the mouth of the polypide are hollow, and
covered with ever-waving cilia, whose beating causes currents of
water to set in towards the animal’s mouth, bringing food with them.
These tentacles appear to be also the only sense organs possessed
by the polypide, and to serve the further purpose of gills. None of
the Polyzoa to which we here make reference possesses a heart or
blood-vessels.