SEA SQUIRTS.
The other day I was down in our porth when some of the fishermen of the
village came in after hauling their trammels. There had been a “good
bit of sea” running during the night, and the trammel had got fairly
filled with weed, so that it was necessary to bring it ashore to clean
it. If the naturalist is about when this happens, he stands a chance
of obtaining some deep-water specimens of interest to him. My eye fell
upon several masses of a clear greenish-white jelly, pear-shaped, and
firm to the touch. I knew what they were, but always anxious to get
local names for natural objects where they exist, I asked the fisherman
what they were. “Oh, I dare say you know, sir; but we always call
_they_ congealed water. Isn’t that right?” I admitted that they were
composed almost entirely of water, but denied that it was congealed.
It would be better, I added, to speak of it as a living leather bottle
full of water--and other things.
“What was it?”
Popularly speaking, it was a Sea Squirt. A naturalist would speak of
it as a simple _Ascidian_--_A. mentula_, to wit; and on being further
pressed, might tell you that the Ascidiaceæ constitute an order of the
Tunicata, which is now included among Vertebrate animals, though no
Tunicate possesses a backbone.
Our description of it as a leather bottle is more to the point, and
equally scientific, for the naturalist who bestowed the name Ascidian
upon this remarkable group of animals got that name from the Greek word
_askos_, a leathern bottle.
[Illustration:
A. ASCIDIA VIRGINEA.
B. CYNTHIA QUADRANGULARIS.
]
Look at these diagrams: they represent two common forms of Ascidians,
and it will be noted that they have a general agreement in shape with
the large specimens of _A. mentula_ we were looking at just now. Like
that, these have each two necks, though those of _mentula_ were closed,
and these are open at their mouths. If we had these in a glass vessel,
but still attached to pieces of the rock upon which they grew, we
should be able to see why one bottle need have two necks. If we were
then to drop a little finely-divided colour-powder such as indigo,
into the water, we should see two currents were in operation, one
flowing to the animal, the other proceeding from it. The first would
be flowing to the neck marked _a_ in the figures, and the second would
be issuing from the mouth of _b_. Naturally, we should at once suppose
that by means of some internal mechanism and system of valves, the same
current that was being induced at _a_, was being continued through the
creature’s body, and pumped out at _b_. Our supposition would be proved
correct by the fact that the colour grains streaming in were also
streaming out. But what happens to them between entering and departing
we cannot clearly see.
By the aid of another diagram (next page) we may get a better notion
of the Ascidian’s internal arrangements than by gazing through its
integuments. Here are all its parts marked with a letter as a guide to
its anatomy. It is a matter of astonishment to many fairly intelligent
people, to find that such soft creatures as Sea-squirts, Jelly-fishes,
Slugs and Caterpillars, are provided with a more or less intricate
machinery for carrying out all the functions of life. But so it is;
and here is the typical plan of arrangements inside our Ascidian. Here
the necks of the bottle are marked _a_ and _n_ respectively, and _a_,
by which the current of water flows in, is called the oral orifice.
Just inside is a series of tentacles (_b_), and below these we are in
the branchial chamber (_c_), where the great work of supplying the
blood with oxygen is carried on. The walls consist of a lattice-work of
blood-vessels, through whose tissues the blood takes up the molecules
of the life-supporting gas. Below this chamber the gullet opens and
is continued into the stomach (_g_), and beyond it is the intestine
(_h_), which in turn opens out through the anus (_l_) into another
roomy chamber, the _atrium_ (_m_) or atrial chamber, with its external
opening (_n_). _O_ is a ganglion or small brain, and _f_ indicates the
heart.
[Illustration: DIAGRAMMATIC SECTION OF AN ASCIDIAN.]
Now in order to get a correct idea of the Tunicates--as the group
in which the Ascidians are included is called--I wish you to note
the figures _d_ and _e_ in the same diagram. You will see that they
indicate two separate envelopes. The outer of these, represented by
the thick dark line, is of a tough, leathery nature, and is much
akin to vegetable cellulose in its character--a fact that caused
some little commotion in scientific circles years ago, when it was
first satisfactorily made out, for prior to that date cellulose was
considered to be purely a vegetable product. This outer coat is known
as the _tunic_, or test, and from the fact that all the species are
enclosed in such a tunic, the group gets its name Tunicata. The inner
coat represented in the diagram by the clear space between the thick
and thin marginal lines, is of a more delicate, more animal nature:
it is composed of soft though powerful muscular tissue, and by its
contraction the water, which always fills the interior of this “leather
bottle,” can be violently spurted forth--a phenomenon which has brought
upon these creatures the name of Sea-squirts. This muscular coat is
known as the _mantle_.
[Illustration: ORANGE-SPOTTED SQUIRT (_Cynthia aggregata_).]
The Ascidian has no proper system of blood-vessels, as we are generally
acquainted with them in higher animals. The blood flows about the
general body cavity, and is not confined to narrow channels as in
our arteries and veins. There is a heart, it is true, but one of the
simplest character, without any elaborate system of ventricles and
auricles, with their regulating valves. The Ascidian’s heart is simply
a tube open at each end, and by its steady pulsation--that is, its
alternate contraction and expansion, it sets the blood flowing to the
blood-vessels that line the walls of the branchial cavity, where it
absorbs oxygen from the continuous flow of fresh sea-water that passes
through it. When this end has been attained, a curious and unique
“reversal of the engine” takes place: there is such an opposite action
of the heart, that all this vivified blood is withdrawn from the
neighbourhood of the branchial chamber and sent flowing to remote parts
of the body.
[Illustration: _Ascidia mentula._
CURRANT-SQUIRTER
(_Styela Grossularia_).]
The flow of water through the branchial chamber is kept up by the
regular and unceasing lashing of eye-lash-like _cilia_, with which the
blood-vessels are fringed. This constant inflow at the oral orifice
forces the water through to the atrial chamber, from which it is pumped
out by the contraction of the mantle. Minute particles of matter that
serve as food are also brought in by the current, and find their way
into special grooves for their reception and digestion. The other
arrangements of the creature are equally simple. The nervous ganglion,
to which we have made reference, is its only brain, and it has no
proper eyes, only some pigment granules near the tentacles appear to be
sensitive to light.
Most of the Ascidians inhabit deeper water than comes within our range,
but we shall find specimens at low-water attached to stones and the
roots of seaweeds. We may even find specimens of _Ascidia mentula_ in
rock-pools, and others we shall discover on smaller stones and shells
that have washed in on sandy shores from greater depths. Among such
will be the Quadrangular-squirter (_Cynthia quadrangularis_), so-called
on account of the squareness of its apertures; and the Currant-squirter
(_Styela grossularia_), a very common form on dead shells, which gets
its name partly from its colour and partly from its form when it has
closed both apertures and become more rounded.
[Illustration: CLAVELINA.]
But there are many other forms of Tunicates that haunt our shores
either in deep water or upon the vegetation of the lower rocks. There
are some of more slender, more elongated form that live together in
bunches, their bases being connected by a kind of running rootstock,
which has the power to produce young individuals by budding from it.
This form is known as _Clavelina lepadiformis_, and is only about an
inch in height, of the form shown in the annexed diagram. In the
figure the reference letters are of the following signification: _a_,
branchial apertures; _b_, atrial apertures; _c_, young individuals
arising from the runners _s_.
[Illustration: SALPA MAXIMA.]
From this form it is an easy transition to the Ascidians known as
_Salpæ_. These have the branchial aperture (_b_) at one end, and the
atrial opening (_a_) at the other. In the figure the heart is shown at
_e_, and the branchial chamber at _d_. These _Salpæ_ are both solitary
and compound Ascidians, for it is a singular fact that the solitary
form as here shown produces buds which develop into a connected series
or chain of individuals. These, in turn, instead of reproducing the
species, in a similar manner produce eggs, each of which gives rise to
a solitary individual. In our figure of _Salpa maxima_, the letters _c_
indicate the points of attachment of the Salpa colony; and the next
figure represents a portion of the Salpa-chain.
[Illustration: PART OF A CHAIN OF SALPÆ.]
Frequently, in gazing down the sides of a still, deep rock-pool, we
shall observe a coating of dark grey jelly, in patches as big as one’s
hand, and on carefully taking off an inch or two of this, and examining
it with the lens, or a low power of the microscope, we shall observe a
number of elegantly-formed jars to be set in the jelly, and as we look
their mouths and necks are raised above the surface of the jelly and
opened. These are the branchial apertures of a colony of Ascidians
(_Leptoclinum gelatinosum_), and if we search around the mass we shall
shortly find a cone-shaped opening in the clear jelly, through which
a current of water flows. This is the common atrial chamber of the
whole colony. The clear jelly is the common outer tunic of the whole
community.
On the walls of overhanging rocks, at low-water, many fleshy clusters,
like pale-coloured strawberries will be found, of firm gelatinous
material, with a clear jelly envelope, through which the crimson dots
of the contained squirts may be seen. One form has a thick trunk, with
but slightly enlarged head, and consisting of a number of groups of
squirts: this is _Aplidium_; it has no common aperture. A more globose
head on a shorter stalk has a distinctly marked common opening: this
is _Polyclinum_. _Amaroecium_ has a corrugated exterior, and is more
cylindrical in form.
[Illustration: BOTRYLLUS.]
Other species will meet us of more symmetrical form, on flat weeds,
smooth stones, and under the overhanging brows of the large rocks
at low-water. These are of varied tints according to species, but
each with a starry pattern worked in with little purple or yellow
Ascidians. It looks as though six or seven of these had agreed to live
together for company’s sake and for economy; and here we find them set
in the jelly, and radiating from a central aperture, the common atrial
opening of the colony.
[Illustration: BOTRYLLUS VIOLACEUS.]
Here is a figure showing part of a patch of _Botryllus violaceus_, such
as you may find abundant on the rocks. _C_ shows the combined tunic
of the colony, _a_ the branchial openings, and _b_ the common atrium.
The general verdict on a patch of Botryllus would probably be that it
was some low form of sea-plant, for a naked-eye view of it reveals
no evidence of animal processes; yet, in spite of its vegetative
condition, this--in common with other Tunicates--is held to approach
nearest to the great back-boned races, the aristocracy of animal life.
But it is a sad story of missed opportunities and consequent
degeneration that the Tunicates have to tell of their race. Some
evolutionists hold that in the primeval Ascidian we must look for the
progenitor of the vertebrates. We know what the primeval Ascidian was
like, for the form is retained, according to a natural law, in the
larval stage of its present-day representatives. Roughly speaking, it
was like a tadpole, with a broad head-and-trunk combined, and a very
long, narrow tail, by the lashing of which from side to side it made
way through the waters, much as the boatman gets along by sculling
from the stern. At the front there was a rudimentary mouth with three
suckers, an optic organ, with a retina, lens, cornea, and so forth; an
auditory organ; the promise of a well-formed brain and nervous system;
and a rod in the tail might be developed into that backbone which is
the distinguishing mark of all the birds, beasts, fishes, reptiles, and
man himself.
[Illustration: LARVA OF A TUNICATE.]
Some of the primeval squirt-larvæ are supposed to have cultivated these
possibilities, and the grand vertebrate division of the animal kingdom
is said to be the result; but others went in for the _status quo_ and
inglorious ease. No developments for us, said they. They may even be
supposed to have anticipated the prayer formerly taught to rural school
children:
“God bless the squire and his relations,
And keep us in our proper stations.”
Then they gave up wandering at random through the waters, and settled
down to a quiet and retired life on a piece of rock at the root of a
branching weed. Taking hold with their suckers, they soon discovered
that tails and sense organs were of no use to those who had forsworn
wandering, so they threw them off, and gradually assumed the wine-skin
shape that has ever since been the ruling fashion among Ascidians. All
that remains of the tail is a few fatty cells in the posterior part
of the trunk. The suckers by which it was attached disappear, and the
test grows over surrounding objects; the auditory organ disappears,
the eye retrogrades into a mere pigment spot, and the nervous system
degenerates into the solitary ganglion to which we have already
referred. It will thus be seen that the life-history of the Tunicates
is a dismal story of degeneration instead of development; but it is
none the less interesting on that account.