NOVEMBER.
Characteristics of the Polyzoa--Details of structure according
to Allman--Plumatella repens--Its great beauty under proper
illumination--Its tentacles and their cilia--The mouth and its
guard or epistome--Intestinal tube--How it swallowed a Rotifer,
and what happened--Curiosities of digestion--Are the tentacles
capable of Stinging?--Resting Eggs, or "Statoblasts"--Tube of
Plumatella--Its muscular Fibres--Physiological importance of
their structure.
During the fag end of last month I observed some fragments of a new
creature among some bits of Anacharis, from the Vale of Heath Pond, and
searched for complete and intelligible specimens without effect. Luckily
one evening a scientific neighbour, to whom I had given some of the
plant for the sake of the beautiful _Stephanoceri_ which inhabited it,
came in with a glass trough containing a little branch, to which adhered
a dirty parchment-like ramifying tube, dotted here and there with brown
oval masses, and having sundry open extremities, from which some
polyp-shaped animals put forth long pearly tentacles margined with
vibrating cilia, and making a lively current. The creatures presented an
organization higher than that of polyps, for there was an evident
_differentiation_ and complication of parts. They belonged to the
_Polyzoa_ or _Bryozoa_,[21] a very important division of the _mollusca_.
The _Polyzoa_ are chiefly marine, and the common "sea-mat," often
erroneously treated as a _sea-weed_, is a well-known form. A species of
another order often picked up on our coasts is the _Sertularia_, or
Sea-Fir, composed of delicate branching stems of a horny-looking
substance, which, under a pocket-lens, is found to contain an immense
number of small cells inhabited by Polyps. It is instructive to compare
the two and note how much more advanced in structure is the Polyzoon
than the polyp.
[21] _Polyzoa_ means "many animals," in allusion to their habit of
living in association. _Bryozoa_, "moss-animals," from some forming
cells having that appearance.
[Illustration: Plumatella repens. Single Polypide enlarged]
Polyzoa were formerly associated with the polyps, to which they bear a
strong superficial resemblance; but they are of a much higher degree of
organization, as will be seen by comparing what has been said in a
former chapter on the _Hydra_, with the description which we now proceed
to abridge from Dr. Allman's splendid monograph on the fresh-water
kinds. In order to get a general conception of a Polyzoon, the Professor
tells us to imagine an alimentary canal, consisting of oesophagus,
stomach, and intestine, to be furnished at its origin with long ciliated
tentacles, and to have a single nervous ganglion on one side of the
oesophagus. We must then conceive the intestine bent back till its
anal orifice comes near the mouth; and this curved digestive tube to be
suspended in a bag containing fluid, and having two openings, one for
the mouth and the other for the vent. A system of muscles enables the
alimentary tube to be retracted or protruded, the former process pulling
the bag in, and the latter letting it out. The mouth of the bag is, so
to speak, tied round the creature's neck just below the tentacles, which
are the only portions of it that are left free. The investing sack has
in nearly every case the power of secreting an external sheath, more or
less solid, and which branches forming numerous cells, in which the
members of the family live in a socialistic community, having, as it
were, two lives, one individual, and the other shared in common with the
rest.
The whole group of tubes and cells, whatever may be the form in which
they are aggregated, is called the _Polypary_, or, as Dr. Allman
prefers, the _Coenoecium_ (common house); the creature he names a
_Polypide_[22] (polyp-like); and the disk which bears the tentacles
_Lophophore_ (crest-bearer). There are some more hard words to be learnt
before the student can enjoy himself scientifically among the Polyzoa,
and we shall be compelled to employ some of them before we have done;
but will now endeavour to describe what was presented to our view by the
specimen obtained from the Hampstead Pond.
[22] _Polyzoon_ is preferable, as avoiding confusion with _polypite_,
used for another class of object.
The general aspect of a branch of _Plumatella repens_--the creature we
have to describe--is given in the drawing annexed. When all was quiet,
the mouths of the bags belonging to each cell were slowly everted, and
out came a numerous bundle of tentacles, which were either spread like
the corolla of a flower, or permitted to hang dishevelled like the
snake-locks of Medusa. We will suppose these organs symmetrically
expanded, and that we are looking down upon them with a magnifying power
of sixty diameters, the light having been carefully adjusted by turning
the reflecting mirror a little on one side, to avoid a direct glare. The
tentacles, each of which curves with a living grace, and displays an
opaline tint in its glassy structure, do not form a complete circle, for
at one place we discern two slightly diverging arms of the disk, or
frame (Lophophore) from which they grow.
These arms support tentacles on each side, and leave a gap between, so
that the whole pattern is _crescentic_, or crescent-shaped, and not
circular. Extending as far as the points of the arms, and carried all
round the crescent, is an extremely delicate membrane, like the finest
gauze, which unites all the tentacles by their basal portions, and makes
an elegant retreating curve between every two. Each tentacle exhibits
two rows of cilia, which scintillate as their vibrations cause them to
catch the light. The motion of the cilia is invariably _down_ one side
and _up_ the other, the current or pattern being carried on from one
tentacle to the other, all through the series. This characteristic, and
the facility with which each cilium can be distinguished, gives great
interest and beauty to the spectacle of this wonderful apparatus, by
which water-currents are made to bathe the tentacles, and assist
respiration, and also to carry food towards the mouth, over which a sort
of finger or tongue is stretched to guard the way, and exercise some
choice as to what particles shall be permitted to pass on. This organ is
called the _epistome_, from two Greek words, signifying "upon the
mouth."
If the cell is an old one, it may be covered with so much extraneous
matter as to obscure the economy within; but we are fortunate in having
a transparent specimen before us, through which we can see all that goes
on. The alimentary tube, after forming a capacious cavity, much longer
than it is broad, turns round and terminates in an orifice near the
mouth, and just below the integuments. When refuse has to be discharged,
this orifice is protruded; and after the operation is over, it draws
back as before. Long muscles, composed of separate threads or fibres,
pull the creature in and out of its cell, and at the part where the
stomach ends, and the intestine turns round, is attached a long flexible
rope, called the _funiculus_, which goes to the bottom of the cell. The
passage of the food down to the stomach, its digestion, and the eviction
of the residue, can all be watched; and when a large morsel is
swallowed, the spectacle is curious in the extreme.
One day a polyzoon caught a large rotifer, (_R. vulgaris_,) which, with
several others of its tribe, had been walking over the _coenoecium_,
and swimming amongst the tentacles, as if unconscious of danger. All of
a sudden it went down the whirlpool leading to the mouth, was rolled up
by a process that could not be traced, and without an instant's loss of
time, was seen shooting down in rapid descent to the gulf below, where
it looked a potato-shaped mass, utterly destitute of its characteristic
living form. Having been made into a bolus, the unhappy rotifer, who
never gave the faintest sign of vitality, was tossed up and down from
the top to the bottom of the stomach, just as a billiard-ball might be
thrown from the top to the bottom of a stocking. This process went on
for hours, the ball gradually diminishing in size, until at last it was
lost in the general brown mass with which the stomach was filled. The
bottom of the stomach seems well supplied with muscular fibres, to cause
the constrictions by which this work is chiefly performed, and by
keeping a colony for a month or two, I had many opportunities of seeing
my Polyzoa at their meals.
When alarmed the tentacles were quickly retracted, but although these
creatures are said to dislike the light, and usually keep away from it
in their native haunts, my specimens had no objection to come out in a
strong illumination, and seemed perfectly at their ease. They were
indeed most amiable creatures, and never failed to display their charms
to admiring visitors, who rewarded them with unmeasured praise. Twice I
had an opportunity of observing an action I cannot explain, except by
supposing either that the tentacles of the _Plumatella_ have some
poisonous action, or that rotifers are susceptible of fear. On these
occasions the common rotifer was the subject of the experiment. First
one and then another got among the tentacles, and on escaping seemed
very poorly. One fellow was, to borrow a phrase from Professor Thomas
Sayers, "completely doubled up," and two or three seconds--long periods
in a rotifer's life--elapsed before he came to himself again.
By keeping a colony of the Plumatella for a few weeks in a glass trough,
and occasionally supplying them with fresh water from an aquarium,
containing the animalcules, they are easily preserved in good health,
and as they develop fresh cells, the process of growth may be readily
watched. This production of fresh individuals enlarges the parent
colony, but could not be the means of founding a new one, which is
accomplished by two other modes. A little way down the cells Professor
Allman discovered an ovary attached to the internal tube by a short
_peduncle_, or foot stalk, while a testis or male generative organ is
attached to the _funiculus_, or "little rope," we have already
described.
July and August are the best times for observing the ovaries, and they
are most conspicuous in the genera _Alcyonella_ and _Paludicella_. True
eggs are developed in the ovaries in a manner resembling this mode of
multiplication in other animals; but there is another kind of egg, or,
perhaps to speak more properly, a variety of bud, which is extremely
curious. In looking at our specimens we noticed brown oval bodies in the
cells; these, on careful examination, presented the appearance of the
sketch. The centre is dark, covered with a network, which is more
conspicuous in the lighter coloured and more transparent margins. These
curious bodies are produced from the funiculus, and act as reserves of
propagative force, as they are not hatched or developed until they get
out and find themselves exposed to appropriate circumstances. Professor
Allman names them _Statoblasts_, or stationary germs, and they bear some
resemblance to what are called the "winter eggs" of some other
creatures. The Professor was never able to discover any mode by which
they were permitted to escape from the cells, and in our colonies none
were allowed to leave their homes until the death of their parent, and
the decomposition of its cell had taken place; a process which went on
contemporaneously with the growth of new cells, until the plant on which
the _coenoecium_ was situated, rotted away, and then unfortunately
the whole concern went to pieces.
[Illustration: Plumatella repens on a leaf.]
The tubes of the _Plumatella_, and of most other Polyzoa, are composed
of two coats, called respectively _endocyst_ and _ectocyst_, that is,
"inner case" and "outer case." The first is vitally endowed, and
exhibits vessels and muscular fibres. The second or outer case is thrown
off by the first. It is a parchment-like substance, strengthened by the
adhesion of dirt particles, and does not appear to exercise any vital
functions, but to be merely a covering for protection. The inner layer
terminates in the neck of the bag before described, as exserted when the
polypide comes out, and inverted when it goes in. This mode of making a
case or sheath by inversion of a bag is technically called
_invagination_, and is readily seen in new and transparent cells.
The movement of _eversion_, or coming out, is chiefly produced by the
contraction of the endocyst; while the _inversion_, or getting in again,
is performed by the long muscles, which, when the animal is extended,
are seen attached to it like ropes. Upon these muscles Professor Allman
remarks that they are "especially interesting in a physiological point
of view, as they seem to present us with an example of true muscular
tissue, reduced to its simplest and essential form. A muscle may here
be viewed as a beautiful dissection far surpassing the most refined
preparation of the dissecting needle, for it is composed of a bundle of
elementary fibres, totally separate from one another through their
entire course." He further adds, "The fibres of the great retractor
muscle are distinctly marked by transverse striae;--a condition, however,
which is not at all times equally perceptible, and some of our best
observers have denied to the Polyzoon the existence of striated fibre."
We can confirm the fact of this sort of fibre being present, but we
fancy a reader not versed in the mysteries of physiology exclaiming,
'What does it matter whether his fibres are striped or not?'
Physiologists used to suppose there was a strong and marked distinction
and separation between _striped_ muscles, that is, muscles the fibres of
which exhibit transverse stripes when magnified, and those which do not.
Koelliker, however, says this decided separation can no longer be
maintained,[23] and he gives instances in proof of the connections that
can be traced between the two forms. In the higher animals the striped
muscles are the special instruments of _will_, and of movements that
follow, or are accompanied by, distinct sensations. Striped fibre must
be regarded as the highest form; and as a muscle of this sort contracts
in length it increases uniformly in breadth.
[23] 'Manual of Human Microscopic Anatomy,' p. 63.
There are many other genera and species of fresh-water polyzoa besides
the _Plumatella repens_, and they are found attached to sticks, stones,
or leaves, generally to the under surface of the latter. They are all
objects of great interest and beauty, which, whatever their diversity,
conform sufficiently to one type that the student who has observed one,
will easily recognise the zoological position of another. They should be
viewed by transmitted and by dark-ground illumination, which produces
very beautiful effects. To observe them in the performance of their
functions, they require more room than the live-box can afford, but are
well shown in the glass trough, whose moveable diaphragm enables them to
be brought near enough to the object-glass, for the use of a power of
about sixty linear for general purposes, and of from one to two hundred
for the examination of particular parts. For a more detailed examination
dissection must be employed, but all that we have mentioned can be seen
without injury to the living animal, if specimens are kept till new
cells are formed in water, which does not contain enough dirt to render
their integuments opaque.