_THE PROTOZOA OF THE SEA SHORE_
We shall now study the principal forms of animal life to be found on the
sea shore; and, in order that the reader may thoroughly understand the
broader principles of classification, so as to be able to locate each
creature observed in its approximate position in the scale of life, we
shall consider each group in its zoological order, commencing with the
lowest forms, and noting, as we proceed, the distinguishing
characteristics of each division.
The present chapter will be devoted to the _Protozoa_--the sub-kingdom
that includes the simplest of all animal beings.
Each animal in this division consists of a minute mass of a jelly-like
substance called _protoplasm_, exhibiting little or no differentiation
in structure. There is no true body-cavity, no special organs for the
performance of distinct functions, and no nervous system.
Perhaps we can best understand the nature of a protozoon by selecting
and examining a typical example:
Remove a small quantity of the green thread-like algous weed so commonly
seen attached to the banks of both fresh and salt water pools, or
surrounding floating objects, and place it in a glass with a little of
the water in which it grew. This weed probably shelters numerous
protozoons, among which we are almost sure to find some _amœbæ_ if we
examine a drop of the water under the high power of a microscope.
[Illustration: FIG. 50.--THE AMŒBA, HIGHLY MAGNIFIED]
The amœba is observed to be a minute mass of protoplasm with an
average diameter of about one-hundredth of an inch, endowed with a power
of motion and locomotion. Its body is not uniformly clear, for the
interior portion is seen to contain a number of minute granules,
representing the undigested portions of the animal’s food. There is a
small mass of denser protoplasm near the centre, termed the _nucleus_,
and also a clear space filled with fluid. This latter is called the
_vacuole_, and is probably connected with the processes of respiration
and excretion, for it may be seen to contract at irregular intervals,
and occasionally to collapse and expel its contents.
As we watch the amœba we see that it is continually changing its
shape, sending out temporary prolongations (_pseudopodia_) of its
gelatinous substance from any part, and sometimes using these extended
portions for the purpose of dragging itself along.
Its method of feeding is as remarkable as it is simple. On coming in
contact with any desired morsel, it sends out two pseudopods, one on
each side of the food. These two pseudopods gradually extend round the
food, till, at last, they meet and coalesce on the opposite side of it,
thus completely enclosing it within the body. Any part of the body of
the amœba may thus be converted into a temporary mouth; and, there
being no special cavity to serve the purpose of a stomach, the process
of digestion will proceed equally well in any part of the body except in
the superficial layer, where the protoplasm is of a slightly firmer
consistence than that of the interior. Further, the process of digestion
being over, any portion of the superficial layer may be converted into a
temporary opening to admit of the discharge of indigestible matter.
[Illustration: FIG. 51.--THE AMŒBA, SHOWING CHANGES OF FORM]
[Illustration: FIG. 52.--THE AMŒBA, FEEDING]
The amœba is an omnivorous feeder, but subsists mainly on vegetable
organisms, especially on diatoms and other minute algæ; and the
siliceous skeletons of the former may often be seen within the body of
the animal, under the high power of a microscope.
The multiplication of the amœba is brought about by a process of
fission or division. At first the nucleus divides into two, and then the
softer protoplasm contracts in the middle, and finally divides into two
portions, each of which contains one of the nuclei. The two distinct
animals thus produced both grow until they reach the dimensions of
their common progenitor.
[Illustration: FIG. 53.--THE AMŒBA, DIVIDING]
All the protozoons resemble the amœba in general structure and
function; but while some are even simpler in organisation, others are
more highly specialised. Some, like the amœba, are unicellular
animals; that is, they consist of a single, simple speck of protoplasm;
but others live in colonies, each newly formed cell remaining attached
to its parent cell, until at last a comparatively large compound
protozoon is formed.
The sub-kingdom is divided into several classes, the principal of which,
together with their leading characteristics, are shown in the following
table:--
1. _Rhizopods:_--Body uniform in consistence.
Pseudopods protruded from any point.
2. _Protoplasta:_--Outer protoplasm slightly firmer in consistence.
Pseudopods protruded from any point.
(Often grouped with the _Rhizopods_.)
3. _Radiolaria:_--Possessing a central membranous capsule.
Usually supported by a flinty skeleton.
4. _Infusoria:_--Outer protoplasm firmer and denser; therefore
of more definite shape.
Possess permanent threadlike extensions of protoplasm
instead of pseudopods.
We shall now observe the principal marine members of the protozoa,
commencing with the lowest forms, and dealing with each in its proper
zoological order as expressed in the above table.
MARINE RHIZOPODS
When we stand on a beach of fine sand on a very calm day watching the
progress of the ripples over the sand as the tide recedes we frequently
observe whitish lines marking the limits reached by the successive
ripples as they advance toward the shore. If, now, we scrape up a little
of the surface sand, following the exact course of one of these whitish
streaks, and examine the material obtained by the aid of a good lens, we
shall in all probability discover a number of minute shells among the
grains of sand.
These shells are of various shapes--little spheres, discs, rods,
spirals, &c.; but all resemble each other in that they are perforated
with a number of minute holes or _foramina_. They are the skeletons of
protozoons, belonging to the class _Rhizopoda_, and they exist in
enormous quantities on the beds of certain seas.
[Illustration: FIG. 54.--A GROUP OF FORAMINIFERS, MAGNIFIED]
We will first examine the shells, and then study the nature of the
little animals that inhabit them.
The shells vary very much in general appearance as well as in shape.
Some are of an opaque, dead white, the surface somewhat resembling that
of a piece of unglazed porcelain; others more nearly resemble glazed
porcelain, while some present quite a vitreous appearance, much after
the nature of opal. In all cases, however, the material is the same, all
the shells consisting of carbonate of lime, having thus the same
chemical composition as chalk, limestones, and marble.
If hydrochloric acid be added to some of these shells, they are
immediately attacked by the acid and are dissolved in a very short time,
the solution being accompanied by an effervescence due to the escape of
carbonic acid gas.
The shells vary in size from about one-twelfth to one three-hundredth of
an inch, and consist either of a single chamber, or of many chambers
separated from each other by perforated partitions of the same material.
Sometimes these chambers are arranged in a straight line, but more
frequently in the form of a single or double spiral. In some cases,
however, the arrangement of chambers is very complex.
We have already referred to the fact that the shells present a number of
perforations on the exterior, in addition to those which pierce the
partitions within, and it is this characteristic which has led to the
application of the name Foraminifera (hole-bearing) to the little beings
we are considering.
[Illustration: FIG. 55.--A SPIRAL FORAMINIFER SHELL]
[Illustration: FIG. 56.--A FORAMINIFER OUT OF ITS SHELL]
The animal inhabiting the shell is exceedingly simple in structure, even
more so than the amœba. It is merely a speck of protoplasm,
exhibiting hardly any differentiation--nothing, in fact, save a
contractile cavity (the _vacuole_), and numerous granules that probably
represent the indigestible fragments of its food.
The protoplasm fills the shell, and also forms a complete gelatinous
covering on the outside, when the animal is alive; and the vacuole and
granules circulate somewhat freely within the semi-solid mass. Further,
the protoplasm itself is highly contractile, as may be proved by
witnessing the rapidity with which the animal can change its form.
When the foraminifer is alive, it floats freely in the sea, with a
comparatively long and slender thread of its substance protruded through
each hole in the shell. These threads correspond exactly in function
with the blunt pseudopodia of the amœba. Should they come in contact
with a particle of suitable food-material, they immediately surround
it, and rapidly retracting, draw the particle to the surface of the
body. The threads then completely envelop the food, coalescing as soon
as they touch, thus bringing it within the animal.
[Illustration: FIG. 57.--THE SAME FORAMINIFER (FIG. 56) AS SEEN WHEN
ALIVE]
[Illustration: FIG. 58.--SECTION OF THE SHELL OF A COMPOUND
FORAMINIFER]
The foraminifer multiplies by fission, or by a process of budding. In
some species the division of the protoplasm is complete, as in the case
of amœbæ, so that each animal has its own shell which encloses a
single chamber, but in most cases the ‘bud’ remains attached to a parent
cell, and develops a shell that is also fixed to the shell of its
progenitor. The younger animal thus produced from the bud gives rise to
another, which develops in the same manner; and this process continues,
the new bud being always produced on the newest end, till, at last, a
kind of colony of protozoons is formed, their shells remaining attached
to one another, thus producing a compound shell, composed of several
chambers, arranged in the form of a line or spiral, and communicating by
means of their perforated partitions. It will now be seen that each
‘cell’ of the compound protozoon feeds not only for itself, but for all
the members of its colony, since the nourishment imbibed by any one is
capable of diffusion into the surrounding chambers, the protoplasm of
the whole forming one continuous mass by means of the perforated
partitions of the complex skeleton.
[Illustration: FIG. 59.--SECTION OF A NUMMULITE SHELL]
Some of the simplest foraminifers possess only one hole in the shell,
and, consequently, are enabled to throw off pseudopods from one side of
the body only. In others, of a much more complex nature, the new
chambers form a spiral in such a manner that they overlap and entirely
conceal those previously built; and the development may proceed until a
comparatively large discoid shell is the result. This is the case with
_Nummulites_, so called on account of the fancied resemblance to coins.
Further, some species of foraminifera produce a skeleton that is horny
in character, instead of being calcareous, while others are protected
merely by grains of sand or particles of other solid matter that adhere
to the surface of their glutinous bodies.
[Illustration: FIG. 60.--_Globigerina bulloides_, AS SEEN WHEN ALIVE,
MAGNIFIED]
We have spoken of foraminifera as floating freely about in the sea
water, but while it is certain that many of them live at or near the
surface, some are known to thrive at considerable depths; and those who
desire to study the various forms of these interesting creatures should
search among dredgings whenever an opportunity occurs. Living specimens,
whenever obtained, should be examined in sea water, in order that the
motions of their pseudopods may be seen.
[Illustration: FIG. 61.--SECTION OF A PIECE OF NUMMULITIC LIMESTONE]
If we brush off fragments from the surface of a freshly broken piece of
chalk, and allow them to fall into a vessel of water, and then examine
the sediment under the microscope, we shall observe that this sediment
consists of minute shells, and fragments of shells, of foraminifers. In
fact, our chalk beds, as well as the beds of certain limestones, consist
mainly of vast deposits of the shells of extinct foraminifera that at
one time covered the floor of the sea. Such deposits are still being
formed, notably that which now covers a vast area of the bed of the
Atlantic Ocean at a depth varying from about 300 to 3,000 fathoms. This
deposit consists mainly of the shells of a foraminifer called
_Globigerina bulloides_, a figure of which is given on the opposite
page.
The structure of chalk may be beautifully revealed by soaking a small
piece of the rock for some time in a solution of Canada balsam, allowing
it to become thoroughly dry, and then grinding it down till a very thin
section is obtained. Such a section, when viewed under the low power of
a compound microscope, will be seen to consist very largely of minute
shells; though, of course, the shells themselves will be seen in
section only.
The extensive beds of nummulitic limestones found in various parts of
South Europe and North Africa are also composed largely of foraminifer
shells, the most conspicuous of which are those already referred to as
nummulites--disc-shaped shells of a spiral form, in which the older
chambers overlap and hide those that enclose the earlier portion of the
colony.
Before concluding our brief account of these interesting marine
protozoons, it may be well to point out that, although the foraminifera
belong to the lowest class of the lowest sub-kingdom of animals, yet
there are some rhizopods--the _Monera_, which are even simpler in
structure. These are mere specks of undifferentiated protoplasm, not
protected by any shell, and not even possessing a nucleus, and are the
simplest of all animal beings.
The second division of the Protozoa--the class _Protoplasta_--has
already received a small share of attention, inasmuch as the amœba,
which was briefly described as a type of the whole sub-kingdom, belongs
to it.
The study of the amœba is usually pursued by means of specimens
obtained from fresh-water pools, and reference has been made to it in a
former work dealing particularly with the life of ponds and streams; but
it should be observed that the amœba inhabits salt water also, and
will be frequently met with by those who search for the microscopic life
of the sea, especially when the water examined has been taken from those
sheltered nooks of a rocky coast that are protected from the direct
action of the waves, or from the little pools that are so far from the
reach of the tides as to be only occasionally disturbed. Here the
amœba may be seen creeping slowly over the slender green threads of
the confervæ that surround the margin of the pool.
The third class--_Radiolaria_--is of great interest to the student of
marine life, on account of the great beauty of the shells; but, as with
the other members of this sub-kingdom, a compound microscope is
necessary for the study of them.
The animals of this group resemble the foraminifers in that they throw
out fine thread-like pseudopods, but they are distinguished from them by
the possession of a membranous capsule in the centre of the body,
surrounding the nucleus, and perforated in order to preserve the
continuity of the deeper with the surrounding protoplasm. They have
often a central contractile cavity, and further show their claim to a
higher position in the animal scale than the preceding classes by the
possession of little masses of cells and a certain amount of fatty and
colouring matter.
[Illustration: FIG. 62.--A GROUP OF RADIOLARIAN SHELLS, MAGNIFIED]
Some of the radiolarians live at or near the surface of the ocean, while
others thrive only at the bottom. The former, in some cases, appear to
avoid the light, rising to the surface after sunset; and it is supposed
that the phosphorescence of the sea is due in part to the presence of
these animals. The latter may be obtained from all depths, down to
several thousand fathoms.
The beauty of the radiolarians as a class lies in the wonderful shells
that protect the great majority of them. These shells are composed not
of carbonate of lime, as is the case with foraminifers, but of silex or
silica, a substance that is not acted on by the strongest mineral acids.
They are of the most exquisite shapes, and exhibit a great variety of
forms. Some resemble beautifully sculptured spheres, boxes, bells, cups,
&c.; while others may be likened to baskets of various ornamental
design. In every case the siliceous framework consists of a number of
clusters of radiating rods, all united by slender intertwining threads.
It is not all the radiolarians, however, that produce these beautiful
siliceous shells. A few have no skeleton of any kind, while others are
supported by a framework composed of a horny material, but yet
transparent and glassy in appearance.
The sizes of the shells vary from about one five-hundredth to one half
of an inch; but, of course, the larger shells are those of colonies of
radiolarians, and not of single individuals, just as we observed was the
case with the foraminifers.
Those in search of radiolaria for examination and study should, whenever
possible, obtain small quantities of the dredgings from deep water.
Material brought up by the trawl will often afford specimens; but,
failing these sources of supply, the muddy deposit from deep niches
between the rocks at low-water mark will often provide a very
interesting variety.
Place the mud in a glass vessel, and pour on it some nitric acid
(aqua-fortis). This will soon dissolve all calcareous matter present,
and also destroy any organic material. A process of very careful washing
is now necessary. Fill up the vessel with water, and allow some time for
sedimentary matter to settle. Now decant off the greater part of the
water, and repeat the process several times. By this means we get rid of
the greater part of the organic material, as well as of the mineral
matter that has been attacked by the acid; and if we examine the final
sediment under the microscope, preferably in a drop of water, and
covered with a cover-glass, any radiolarians present will soon reveal
themselves.
It is often possible to obtain radiolarian shells, as well as other
siliceous skeletons, through the agency of certain marine animals. The
bivalve molluscs, for example, feed almost entirely on microscopic
organisms; and, by removing such animals from their shells, and then
destroying their bodies with aqua-fortis, we may frequently obtain a
sediment composed partly of the skeletons referred to.
There remains one other class of protozoons to be considered, viz. the
_Infusorians_--the highest class of the sub-kingdom. In this group we
observe a distinct advance in organisation; for, in the first place, the
infusorians are enclosed in a firm cuticle or skin, which forms an
almost complete protective layer. Within this is a layer of moderately
firm protoplasm, containing one or more cavities that contract at
intervals like a heart. Then, in the interior, there is a mass of softer
material with cavities filled with fluid, two solid bodies, and numerous
granules.
[Illustration: FIG. 63.--THREE INFUSORIANS MAGNIFIED]
In these creatures we find, too, a distinct and permanent mouth, usually
funnel-shaped, leading to the soft, interior substance, in which the
food material becomes embedded while the process of digestion proceeds.
Here, then, for the first time, we meet with a special portion of the
body set apart for the performance of the work of a stomach; and,
further, the process of digestion being over, the indigestible matter is
ejected through a second permanent opening in the exterior cuticle.
Again, the infusorian does not move by means of temporary pseudopods, as
is the case with the lower protozoons, but by means of minute hair-like
processes which permanently cover either the whole of the body, or are
restricted to certain portions only. These little processes, which are
called _cilia_, move to and fro with such rapidity that they are hardly
visible; and, by means of them the little infusorian is enabled to move
about in its watery home with considerable speed.
In some species a few of the cilia are much larger than the others, and
formed of a firmer material. These often serve the purpose of feet, and
are also used as a means by which the little animal can anchor itself to
solid substances.
As with the lower protozoons, the infusoria multiply by division; but,
in addition to this, the nucleus may sometimes be seen to divide up into
a number of minute egg-like bodies, each of which, when set free, is
capable of developing into a new animal. Should the water in which
infusorians have been living evaporate to dryness, the little bodies
just mentioned become so many dust particles that may be carried away by
air currents; but, although dry, they retain their vitality, and develop
almost immediately on being carried into a suitable environment.
Infusorians are so called because they develop rapidly in infusions of
various vegetable substances; and those who desire to study their
structure and movements with the aid of a microscope cannot do much
better than make an infusion by pouring boiling water on fragments of
dried grass, and leaving it exposed for a few days to the warm summer
atmosphere. The numerous germs floating in the air will soon give rise
to abundance of life, including several different species of infusoria,
varying from 1/30 to 1/2000 of an inch in length.
Fresh-water pools and marshes provide such an abundance of infusoria
that the animals are generally obtained for study from these sources,
and a few of the common and most interesting species inhabiting fresh
water have already been described in a former work. Nevertheless, the
sea is abundantly supplied with representatives of the class, and it is
certain that the beautiful phosphorescence sometimes observed in the sea
at night is in part due to the presence of luminous infusoria, some of
which appear to have an aversion to sunlight, retiring to a depth during
the day, but rising to the surface again after sunset.
[Illustration: FIG. 64.--A PHOSPHORESCENT MARINE INFUSORIAN
(_Noctiluca_), MAGNIFIED]