The Sub-kingdom Protozoa.
The consideration of the whole special group of organisms forming the
subject matter of this chapter, under the heading of Protozoa, were
formerly included among Infusoria, which also embraced every kind
of microscopical aquatic body, whether belonging to the vegetable
or animal series. A more critical survey of the organisation and
affinities of Infusoria and the members which constituted the group
led to a re-arrangement, which has been very generally accepted as
forming a sub-kingdom, Protozoa. This may be defined as embracing
all those forms of life, referable to the lowest grade of the animal
kingdom, whose members for the most part are represented by organisms
possessing a single cell or aggregation of cells (and also included
under the general term of unicellular organisms) the whole of which are
engaged in feeding, moving, respiring, and reproducing by segmentation
or fission much in the same way as that of the unicellular plants
described in a previous chapter. Following out this sub-division of the
entire series of Protozoa, the several groups range themselves into
four readily distinguishable sections. In the first, the most lowly
organised and most abundant have no oral orifice in the literal meaning
of the word, food being intercepted at any point of the surface of the
body. This most simple elementary type of structure of the Protozoa is
represented in the Amœba and Actinophrys, the various representatives
of the Foraminifera, and certain Flagellata, as Spumella and
Anthrophysa. Next in the ascending scale is a group of Protozoa, in
which, though differentiation has not proceeded so far as to arrive
at the constitution of a distinct oral aperture, the inception of
food substance is limited to a discoidal area occupying the anterior
extremity of the body and is associated with the special food-arresting
apparatus. To this section of the Protozoa are relegated the minuter
flagellate, “collar-bearing” animals, and also the entire group of
sponges or Porifera.
[Illustration: GREGARINIDA, POLYCYSTINA, FORAMINIFERA, ROTIFERA, ETC.
Tuffen West, del. Edmund Evans.
PLATE III.]
In the third section the highest degree of organisation is arrived
at. Here is represented a single, simple, often highly-differentiated
oral aperture or true mouth. Associated with this section are found
the majority of those organisms that collectively constitute the class
Infusoria in the proper acceptation of the term, and it embraces the
majority of the Ciliata, the Cilio-flagellata, as Euglena, Chilomonas,
&c., in which the presence of a distinct and circumscribed oral
aperture is clearly seen. With the fourth and remaining section
of Protozoa, the oral or inceptive apparatus exhibits a highly
characteristic structural modification. This is not restricted to
a definite area, nor is it associated with the entire surface of
the body, but it consists of a number of flexible, retractile,
tentacle-like organs radiating from diverse and definite regions of
the periphery, each of which subserves as a tubular sucking-mouth, or
for the purpose of grasping food. These may be literally described as
many-mouthed, and have been appropriately designated Polystomata. The
true zoological position of the Spongida or Porifera is not finally
settled, the members of this important section having been formerly
regarded as a subordinate group of the Rhizopoda or an independent
class of the Protozoa; consequently a tendency has been shown to
assign to them a position more nearly approximating to that of the
Cœlenterata, or zoophytes and corals, or place them among the more
highly organised tissue-constructed animals, the Metazoa, these being
characterised by groups of cells set apart to perform certain functions
for the whole animal. A division of labour is seen to be marked in
these lower animals as the organism becomes more specialised, and the
number of functions a cell performs becomes more and more limited as
the body becomes more complex.
It has been found convenient to adopt the following definition of the
Infusoria as one more generally acceptable. The Protozoa in their adult
condition are furnished with prehensile or locomotive organs, that take
the form of cilia, flagella, or of adhesive or suctorial tentacula, but
not of simple pseudopodia; their zooids are essentially unicellular,
free swimming or sedentary; they are either naked, loricate, or inhabit
a simple, mucilaginous matrix; single or united in aggregations, in
which the individual units are distinctly recognisable; not united and
forming a single gelatinous plasmodium, as in Mycetozoa, nor immersed
within and lining the interior cavities of a complex protoplasmic
and mostly spiculiferous skeleton, as in the Spongida, their food
substances being intercepted by a single distinct oral aperture, or
by several apertures through a limited terminal region or through the
entire area of the general surface of the body. They increase by simple
longitudinal or transverse fission, by external or internal gemmation
or division, preceded mostly by a quiescent or encysted state, into
a greater or less number of sporular bodies. Sexual elements, as
represented by true ova or spermatozoa, are entirely absent, but two
or more zooids frequently coalesce as an antecedent process to the
phenomena of open formation.[63]
The infusorial body in its simplest type of development, as in
Amœba, exhibits a structural composition substantially corresponding
with that of the lowest organised tissue cell. There is no distinct
bounding membrane, or cell-wall, and it is throughout, and apart from
the nucleus or endopart, one continuous mass of granular matter, but
otherwise homogeneous and undifferentiated protoplasm. Professor
Greef, who has made a study of the Amœba, describes motor fibrils in
the exoplasm which are active and large in _A. terricola_. These are
readily seen by staining with osmic acid, and, after washing this
out with water, immersing in a weak alcoholic solution. In Amœba so
prepared and examined with a high power, the whole body will be seen
to be surrounded by a distinct double integumentary layer. Highly
refractive bodies may also be seen in the interior, connected together
by extremely fine filaments. Professor Greef concludes that here we
have to do with muscular fibrillæ, which traverse the contractile outer
zone in a radial direction and there terminate for the time being. By a
similar method, axial filaments can be demonstrated in Heliozoa; these,
it is believed, are the true motors of their pseudopodia, and also the
axial structures of the Acineta, a marine animal related to ciliate
infusoria.
In the Amœba, at one time well known as the _Proteus animalcule_,
Fig. 325, the marvellous body creeps onward in a flowing manner,
occasionally and languidly emitting a single pseudopod first on one
side, then on the other. More commonly it puts on a dendroid or palmate
form; then again it assumes more or less grotesque shapes in which
almost any conceivable image may be imagined. The body, as will be
seen in this highly-magnified figure, is full of granules (with the
exception of a thin clear outer hyaline zone), and near the centre is
a globular or discoid body known as the nucleus, composed of slightly
denser material than that which surrounds it. The division of the body
into two is preceded by a division of this nucleus. Near the latter
is a clear spherical space--the contractile vacuole--which gradually
expands, and then rather suddenly collapses and reappears at the
same spot, the systole and diastole being slow and continuous. The
contractile vacuole contains a clear liquid which is expelled on the
collapse of the vacuole. This organ probably serves the double function
of respiration and excretion. The Amœba is omnivorous, chiefly a
vegetarian, and, therefore, found on the ooze of ponds or on the under
surface of the leaves of aquatic plants, especially among Confervæ. It
can be readily produced by placing a few fibres of fresh meat in an
infusion of hay.
[Illustration: Fig. 325.--Amœba, _Proteus animalcule_; magnified 600
diameters.--(Warne).]
The Gregarinæ consist of a remarkable group of organisms, but these,
although unicellular, are, for the most part, confined to the
intestinal tract of worms and of the higher animals, and will therefore
be described among internal parasites.
Tho fungus-animals, Mycetozoa, have already been referred to in a
previous chapter. The best known species, however, is found in tan
yards in the form of creeping masses of naked protoplasm, termed
Plasmodia. Cakes of protoplasm become segregated from the main mass,
and break up into Amœba-like spores, which unite again to form
Plasmodia.
[Illustration: Fig. 326.--Rhizopoda lobosa.
A. _Difflugia proteiformis_; B. _Difflugia oblonga_; C, D. _Arcella
acuminata_ and _dentata_]
The Rhizopoda, or root-footed class of animals, are among the most
interesting simple organisms with which the microscope has made us
acquainted. In the living state they have the power of protruding
pseudopodia from the body, by which they creep about, or cling to
plants when in search of food. This group, in fact, includes Amœba,
Foraminifera, Sun-animalcules, and Radiolarians. In the first the
pseudopodia are simple and lobose; in the second they are slender,
confluent and reticulate; while in the two last they are simple,
radiating and somewhat stiff, and partake of a calcareous formation.
Of the Lobosa, we may take a well-known representative of the group,
the Protomyxa, found at the bottom of fresh-water pools, especially
those near bog-moss, where its minute orange-coloured particles of
jelly-like substance are seen creeping over stones or shells. If
quietly watched the pseudopodia, some of which are broad and others
slender, become quiescent spheres, which break up into numerous
portions, each of which becomes a new animal.
This group is divided into the shell-less (Nuda) and shell-formed
(Testacea). The brown, horny covering is often finely faceted, and is
either shaped like a dome, semi-circular, or flat as a box, through
which they protrude their few or many pseudopodia (seen in Fig. 326).
[Illustration: PLATE XV.
GROMIA.]
In the Difflugia the lorica or shell is strengthened by the addition of
silicious particles; in Euglypta it is sac-shaped, with a jagged free
margin, the surface being covered by overlapping scales; while Arcella
are capable of secreting vesicles of air in their interior, whereby
they are enabled to rise to the surface. On some parts of our coast, if
the sea sand be carefully looked over with a pocket lens, there will
often be found minute grains of a porcelain oval kind, belonging to the
Miliolina, segmented or strung together not quite in the same plane.
[Illustration: Fig. 327.--Section of Rotalia.
_a,a_, Radiating interceptal canals; _b_, Internal bifurcations; _c_,
Transverse branch; _d_, Tubular wall of chambers.]
[Illustration:
Fig. 328.--_Rosalina varians_ or _Discorbina globularia_, with
pseudopodia protruding.]
The Foraminifera are rhizopods, whose simple protoplasmic bodies
send forth, through perforations in the membrane or outer covering
of calcium carbonate and silica, branching rays of pseudopodia. The
order is divided into two groups, the Imperforata and the Perforata;
in the former the shell or harder structure possesses only one or more
apertures, whereas in the latter, in addition to the main opening,
the shell has its walls perforated throughout, which admits of minute
pseudopodia or fine threads being protruded (Fig. 328). (See also Plate
III., Nos. 75-85.) The vast majority of Perforata form their shells, or
rather skeletons, of calcium carbonate and silica, which renders them
almost indestructible. Consequently the form is preserved through ages,
and they present objects of the greatest interest to the microscopist.
A curious and interesting feature of the Foraminifera--often an element
of difficulty to the student--is the tendency of modifications of types
comprising the larger groups to run into parallel isomorphous series.
Thus, if the entire class be roughly divided, as it sometimes has been,
into three orders, comprising respectively the forms characterised
by porcellaneous, arenaceous, and hyaline “tests,” the same general
conformation and arrangement of chambers will be found in each of the
three series. The most remarkable example, even among the smaller
groups, is the Rotaliidæ, of which three or four genera may be arranged
in parallel lines, and in more or less closely isomorphous series.
In the report appended to the “Challenger” scheme of classification
many examples are enumerated. In Arenacea we have a small family of
Foraminifera, the external surfaces of which present a ridge and furrow
arrangement, and the incrustations are entirely of a sandy nature
held together by a cement secreted by the animal. (Plate XV., No. 1,
_Astrorhiza limicola_.)
_Gromia._--Among the more remarkable of the Perforata group the Gromia
have a foremost place. They are very minute globular or oval-shaped
bodies, about one-twenty-fourth of an inch in length, found in fresh,
brackish, and salt water. The forms brought up in Dr. Wallich’s deep
sea soundings of 1860 were taken attached to pieces of corallines,
or found loose among Globigerina ooze. At first there appears to be
nothing peculiar about these tiny specks of matter resembling the ova
of a zoophyte, but presently, at the smaller end, a very fine thread
is protruded, and then another, dividing into finer branches, and,
ultimately, a complete network of filaments extends on all sides, and
become attached to the side of the glass jar that contains them. Now,
on employing magnifying power, every thread exhibits a circulatory
motion, an up and down stream or cyclosis of granules suspended in a
fluid mass. It is by means of these pseudopodia, as the threads are
termed, that the Gromia moves its body along and clings to the glass.
We may surmise, then, that these pseudopodia are either gelatinous,
glutinous, or terminate in sucker-like processes. Increase in the
“test,” integument, is brought about, as in Difflugia, by the secretion
of calcareous matter or by cementing fine silicious particles to the
outer wall, as the protoplasm is seen to flow over the test, so that
when it comes in contact with a diatom it is thereby drawn towards the
oral opening and slowly digested.
Some considerable time elapsed between the discovery of Gromia by Mr.
W. Archer, F.R.S., and the demonstration of a nucleus and contractile
vesicle by Dr. Wallich. It was thought that in the whole of the Monozoa
the nucleus was absent, but it is now known that this important body is
embedded in the protoplasmic substance, and the reproduction of these
curious animals is thereby secured. Among the better known species of
Gromia is _G. Dujardinii_, chiefly distinguishable by the darker colour
of the “test,” by the greater quantity of silica that enters into
the formation of its pseudopodia, and by the formation of isogamous
zoospores, two of which are seen in conjugation in Plate XV., No. 2.
An excess of protoplasm must also be secreted to admit of so large a
protrusion outside the testa.
_G. Lieberkühnia_ (of Claparède and Lachman), No. 5, differs in
formation. Its shape is pyriform, and the opening whence the
pseudopodia streams out is situated in a lateral depression about
midway in the testa, _c, o_. Hence a trunk branch is seen to issue
forth, and from this a ramification of threads, _psdp_, extends to a
considerable distance in all directions.
The Micro-gromia of Hertwig, No. 4, is the minutest form of the genus
yet discovered, and differs from those already described in the mode
of reproduction. The individual takes the shape of a water bottle with
a short neck, whence issue forth a limited number of very slender
threads. The test is quite transparent, and it was in this species that
the nucleus and contractile vesicle, which lie embedded near the mouth,
were first clearly made out.
The zoospores of Micro-gromia have a curious habit of uniting with
their neighbours to form a colony, No. 4. Their colonisation is
apparently intended to facilitate multiplication. Reproduction is
carried on somewhat after the manner of Volvox. The globular bodies
formed sink to the bottom of the glass vessel, and there remain for
a time in a quiescent state. In the course of a day or two the mass
assumes a motive appearance, increases in bulk, becomes more ovoid in
shape, and ultimately the nucleus shows the first sign of division.
Vertical segmentation takes place, as at A, into two equal parts; each
half is seen to possess its fair share of the nucleus and contractile
vesicle. It then turns in the horizontal direction, and now there
appears to be an upper and a lower division, the uppermost having a
neck-like attachment, and this is making its way to the narrow oral
opening in the parent testa, as at B. Here it is seen pressing forward,
and at C the neck is protruding some distance, and the second half
assumes a bottle shape; at D the greater part of the animal is nearly
set free, and after a short rest it fully launches forth. It finally
pulls itself together, as at E, and either develops a pair of flagella
and swims off, or assumes the form of an Actinophrys. In either case,
and in a very short space of time, the separated young animal is quite
ready to re-unite, as at F, and assist in forming a new colony of the
species.
The Polymorphina belong to a low genus of the Foraminifera. They
consist of a number of forms and exhibit a rather extensive series of
variations, although consisting of a few simple types, and showing
transitions between forms which at first seem to be distinct. The
majority of species keep to the sea bottom; some few are pelagic,
and occur in abundance on the surface of the ocean. Among the latter
are the Globigerina: its shell is about one-fortieth of an inch in
diameter, and usually composed of seven globular chambers arranged
spirally in such a manner that all are visible from above, each chamber
opening by a crescentic-shaped orifice into a depression in the middle
of the next. Perfect specimens bristle with long slender spines, the
pores affording passage to pseudopodia, which stream out along the
spines. The more carefully-conducted deep-sea investigations have
brought to light the fact that the floor of the ocean, at great depths,
and over a vast area, is formed of these white or pinkish coloured
bodies, all containing on an average about 60 per cent. of calcium
carbonate. It is a question whether the Globigerinidæ which make up the
bulk of the ooze actually live at the bottom as well as the surface of
the sea. This question has given rise to much discussion. Dr. Murray
came to the conclusion that pelagic species do not live near the ocean
floor. This opinion is partly based on the fact that the area of the
Globigerina ooze coincides with the area of surface of temperature at
which these bodies are found to exist. When the surface water is too
cold for them, they are not to be found, neither are they found below.
Major S. R. J. Owen, while dredging the surface of mid-ocean--the
Indian, and the warmer portion of the Atlantic--found attached to his
nets a number of these interesting bodies, and which always made their
appearance just about sunset. In Plate III., Nos. 43-52, a number of
these interesting and variously-formed bodies are given, and an attempt
is also made to show the richly-tinted colour appearances presented by
the sarcode or protoplasm of the Globigerina.
[Illustration: Fig. 329.--Globigerina and other bodies taken in deep
sea soundings (Atlantic).]
“Many of the forms,” writes Major Owen,[64] “have hitherto been claimed
by the geologist, but I have found them enjoying life in this their
true home, the silicious shells filled with coloured sarcode, and
sometimes this sarcode in a state of distension somewhat similar to
that found projecting from the Foraminifera, but not in such slender
threads. There are no objects in nature more brilliant in their
colouring or more exquisitely delicate in their forms and structure.
Some are of but one colour, crimson, yellow, or blue; sometimes two
colours are found on the same individual, but always separate, and
rarely if ever mixed to form green or purple. In a globular species,
whose shell is made up of the most delicate fretwork, the brilliant
colours of the sarcode shine through the little perforations very
prettily. In specimens of the triangular and square forms (Plate III.,
Nos. 43, 44, 45 and 46), the respective tints of yellow and crimson
are vivid and delicately shaded; in one the pink lines are concentric;
while another is of a stellate form, the points and uncoloured parts
being bright clear crystal, while a beautiful crimson ring surrounds
the central portion. A globular form resembles a specimen of the
Chinese ball-cutting--one sphere within another; this, however, appears
to belong to a distinct species.
[Illustration: Fig. 330.--Globigerina and other bodies taken in deep
sea soundings, 1856 (Atlantic).]
“The shells of some of the globular forms of these Polycystina,
whose conjugation I believe I have witnessed, are composed of a fine
fretwork, with one or more large circular holes; and I suspect the
junction to take place by the union of two such apertures. That the
figures of these shells become elongated, lose their globular form
after death, and present a disturbed surface is seen in some of the
figures represented in Plate III., Nos. 82-85.” Those without internal
chambers have been described as _Orbulina universa_, Plate III., Fig.
78, while Nos. 75 and 76, although members of the same family, have
been separated, but all should certainly be united under Globigerina.
“The minute silicious shells of Polycystina present wonderful beauty
and variety of form; all are more or less perforated, and often
prolonged into spines or other projections, through which the sarcode
body extends itself into pseudopodial prolongations resembling those
of Actinophrys. When seen disporting themselves in all their living
splendour, their brilliancy of colouring renders them objects of
unusual attraction. It will appear that they wish to avoid the light,
as they are rarely found on the surface of the sea in the daytime;
it is after sunset and during the twilight that they make their
appearance.”
Many forms of Globigerina and Foraminifera are represented in Figs.
329 and 330. These varied and beautiful forms were dredged up with
soundings made in 1856 for the purpose of ascertaining the depth of the
Atlantic, prior to the laying down of the electric telegraph wire from
England to America, and taken at a depth of 2,070 fathoms.
_Heliozoa._--_Actinophrys-Sol_, “sun-animalcules,” belong to this
group; most of them inhabit fresh water (Plate III., No. 66). The
chief characteristic, and the one to which they owe their name, is
the possession of long, slender, somewhat stiff pseudopodia; these
radiate from all parts of the body. The living animal usually contains
green-coloured particles within a minute translucent spherical globule
of about 1/250th of an inch in diameter. It is, therefore, variously
designated the green sun-animalcule, Acanthocystis, or Actinophrys-Sol.
It is commonly found amongst the weeds in clear pools of water, where
desmids abound. The pseudopodia appear to be stiff; they are, however,
quite flexible, and the body contains more than one clear vesicle with
a nucleus; reproduction is secured by the simple division commencing
in the nucleus. The little animal can move over a hard surface by
the alternate relaxation and stiffening of its pseudopodia; when one
of these touches a small organism, it is believed to paralyse it,
then envelop, and deliberately digest it. In another species, the
lattice-animalcule (Cathrulina), the pseudopodia or silicious threads
are arranged tangentially. It grows on a long flexible stalk, attached
to an aquatic plant, the total length of which is about 1/200th an
inch. The globular body is perforated in all directions, through which
the fine stiff pseudopodia are thrust out; it is often known to form
colonies.
In this order may well be placed the Radiolaria; they are, however,
usually separated. But Radiolarians, whether seen alive or in their
skeleton form, are surpassingly beautiful. By the favour of Messrs.
Warne, I am enabled to append a frontispiece plate to this volume taken
from their “Royal Natural History.” These bodies are all marine, and
live in zones of several thousand fathoms, and like their congeners,
the Globigerina, they avoid a strong light, and only appear after
sunset. Their bodies are supposed to emit a phosphorescent glow, but
more is known of their silicious skeletons than of their living forms;
yet it is not this feature that separates them from other orders of
rhizopods, but the possession of a membranous central capsule enclosing
the nucleus. The body substance outside this capsule is highly
vacuolated in some species, especially in surface forms. A few are
without a skeleton, and these consist of oval masses of protoplasm,
with slender pseudopodia. In a few species the skeleton is formed of
a glassy horny substance, termed acanthin, arranged in the form of
radiating spines.
Radiolarians secrete a silicious skeleton, which assumes a variety of
forms, as trellis-work, boxes joined by radiating spines, helmets,
baskets, bee-hives, discs, rings, and numerous other forms. Haeckel
has described upwards of four thousand species, and possibly as many
more could be added to this number. Radiolaria are divided into two
groups. In the one there is either no skeleton or one of silex; in the
other the skeleton is formed of radiating spines of a horny nature.
These are again subdivided according to the characters of the central
capsule. In those forms with a silicious skeleton the geometrical
pattern conforms more or less to the shape of the central capsule,
being either spherical or conical. The central capsule is regarded as
being homologous with the calcareous shell of Globigerina. Reproduction
takes place by simple division into two, or by the body breaking up
into spores, each provided with a flagellum, or two spores may fuse
together, and the result will be an adult Radiolarian. Certain yellow
corpuscles present in the outer part of their body-surface change into
unicellular parasitic algals; these can be separated and cultivated
independently of their host. The Radiolarians live floating at all
depths from 1,000 to 2,500 fathoms, and are distributed over areas in
the central Pacific and the south-eastern part of the Indian Ocean,
the ooze forming the ocean bed being made up of their skeletons to
an extent of 80 per cent. of the deposit; hence it has become known
as Radiolarian ooze. The chalky-looking Barbadoes earth, a Tertiary
formation, is composed almost entirely of their skeletons. Somewhat
similar deposits exist in the Nicobar Islands, in Greece, and in Sicily.
It will have been noticed that by far the greater number of
Foraminifera are of marine origin, and these occur in such widespread
profusion that the finest calcareous particles which constitute the
seashore in some places consist almost wholly of their microscopic
remains. At former periods of the earth’s history they appear to have
existed even in greater profusion than at the present time. This is
evidenced by their remains forming the principal constituent of our
largest geological formations.
Moreover, during the Canadian Geological Survey large masses of what
appeared to be a fossil organism were discovered in rocks situated
near the base of the Laurentian series of North America. Sir William
Dawson, of Montreal, referred these remains to an animal of the
foraminiferal type; and specimens were sent by Sir W. Logan to the late
Dr. Carpenter, requesting him to subject them to a careful examination.
As far back as 1858 Sir W. Logan had suspected the existence of organic
remains in specimens from the Grand Calumet limestone, on the Ottawa
River, but a casual examination of the specimens was insufficient to
determine the point. Similar forms being seen by Sir W. Logan in blocks
from the Grenville bed of the Laurentian limestone were in their turn
tried, and ultimately revealed their true structure to Sir William
Dawson and Dr. Sterry Hunt, who named the structure _Eozoon Canadense_.
The masses of which these fossils consist are composed of layers of
serpentine alternating with calc spar. It was found by these observers
that the calcareous layers represented the original shell, and the
silicious layers the softer parts of the once living Foraminifera.
The results were arrived at through comparison of the appearance
presented by the Eozoon with the microscopic structure which Dr.
Carpenter had previously shown to characterise certain members of the
Foraminifera. The Eozoon not only exceeded other known Foraminifera
in size to an extent that might have easily led observers astray,
but, from its apparently very irregular mode of growth and general
external form, no help was derived in its identification, and it was
only by microscopical examination of its minute structure that its true
character was ascertained. Dr. Carpenter wrote:--“The minute structure
of Eozoon may be determined by the microscopic examination either of
thin transparent sections, or of portions which have been subjected to
the action of dilute acids, so as to remove the calcareous portion,
leaving only the internal casts, or models, in silex, of the chambers
and other cavities originally occupied by the substance of one animal.”
Subsequently he found portions of minute structure so perfect that he
was able to say that “delicate pseudopodial threads were originally
put forth through openings in the shell wall of less than 1/10000th of
an inch in diameter” (Plate III., Nos. 64, 65). In a paper read at the
meeting of the Geological Society he stated that he had since detected
Eozoon in a specimen of ophicalcite from Bohemia, in a specimen of
gneiss from near Moldau, and in specimens of serpentine limestone sent
to Sir C. Lyell by Dr. Gümbel, of Bavaria. These also were found to be
parts of the great formation of the “fundamental” gneiss, considered
by Sir Roderick Murchison as the equivalent of the Laurentian rocks of
Canada.[65]
If the remains of Foraminifera be dissolved in dilute hydrochloric
acid, an organic basis is left, after the removal of the calcareous
matter, accurately retaining the form of the shell with all its
openings and pores. The earthy constituent is mainly calcium carbonate;
but there is also a small amount of phosphate of lime in the shells of
many of them.
[Illustration: Fig. 331.
1. Separated prisms from outer layer of Pinna shell; 2. Skeletons of
Foraminifera from limestone; 3. Recent shell of _Polystomella crispa_;
examined under dark-ground illumination.]
Infusoria.
We are now brought face to face with animals which possess considerable
variation of structure, _Infusorial animalcules_, as they are termed.
It was Ehrenberg who attributed to them a highly complex organisation,
but later observations negatived these views and showed them to
be animals formed of one or more cells, or colonies of so-called
individuals. It is true that this cell or united protoplasm may show
a wonderful amount of differentiation, what with its nucleus and
vacuole, mouth and gullet, its variously-arranged cilia or flagella,
its contractile fibres, its separation into an outer denser and a more
fluid inner protoplasm, and its horny cup and stalks.
In these few lines we have a condensed summary of the special
qualities of minute forms of life that afford much interesting work for
the microscope.
[Illustration: Fig. 332.--Acineta, magnified 600 diameters (_Warne_).]
Among those widespread, and in some respects heterogeneous, forms
of life associated under the comprehensive title of Infusoria, we
encounter types that not only differ very widely from one another, but
which occupy a different rank or position, so to speak, with regard
to the relation they bear to each other, and also to the outlying
representatives of the series--differences that permeate throughout
the ranks of this extensive group. Furthermore, a considerable number
of Infusorial animalcules foreshadow or typify, in a corresponding
degree, the separate or associated cell elements out of which higher
tissue structures--metazoic organisms--are built up. We may take
the well-known example _Euglena viridis_ (Plate III., No. 67), or
Paramecium (No. 74), and their allies; these would appear to be the
prototypes of Turbellaria. Another more lowly organised group of
the Ciliata exhibits a distinct and highly-interesting affinity to
the Opalinidæ. There are many other species (Acineta, Plate III.,
No. 68, for instance), which at first sight would seem to stand by
themselves and present no marked agreement with any metazoic type.
Indeed, the function of these and other polypites consists simply in
seizing food and conveying it through perforations at the extremity of
each separate tentaculum to its interior. In Acineta certain of the
tentacles only are suctorial, and these, being the inner ones, fulfil
the ingestive function, while the peripheral series are prehensile.
This stalked club-shaped body (Fig. 332), which fixes itself to
seaweeds or Bryozoa, is seen to have a nucleus, and also clear vesicles
in the body-substance; its embryos are ciliated. It is an object of
considerable interest even among curious marine animalcules; one or two
species inhabit fresh water. The spiral-mouthed Spirostomum are among
the largest of the class, and in sunlight are visible to the naked eye
as slender golden threads of about 1/10th of an inch in length. The
mouth slit, extending half the length of the body, is bordered on one
side by cilia. The body is cylindrical and the surface covered with
rows of cilia. Its multiplication takes place by transverse fission
through the middle.
_Flagellate Infusoria._--The characteristic of this group, as its
name implies, is the possession of one or more flagella or whip-like
appendages, at the base of which is an opening in the denser surface
layer of protoplasm, and in the interior a nucleus and one or more
contractile vacuoles, and not infrequently a brilliant red spot of
pigment known to microscopists as the eye-spot. The Monads, which
constitute the simplest members of the group, are commonly found in
fresh-water pools and vegetable infusions. The typical form consists
simply of a spherical or oval cell provided with a flagellum.
The Volvox was formerly placed in this group, but as it contains
chlorophyll it is properly claimed by the botanist. The collared
group possesses cup-like collars, and these frequently secrete horny
receptacles or cups, and form elegant tree-like colonies.
The mail-coated group are of very varied form, the body being often
prolonged into spiny processes. They have two long flagella which
fit into grooves purposely provided. But the most interesting and
remarkable are the phosphorescent animalcules (Noctiluca), whose
beautiful bluish-green luminosity on the surface of the sea has
attracted attention from very early periods. It was, however, not
until the first half of the present century that the luminosity was
discovered to be due to the presence of multitudes of these minute
jelly-like spheres.
[Illustration: Fig. 333.--_Noctiluca miliaris_; magnified 150
diameters.]
[Illustration: Fig. 334.--Pyrocystis; magnified 150 diameters.]
The body of the Noctiluca (Fig. 333) is a nearly globular-shaped cyst,
enclosed in a tough membranous wall, from a grooved opening in which
a striated muscular flagellum or proboscis is projected forth, and it
is by means of this the animal swims away even in rough seas. A fine
whip-like flagellum is also located in the same groove. At the apex
of the funnel there is a mass of protoplasm which extends itself as a
widely-meshed, highly-vacuolated network to the inner wall of the cyst,
whence it is believed the phosphorescent light emanates. It multiplies
by self-division, first becoming encysted after withdrawing its
flagellum, and then breaking up into numerous ciliated helmet-shaped
swarm spores. Frequently two organisms fuse into one and then divide
into spores.
Noctiluca mainly confines itself to the shallower seas, but there are
related forms met with in the warmer open seas; these belong to the
genus Pyrocystis (Fig. 334). In one variety the body is perfectly
spherical and without the big flagellum or proboscis. Professor
Butschli, however, regards this species as an encysted or resting phase
of the commoner and better-known form.
The late Mr. Philip Gosse, F.R.S., was the first microscopist to
describe the Noctiluca. After careful observation, he wrote in his
“Naturalist’s Rambles” as follows:--“I had an opportunity of becoming
acquainted with the minute animals to which a great portion of the
luminousness of the sea is attributed. One of my large glass vases of
sea-water I had observed to become suddenly at night, when tapped with
the finger, studded with minute but brilliant sparks at various points
on the surface of the water. I set the jar in the window, and was not
long in discovering, without the aid of a lens, a goodly number of
the tiny jelly-like globules of _Noctiluca miliaris_ swimming about
in various directions. They swam with an even gliding motion, much
resembling that of the _Volvox globator_ of our fresh-water pools.
They congregated in little groups, and a shake of the vessel sent
them darting down from the surface. It was not easy to keep them in
view when seen, owing rather to their extreme delicacy and colourless
transparency than to their minuteness. They were, in fact, distinctly
appreciable by the naked eye, measuring from 1/50th to 1/30th of an
inch in diameter.”
Among the numerous fresh-water members of the flagellate infusoria,
there is one which especially calls for notice, Codosiga, discovered
by the late Professor H. J. Clark. This minute body bears a delicate
funnel-shaped protoplasmic expansion or collar, common to the several
members of this organic series. The flagellum is placed at the base
of the oral opening, and within the circumscribed area of the collar,
which is of such extreme tenuity that its true form and nature can
only be determined by a very careful adjustment of the achromatic
condenser and accessory apparatus employed, together with a wide-angled
objective. It is seen to greater advantage by supplying the animal
with very fine particles of colouring matter. In this way it is found
that the infundibuliform cup consists of protoplasm, through which
the flagellum is protruded and withdrawn into the general substance
of the Monad’s body (Fig. 335). As many as twenty or more zooids are
attached to the extremity of a slender footstalk. The length of the
body, exclusive of the collar, is 1/2500th to the 1/1200th of an inch.
The habitat of these bodies is fresh water. Mr. Saville Kent in 1869
discovered some of these interesting infusoria in the London Docks.
“The more exact significance of the special organ, the collar, is
manifest by the circulatory currents or cyclosis induced, and there can
be no room for doubt that this structure finds its precise homologue
in the pseudopodia of the foraminiferous group of the Rhizopoda, in
which a similar circulation or cyclosis of the constituent sarcode is
exhibited. The whole of this highly-interesting flagellate order, a
comparatively small one as yet, are remarkable for their pale glaucous
green or florescent hue, such colour assisting materially in their
recognition, even when the magnifying power employed is insufficient
for the detection of the very characteristic collar with its enclosed
flagellum.”[66]
[Illustration:
Fig. 335.--_Codosiga umbellata_; a few colonies of Zooids diverging
from the parent foot-stalk with flagella extended, magnified 650
diameters.]
_Ciliata._--Types of Ciliata obtained from hay infusions are very
numerous. Ehrenberg’s animalcules were mainly of a large size, and
of those belonging to the higher order of the Ciliata, pertaining
to such genera as Paramecium, Colpoda, Cyclidium, Oxytricha, and
Vorticella. These, however, represent but an insignificant minority
of the hosts of flagellate forms which abound in our humid climate,
and in hay infusions in particular. In such infusions, watched from
day to day and produced from hay obtained from different localities,
the number of types developed in regular sequence is found to be
perfectly marvellous, commencing with the _Monas_ proper, Amphimonas
and Heteromita; while Bacteria, in their motile and quiescent forms,
are invariably present and furnish an abundant supply of material for
the microscope.[67]
Vorticellidæ constitute one of the most numerous families of the
ciliate infusoria. All its members are at once recognised by their
normal stationary condition, and by the structure of their oral
system. In but few of the genera is there any marked divergence
from this formula, and when any exists it is made manifest by an
increase in development of some one of its elements at the expense
of another. For instance, in the genus Spirochona, the external edge
of the encircling border or peristome is suppressed, while the inner
portion is abnormally developed into a transparent and highly elevated
spiral membrane. The bell-animalcules usually possess stalks, and are
either solitary or form branching colonies. _Conichilus vorticella_
(Plate III., No. 80) is a well-known member of the colony stock, all
the zooids of which are united on a slender branching pedicle, which
consists of a central contractile cord enclosed within a tubular
hyaline sheath. There are many other shrub-like colonies all variously
modified in form and character. The _Epistylis opercularia_, or
nodding-bell animalcule, is an interesting member of a numerous host of
solitary short-stalked forms (Fig. 337). When the animal is disturbed,
the heads drop down towards the stalk. This animalcule has been
found to form a colony; and another, Carchesium, whose tiny branched
tree-like colonies resemble little white globular masses of moulds, are
seen at once to drop down towards the base of the colony with a jerky
movement if the cell be touched. By a process of encysting, all the
Vorticellæ and many of the more highly-organised ciliata have the means
of what may be termed self-preservation. Should the water dry up in
which they have been living, the little animal encases itself in mud at
the bottom of the pool. Should this be baked by the sun not the least
injury arises, for at this stage it crumbles into dust, and is carried
by the wind to long distances, but the first shower of rain calls it
back to active life, and soon after it is seen to issue forth as a free
swimming bud.
[Illustration: Fig. 336.--_Vorticella microstoma._]
_Thuricola valvata_ (Plate III., No. 72) possesses a hinge-like process
which closes up like a door when the animal contracts itself into its
case. This very effectually protects it from assault. Both portions of
the valve are capable of extension. Another group of ciliate infusoria
also possess a limited number of cilia, but these, although restricted
to the under surface of their bodies, have an unrestricted range
of motion. The group are all free swimmers, belonging to the genus
Oxytricha. They possess two separate alimentary orifices, neither of
which are situated at the extremities or encased by a dense integument.
Their locomotive organs consist either of setæ, vibratile cilia, or
non-vibratile styles or uncini, variously situated, and all serving
to make these infusorial animals very active (Plate III., Nos. 73 and
77). A typical species is the mussel-animalcule (Stylonychia, Fig.
338), common in all infusions and pools of water. Its body is oval
and flattened, and about 1/100th of an inch in length. At one end a
funnel-shaped depression or mouth, with a ciliated margin, leads to
the inner part of the body, in which are two oval bodies, a nucleus
and a contractile vacuole, which is seen to contract rhythmically. The
creature can also stalk along by means of its cilia or setæ, and set
up currents to the mouth. Plate III., Nos. 70, 71, 72, 73, and 74, are
types of these interesting bodies.
[Illustration: Fig. 337.--Nodding-bell animalcule (_Epistyles
operculata_) × 250 (Warne).]
[Illustration: Fig. 338.--Mussel-animalcule (_Stylonychia mytilus_)
under surface.
_a._ Mouth; _b._ Contractile vacuole; _c._ Nucleus. (Magnified 150
diameters.)]
Dr. Balbini believes a true sexual generation occurs among these
organisms, but, with the exception of the Paramecium, this has not been
seen to take place; even Gruber’s more recent investigations appear to
be inconclusive on this point. Conjugation, however, it is said takes
place among some attached forms, as in the Stentors. These have been
seen to put forth a bud from the body base, and soon after become free
swimming bodies. The trumpet-animalcule (Stentor), a conspicuous member
of the ciliata, is comparatively large, being about the 1/25th of an
inch in length when extended to the full size. It is usually found
attached to the under sides of duckweed, and is continually changing
its form from that of a small knob when contracted, to the trumpet
shape seen in Fig. 339, No. 6, when fully extended, and from which it
derives its name. The long cilia projected from the upper part form a
spiral within the margin of the open mouth leading to the digestive
sac. A contractile vacuole lies to the right of the oral opening. New
individuals are produced by the process of budding, and in the form of
ciliated embryos from the nucleus. Stentors are commonly met with in
fresh water, and are usually of a brilliant green colour. These little
bodies will bear cutting up: if only a fragment of the nucleus be
included in the section, the injury is soon repaired.
_Rotifera_, or _Wheel-animalcules_ (Fig. 339).--In this group we have
a higher type of animal, with a more complex organisation than those
previously noticed. The great majority inhabit fresh water, and are
readily developed in hay infusions, in bog-moss, in house-top gutters,
everywhere if looked for after a shower of rain. The rotating organs
from which these fascinating animalcula derive their name consist of
two disc-like bodies whose margins are fringed with rows of cilia,
which create currents toward the oral aperture, and which have given
rise to the optical delusion of rotating wheels. The disposition of the
cilia is so arranged as to bring food to the rotifer and conduct it to
the mastax or digesting apparatus--a muscular bulb moved by a series
of muscles--the gastric glands and stomach. The great transparency of
the whole structure permits of the animal economy being easily studied.
The body is covered with a horny envelope of two layers, and is divided
into segmental divisions, which slide into each other telescopic
fashion. Consequently, as the water dries up, the animal is for a
long time rendered indestructible and capable of resisting varying
temperatures and the action of caustic reagents.
Rotifers are oviparous, and their eggs are conspicuous and of three
kinds. The common soft-shelled eggs produce females, the smaller and
more spherical produce males. The ephippial, or summer eggs, are often
beset with spines or bosses; these have only a membranous covering,
and are hatched soon after they are laid, or before leaving the ova
sac. The male rotifer is but a third of the length of the female, often
without cilia, and appears to have no alimentary tract; indeed, the
only internal organ is a large sperm sac. Rotifers have been divided
by Dr. Hudson and the late Mr. Gosse in their charming work on these
very interesting “Wheel-animalcules” into four orders, according to
their powers of locomotion, as follows:--(1) Rhizota, the rooted;
(2) Bdelloida, the leech-like, that swim and creep like a leech; (3)
Ploïma, the sea-worthy, that only swim with their ciliary wreath; (4)
Scirtopoda, the skippers, that swim with their cilia and skip with
arthropodous limbs. These, again, are subdivided into families. With
such hardy creatures as Philodina, Adineta, Brachionus, &c., creatures
to whom extremes of cold, heat, and drought are the ordinary conditions
of life, nothing can be easier to keep going throughout the year. Mr.
C. F. Rousselet, who has so thoroughly succeeded in mounting Rotifers
with their cilia fully extended, recently exhibited at one of the
evening meetings of the Royal Microscopical Society, London, no less
than four hundred specimens in a natural and perfect condition, the
nervous system being seen more clearly from its successful staining
throughout the body than in the living rotifer.
[Illustration: Fig. 339.
1. _Rotifer vulgaris_ with its cilia; _b._ rotating; _c._ horn;
_d._ œsophagus; _f._ outer case; _g._ ova, foot protruding through
outer case. 2. Same in the contracted state and at rest, showing the
segmentation of the body and development of young. 3. Pitcher-shaped
Brachionus, furnished with two horny projections; _a._ mastax; _b._
shell; _c._ cilia, rotating disc; _d._ foot. 4. Baker’s Brachionus,
with six horny setæ; these are retracted when the cilia are in action;
the letters relate to the same internal organs as in the former; the
ova sac seen filled with eggs. 5 and 6. _B. ovalis_, closed, and with
cilia displayed.]
There is also a family of Rotatoria with a single rotatory organ,
disposed around the margin of the case. This comprises at present a
very small group. The Œcistes is a member of the family (Plate III.,
No. 69). A single ciliary wreath leads to the alimentary canal, and a
pharyngeal bulb or mastax comprises the apparatus of nutrition. The
visual organs are red, as in other rotifers, and the ovarium contains
several ova, shown in No. 69. The envelope is a gelatinous transparent
sheath, into which the animalcule can withdraw itself, its attachment
to the bottom being by the end of the foot-like tail. The most
interesting among this genus are the Floscularians. These creatures may
undoubtedly be described as among the most beautiful and interesting of
infusorial animals.
The Stephanoceros, “crowned animalcule,” as it is termed, is about
1/36th of an inch in length, and enclosed in a transparent cylindrical
flexible case, beyond which it protrudes five long arms in a graceful
manner. These, touching at their points, give a form from which it
derives its name. These arms are furnished with several rows of short
cilia, which seize the food brought within their grasp until it can be
swallowed. In addition to the rotatory organs, they have short flexible
processes, or cornu, attached to the outside of one or more of their
lobes. The water vascular system consists of two canals arising from
a small pyriform contractile vesicle, situated below the stomach. The
ova, after leaving the ova sac, remain quiescent until their cilia are
developed. Floscularians, like Melicertans, have a certain affinity in
form with Vorticellians and Stentors, and also with Campanulariæ, among
polypes. Their cilia are less regular when in action than in other
Rotatoria. When they retreat into their transparent cells they appear
to fold themselves up. Their internal structure can be seen through the
external case, and ova are observed enclosed in an ova sac; when thrown
off they remain quiescent until the formation of their cilia. The whole
family furnish interesting objects for microscopic investigation.
_Melicerta ringens_ (“beaded Melicerta”).--Of all the Melicerta, or
“horny floscularia,” this is the most beautiful. Its crystalline body
is enclosed in a pellucid covering, wider at the top than the bottom,
of a dark yellow or reddish-brown colour, which gradually becomes
encrusted by zones of a variety of shapes, cemented together with a
peculiar secretion that hardens in water. It derives its name from
these pellets, which have the appearance of rows of beads. Mr. Gosse
furnished an excellent account of the architectural instincts of
_Melicerta ringens_: “An animalcule so minute as to be with difficulty
appreciable by the naked eye, inhabiting a tube composed of pellets,
which it forms and lays one by one. It is a mason who not only builds
up his mansion brick by brick, but makes his bricks as he goes on, from
substances which he collects around him, shaping them in a mould which
he carries on his body.
“The pellets composing the case are very regularly placed in position;
in a fine specimen, about the 1/30th of an inch in length, when fully
expanded, as many as fifteen longitudinal rows of pellets were counted,
which gave about thirty-two rows in all. As it exposes itself more and
more, suddenly two large rounded discs are expanded, around which,
at the same instant, a wreath of cilia is seen performing surprising
motions.
“On mixing carmine with the water, the course of the ciliary current
is readily traced, and forms a fine spectacle. The particles are
hurled round the margin of the disc, until they pass off in front
through the great sinus, between the larger petals. If the pigment be
abundant, the cloudy torrent for the most part rushes off, and prevents
our seeing what takes place; but if the atoms be few, we see them
swiftly glide along the facial surface, following the irregularities
of outline with beautiful precision, dash round the projecting chin
like a fleet of boats doubling a bold headland, and lodge themselves,
one after another, in the little cup-like receptacle beneath. Mr.
Gosse, believing that the pellets of the case might be prepared in
the cup-like receptacle, watched the animal, and presently had the
satisfaction of seeing it bend its head forward, as anticipated, and
after a second or two raise it again; the little cup having in the
meantime lost its contents. It immediately began to fill again; and
when it was full, and the contents were consolidated by rotation,
aided probably by the admixture of a salivary secretion, it was again
bent down to the margin of the case, and emptied of its pellet. This
process he saw repeated many times in succession, until a goodly array
of dark-red pellets were laid upon the yellowish-brown ones, but
very irregularly. After a certain number were deposited in one part,
the animal would suddenly turn itself round in its case, and deposit
some in another part. It took from two-and-a-half to three-and-a-half
minutes to make and deposit a pellet.”
Melicerta may be found in clear pools, mill-ponds, and other places
through which a current of water gently flows. If a portion of
water-weed be brought home and placed in a small glass zoophyte-trough,
and carefully examined with a magnifying power of about fifty
diameters, a few delicate-looking projections of a reddish-brown colour
will probably be seen adhering to the plant; these are the tubular
cases of Melicerta, which, after a short period of rest, will be seen
to be animals of 1/12th of an inch or more in length.
Porifera. Spongiadæ.
[Illustration: Fig. 340.--_Spongia panicea._
Bread-crumb Sponge, showing currents entering surface _a_, and leaving
by oscules _b_.]
_Sponges._--The term Porifera, or “canal-bearing zoophytes,” was
applied by the late Dr. Grant to designate the remarkable class of
organisms known as sponges, met with in every sea, and numbering
about two thousand species, varying in size from a pin’s head to
masses several feet in height; and weighing from a few grains to
over a hundred pounds. Sponges assume an endless variety of shapes,
as cups, vases, spheres, tubes, baskets, branched-like trees, but
often as shapeless masses. When living they are all colours and all
consistences, soft and gelatinous, fleshy, leathery or stony. A fuller
knowledge of sponges was gained in 1825, when Dr. Robert Grant examined
a fragment of living sponge under the microscope. On bringing it to
the side of the glass cell in which he had preserved it, he beheld
this living fountain pouring forth a torrent of liquid matter in rapid
succession, and he was at once convinced that a current flowed out of
the larger orifices. He introduced a small portion of fine chalk, and
saw particles driven into the interior, and pass out again by different
ways. To determine the cause of the currents, it was necessary to make
a closer examination of the anatomy of the sponge. For this purpose
he cut or peeled off thin sections, and saw that the whole substance
was divided into flagellated chambers, enclosing spherical and other
bodies, and perforated by pores. Each chamber proved to be about
1/500th of an inch in diameter, groups of them opening by a wider
orifice into a common space, or canaliculus, and joining others to
form canals terminating in larger oscular canals. The walls throughout
are lined with flat cells, but in the flagellated chambers the living
cells are more or less cylindrical, and each is provided at the free
end with a whip-like appendage, or flagellum. Furthermore the upper
margin was seen to be expanded into a thin hyaline collar, so that the
whip appeared to have its origin in the centre of a basin or funnel.
The currents of water traversing the body of the sponge are kept up by
the movements of the flagella of the collar-cells. These beat the water
in the flagellated chambers into the rootlets of the canals leading to
the oscules. To replace this, water flows into the flagellated chambers
from the rootlets of the canals passing down from the groups of pores
in the skin. The currents entering the sponge bring in oxygenated
sea-water and minute food particles, such as diatoms and infusorial
organisms; the currents from the oscules contain an excess of carbonic
acid of waste products, resulting from vital activity and indigestible
remains. The cells lining the canals effect the exchange of gases, and
take up food particles.
[Illustration:
Fig. 341.--A section of a flagellate chamber of a Fresh-water Sponge,
showing collar-cells (Vosmaer).]
Professor Grant’s careful and instructive researches were begun on
the smaller kind of British sponges hanging down from rocks (_Spongia
coalita_), and on which he gazed for “twenty-five minutes, until
obliged to withdraw his eyes from fatigue.” This sponge fixes itself
by a root; and the currents enter through the stem and body, and leave
principally by oscules placed on the branches.
[Illustration: Fig. 342.--An Ascon Sponge.
A. Magnified × 20 diameters; B. × 80 diameters; C. Transverse section;
D. Collar-cells, × 700 diameters. The embryo, an extremely minute oval
cyst, is furnished with a flagellum for swimming; in the third it
assumes an amœboid form (Warne.)]
At present too little is known as to the physiology of digestion in
sponges to permit of a definite statement on the subject. In specimens
fed upon carmine the collar-cells have been found loaded with granules;
in others, again, the flat cells lining the subdermal cavities have
been found gorged with colour granules. From Bowerbank’s monograph
on the British Spongiadæ (1864 and 1874) nothing of importance can
be gained on the subject; in fact, it relates almost entirely to the
structure and organisation of sponges in their dried or preserved
condition, and therefore is only of value for purposes of specific
identification. One of the simplest of living sponges, the microscopic
structure of which it is possible to trace, _Ascetta primordialis_,
is found on seaweeds in the Mediterranean. In its simple unbranched
condition it forms a minute white sac about one twenty-fifth of an inch
in height, opening above by a wide round oscule and narrowing below to
a stalk (Fig. 342). The walls are very thin and perforated by pores,
through which the water passes into the interior. The walls of the
sac are composed of two layers, an inner lining of collar-cells, and
an outer layer consisting of a gelatinous matrix containing amœboid
bodies and transparent three-rayed spicules. These serve to support
the walls and as a frame-work for the pores, as in all the sponges. By
eliminating the spicular skeleton, and by supposing the tube to be more
globular, the “olynthus form” will be obtained, which has been regarded
as the hypothetical ancestor of all sponges. A canal system arises when
the walls grow thick or form folds, or give off pouches or tubes. From
these channels arise incipient in-current canals, between the inside
or lumen of the folds and that forming the out-current canal system.
There is a common ciliated Sycon found on seaweed round the British
coast; it has the appearance of a white sac about an inch in height,
with a crown of glassy spicules around the orifice. The vertical cavity
of the sac is surrounded by a wall of closely-packed horizontal tubes,
opening at their inner ends into the central cavity, but externally
ending blindly. The central cavity of the sac is surrounded or lined
with flat-cells, and the radial tubes with collar-cells, and the walls
of the tubes are perforated. Here the spaces between and outside
the densely-packed tubes are the in-current canals. In an equally
common British sponge, Grantia, which forms small flat white bags, a
rudimentary cortex covers the outer ends of the tubes. In Grantiopois,
the cortex becomes quite thick; as the radial tubes in this species
become more branched and the mesoderm thicker, so the passages or
in-current canals become more complicated. Common silicious, sponges
develop in a different manner from the calcareous ones, namely, from a
hollow conical sac open at the top and with a flat base; the spherical
flagellated chambers at a very early stage forming a mammillated
layer in the walls. Plakina, one of the simplest silicious sponges,
encrusts stones with a fleshy crust, consisting of a sac with a flat
base attached to the stone in sucker-like fashion, and with the rest
of the walls forming simple folds. The spaces between and outside the
folds form the in-current, and those in the lumen of the folds the
out-current, channels. Each of the flagellated chambers in the walls
of the folds communicates with the in-current spaces through several
pores, and opens into the out-current spaces by one large pore, the
currents of water passing out by the central oscule. Here we have a
general idea of the formation of all the commoner forms of sponges. In
the more delicate species, as that of Venus’ flowerbasket, the cells
are formed by a trellis work of large spicules of silica. Groups of
cells congregate in the ground substance and secrete a network of
cylindrical fibres and spicules, which, although they remain to a
certain extent separate, are always beautifully adapted for purposes of
support. In addition to the support these afford, the skeleton spicules
afford a means of defence against the attacks of small animals.[68]
A fairly good idea will be gained of the internal structure of sponges
from the section made of a _Geodia Barretti_, Fig. 343.
[Illustration: Fig. 343.--_Geodia Barretti_ (Bowerbank).
A tangential section of geodia sponge exhibiting the radial d
isposition of the fasciculi of the skeleton, and a portion of the
mesoderm of the sponge, magnified 50 diameters; _a._ intermarginal
cavities; _b._ a basal intermarginal cavity; _c._ ova imbedded in the
dermal crust of the sponge; _d._ large patentoternate spicula, the
heads of which form areas for the valvular bases of the intermarginal
cavities; _e._ recurvo-ternate defensive and aggressive spicula within
the summits of the intercellular spaces of the sponge; _f._ portion
of the interstitial membrane of sponge, crowded with minute stellate
spicula; _g._ portions of the secondary system of external defensive
spicula.]
_Reproduction._--As regards the modes of reproduction, both male and
female cells are found in the mesoderm. The male cells generally
give rise by division of the nucleus to masses of spermatozoa, each
of which possesses a conical head and a long vibratile filament. The
ova appear as large round cells, and when conglomerated in masses,
resemble those of Micro-gromia, which, after fertilisation, undergo
segmentation or division, first into two cells, and again dividing and
sub-dividing, until a cluster or mass of cells results (as seen
in Fig. 343). The outer layer of the egg-shaped embryo becomes more
cylindrical in shape, and is now provided with cilia, and soon appears
as an independent minute oval body. If a bread-crumb sponge be cut open
in the autumn, the embryos will be seen as bright yellow spots within
the body-substance. By keeping specimens in a vessel of water, the
embryos will be seen to escape from the oscules, and swim freely about
with the broad end forwards. After twenty-four hours of independent
existence, the embryo remains stationary, and fixes itself by its broad
end, which becomes flattened out. By a remarkable transformation, the
larger granular cells of the interior burst out and grow over the outer
flagellate layer of cells, and the latter become the collar-cells of
the adult sponge. A minute sponge with one oscule results from the
development of the fertilised ovum. An extensive crust with numerous
oscules may be regarded either as a colony in which each oscule
represents an individual, or simply as one individual in which the
growth of the body necessitates the formation of new channels for the
conveyance of food materials. The embryos of some of the fresh-water
sponges (Spongillidæ) living in ponds, canals, lakes and rivers all
over the world, as soon as they become fertilised undergo segmentation,
and form oval ciliated bodies, in appearance somewhat resembling the
gastrula of Monoxenia, one of the simplest kinds of corals. Fresh-water
sponges are green in colour, due to the granular bodies which crowd the
cells near the surface of the sponge; that this colour is not due to
the formation of chlorophyll is seen on keeping them in a shady place,
when they become pale grey or yellowish-brown, and if kept quite in the
dark they entirely lose all colour.
[Illustration: PLATE XVI.
SKELETONS AND SPICULA OF SPONGES.]
A few sponges possess no skeleton whatever, excepting the gelatinous
ground substance; in some specimens the skeleton is mainly or entirely
composed of foreign particles of sand or the remains of Foraminifera.
Others are composed of calcium carbonate, and form the class Calcarea,
the spicules of which are white, and opaque in mass; but on placing
portions in hydrochloric acid, the skeleton is dissolved away with
effervescence, and the spicules are left behind transparent and glassy.
A great variety is seen in the different species, as will be gathered
from the few typical forms shown in Plate XVI., and which even in
their fossilised state remain unaltered, the silica which enters so
largely into their composition being indestructible, the calcareous
matter alone becoming separated in exposure to the action of air, or by
boiling in hydrochloric acid. The only perceptible difference noticed
is an increase in transparency, and this, on mounting them in Canada
balsam, adds to their beauty when examined by polarised light.
Hyalonema, the “glass-rope” sponge of Japan, consists of a bundle
of from 200 to 300 threads of transparent silica, glistening with a
satiny lustre like the most brilliant spun glass; each thread is about
eighteen inches long, in the middle the thickness of a knitting-needle,
and gradually tapering towards either end to a fine point; the whole
bundle coiled like a strand of rope into a lengthened spiral, the
threads of the middle and lower portions remaining compactly coiled
by a permanent twist of the individual threads; the upper portions of
the coil frayed out, so that the glassy threads stand separate from
each other. The spicules on the outside of the coil stretch its entire
length, each taking about two and a half turns of the spiral. One of
these long needles is about one-third of a line in diameter in the
centre, gradually tapering towards either end. The spirally-twisted
portion of the needle occupies rather more than the middle half of its
entire length. In the lower portion of the coil, which is embedded in
the sponge, the spicule becomes straight, and tapers down to an extreme
tenuity, ultimately becoming so fine that it is scarcely possible to
trace it to its termination.
Within the mesoderm, and in oscule, was noticed a deep brownish-orange
coloured shrunken membrane; this was traced to a parasitic polyp.
Since this was first observed on an early specimen of the Japanese
glass-sponge, the same parasite has always been found growing on and
in all these curious sponges. The surface of the stalk above the
portion embedded in the mud is seen to be covered with a warty crust
of parasitic polyps. All the specimens of Hyalonema in the European
museums in 1860 had their stalks overgrown with Palythoa, while many
had their bodies also covered with another parasite, and which,
fortunately for the sponge, did not form a sandy crust. The polyps,
having no skeleton, dry up entirely, and leave behind no trace except
the stain first referred to. Unlike a parasite, however, the polyps do
not feed upon the juices and soft parts of the sponge, nor indeed do
they share its food, but simply settle upon the sponge and feed upon
any food that may chance to come within their reach.
The dredgings of the _Challenger_ brought to the surface many entirely
new forms of glass-sponges and from great depths. One of the most
beautiful, known as Carpenter’s glass-sponge (Pheronema), is composed
of concentric laminæ of silica deposited around a fine central axial
canal. These form a gauze-like network throughout, but with no
regularity of structure.
_Clionæ._--Not the least wonderful circumstance connected with the
history of sponges is the power possessed by certain species of boring
into substances, the hardness of which might be considered as a
sufficient protection against such apparently contemptible foes. Shells
(both living and dead), coral, and even solid rocks are attacked by
these humble destroyers, gradually broken up, and, no doubt, finally
reduced to such a state as to render substances which would otherwise
remain dead and useless in the economy of nature available for the
supply of the necessities of other living creatures.
These boring sponges constitute the genus Cliona of Dr. Grant. They are
branched in form, or consist of lobes united by delicate stems, and
after having buried themselves in shells or other calcareous objects,
preserve their communication with the water by means of perforations in
the outer wall of the shell. The mechanism by which a creature of so
low a type of organisation contrives to produce effects so remarkable
is still doubtful, from the great difficulties which lie in the way of
coming to any satisfactory conclusions upon the habits of an animal
that works so completely in the dark as the _Cliona celata_. Mr.
Hancock, in his valuable memoir upon the boring sponges, attributes
their excavating power to the presence of the multitude of minute
silicious crystalline particles adhering to the surface of the sponge;
these he supposes are set in motion by ciliary action. In whatever
way this action may be produced, however, there can be no doubt that
these sponges are constantly and silently effecting the disintegration
of submarine calcareous bodies--the shelly coverings, it may be, of
animals far higher in organisation, and in many instances they prove
themselves formidable enemies even to living molluscs, by boring
completely through the shell. In this case the animal whose domicile it
so unceremoniously invades has no alternative but to raise a wall of
new shelly matter between himself and his unwelcome guest, and in this
manner generally succeeds in barring him out.
From a close examination of the structural and developmental
characters of the Spongideæ, it must be conceded that they belong
rather to the flagellata Protozoa than to any other order. This was
the view held by the late Professor Clark, and Mr. Saville Kent quite
concurs in it.[69] Summing up the entire evidence adduced, scarcely
a shadow of doubt is admissible concerning the intimate relationship
that subsists between the Choano-flagellata and other flagellate
Protozoa and that of sponges. The primary and essential element of
the apparently complex sponge stock is the assemblage of collared
flagellate zooids that inhabit its interstitial cavities under various
plans of distribution. Individually these collared zooids correspond
structurally and functionally in every detail with the collared units
of such genera as Codosiga, Salpingœca, and Proto-spongia. The collar
in either case presents the same structure and functions, exhibits
the same circulatory currents or cyclosis, and acts in the same way
for the capture of food. The body contains an identical centrally
located spheroidal nucleus or endoplast, and a corresponding series
of rhythmically pulsating contractile vesicles. The developmental
reproductive phenomena are also strictly parallel. Both originate as
simple Amœba or simple flagellate Monads, exhibiting no trace in their
earliest stage of the subsequently acquired characteristic collar. Both
again after a time withdraw their collar and flagellum, and assume the
amœboid state; then, coalescing, enter upon a quiescent or encysted
condition, and break up into a number of sporular bodies, and thus
provide for the further existence and distribution of the species.
The whole process again is much akin to that which obtains in the
protophytic type, _Volvox globator_, which liberates from its interior
free swimming gemmules that take the form of spherical aggregation of
biflagellate daughter-cells. In their isolated state, on the other
hand, the swarm gemmules of the sponge stock are directly comparable
with the free swimming subspheroidal colony stock of the flagellate
infusoria Synura, Syncrypta, and Uroglena, or with the attached
subspheroidal clusters of Codosiga and Anthophysa.
[Illustration: ECHINODERMATA, HYDROZOA, POLYZOA, HELMINTHOIDA.
Tuffen West, del. Edmund Evans.
PLATE IV.]