DETAILS OF PROTOZOA.
The Protozoa are divided into three orders.
I.--_Gregarinidæ._
II.--_Rhizopoda._
III.--_Infusoria._
I. The _Gregarinidæ_ consist of minute protozoa, parasitic in the
interior of insects, &c., and like other internal parasites are
colourless, as we should expect.
II. The _Rhizopoda_ may, for our purpose, be divided into the naked
forms like _Amoeba_, and those which possess a skeleton, such as the
Radiolaria, the Foraminifera and the Spongia.
Of these the naked forms are colourless, or uniformly tinted, excepting
the flush already described as emphasizing the contractile vesicle.
The _Foraminifera_ are the earliest animals that possess a skeleton or
shell, and though generally very small, this shell is often complex, and
of extreme beauty, though their bodies retain the general simplicity of
the protozoa, indeed, they are said to possess no contractile vesicle.
Still the complexity of their shells places them on a higher level than
the naked rhizopoda.
In these animals we find the first definite colour, not as a pattern,
but as simple tinting of the protoplasm. The general hue is
yellowish-brown (as in _Amoeba_), but deep red is not uncommon. The
deepest colour is found in the oldest central chambers, becoming fainter
towards the periphery, where it is often almost unrecognisable.[19]
The _Radiolaria_ are minute organisms with still more complex skeletons,
and are considered by Haeckel[20] to be more highly organized than the
preceding order. They consist of a central portion containing masses of
minute cells, and an external portion containing yellow cells. Here we
have the first differentiation of parts in the external coating and
internal capsule, and side by side with this differentiation we find
colour more pronounced, and even taking regional tints in certain forms.
We may notice the following genera as exhibiting fine colour:--
_Red._ Eucecryphalus, Arachnocorys, Eucrytidium, Dictyoceras.
_Yellow._ Carpocanium, Dictyophimus, Amphilonche.
_Purple._ Eucrytidium, Acanthostratus.
_Blue._ Cyrtidosphæra, Coelodendrum.
_Green._ Cladococeus, Amphilonche.
_Brown._ Acanthometra, Amphilonche.
Examples of these may be seen in the plates of Haeckel's fine work, and
as an illustration of regional decoration we cite _Acanthostratus
purpuraceus_, in which the central capsule is seen to run from red to
orange, and the external parts to be colourless, with red markings in
looped chains.
_Spongocyclia_ also exhibits this regional distinction of colour very
clearly, the central capsule being red and the external portion yellow.
The _Spongida_, or sponges, are, broadly speaking, assemblages or
colonies of amoeba-like individuals, united into a common society.
Individually the component animals are low, very low, in type, but their
union into colonies, and the necessity for a uniform or common
government has given rise to peculiarities that in a certain sense raise
them even above the complex radiolaria. Some, it is true, are naked, and
do not possess the skeleton that supports the colony, which skeleton
forms what we usually call the sponge; but even amongst these naked
sponges the necessity for communal purposes over and above the mere
wants of the individual, raises them a step higher in the animal series.
A multitude of individuals united by a common membrane, living in the
open sea, it must have happened that some in more immediate contact with
the food-producing waters, would have thriven at the expense of those in
the interior who could only obtain the nutriment that had passed
unheeded by the peripheral animals. But just as in higher communities we
have an inflowing system of water and an out-flowing system of effete
sewerage quite uncontrolled, and, alas, generally quite unheeded by the
individuals whose wants are so supplied; so in the sponges we have a
system of inflowing food-bearing water and an out-flowing sewage, or
exhausted-water system. This is brought about by a peculiar system of
cilia-lined cells which, as it were, by their motion suck the water in,
bringing with it the food, and an efferent system by which the exhausted
liquid escapes. These cilia-lined cells are the first true organs that
are to be found in the animal kingdom, and according to the views we
hold, they ought to be emphasized with colour, even though their
internal position renders the colouration less likely. This we find
actually to be the case, and these flagellated cells, as they are
called, are often the seat of vividest colour.
The animal matter, or sarcode, or protoplasm of sponges falls into three
layers, just as we find the primitive embryo of the highest animals; and
just as the middle membrane of a mammalian ovum develops into bone,
muscle and nerve, so the middle membrane (mesosarc) of the sponges
develops the hard skeleton, and in this most important part we find the
colour cells prevail. Sollas, one of our best English authorities upon
sponges, writes, "The colours of sponges, which are very various, are
usually due to the presence of pigment granules, interbedded either in
the _endosarc of the flagellated cells_, or in the mesodermic cells,
usually of the skin only, but sometimes of the whole body."[21]
We can, then, appeal most confidently to the protozoa as illustrating
the morphological character of colouration.
[Illustration]
[19] Leidy. Rhizopoda of N. America, p. 16.
[20] Haeckel. Die Radiolarien, Berlin, 1862.
[21] Sollas. Spongidæ. Cassell's Nat. Hist. Vol. vi., p. 318.