TARDIGRADA
OCCURRENCE—ECDYSIS—STRUCTURE—DEVELOPMENT—AFFINITIES—BIOLOGY—DESICCATION—
PARASITES—SYSTEMATIC
The animals dealt with in this chapter lead obscure lives, remote from
the world, and few but the specialist have any first-hand acquaintance
with them. Structurally they are thought to show affinities with the
Arachnida, but their connexion with this Phylum is at best a remote one.
Tardigrades are amongst the most minute multicellular animals which
exist, and their small size—averaging from ⅓ to 1 mm. in length—and
retiring habits render them very inconspicuous, so that as a rule they
are overlooked; yet Max Schultze[374] asserts that without any doubt
they are the most widely distributed of all segmented animals. They are
found amongst moss, etc., growing in gutters, on roofs, trees or in
ditches, and in such numbers that Schultze states that almost any piece
of moss the size of a pea will, if closely examined, yield some members
of this group, but they are very difficult to see. The genus
_Macrobiotus_ especially affects the roots of moss growing on stones and
old walls. _M. macronyx_ lives entirely in fresh water, and _Lydella
dujardini_ and _Echiniscoides sigismundi_ are marine; all other species
are practically terrestrial, though inhabiting very damp places.
In searching amongst the heather of the Scotch moors for the ova and
embryos of the Nematodes which infest the alimentary canal of the
grouse, I have recently adopted a method not, as far as I am aware, in
use before, and one which in every case has yielded a good supply of
Tardigrades otherwise so difficult to find. The method is to soak the
heather in water for some hours and then thoroughly shake it, or to
shake it gently in a rocking machine for some hours. The sediment is
allowed to settle, and is then removed with a pipette and placed in a
centrifugaliser. A few turns of the handle are sufficient to concentrate
at the bottom of the test-tubes a perfectly amazing amount of cryptozoic
animal life, and amongst other forms I have never failed to find
Tardigrades.
[Illustration:
FIG. 249.—Dorsal view of _Echiniscus testudo_, C. Sch., × 200, showing
the four segments 1, 2, 3, 4. (From Doyère.)
]
[Illustration:
FIG. 250.—Cast-off cuticle of _Macrobiotus tetradactylus_, Gr., ×
about 150, containing four eggs in which the boring apparatus of the
embryo can be distinguished. (From R. Greeff.)
]
Many Tardigrades are very transparent; their cells are large, and
arranged in a beautifully symmetrical manner; and since those of them
that live in moss, and at times undergo desiccation, are readily thrown
into a perfectly motionless state, during which they may be examined at
leisure, it is not surprising that these little creatures have been a
favourite object for histological research. One way to produce the
above-mentioned stillness is partly to asphyxiate the animals by placing
them in water which has been boiled, and covering the surface of the
water with a film of oil.
[Illustration:
FIG. 251.—_Echiniscus spinulosus_, C. Sch., × about 200, seen from the
side. (From Doyère.)
]
The whole body is enclosed in a thin transparent cuticle, which must be
pierced by a needle if it be desired to stain the tissues of the
interior. As a rule the cuticle is of the same thickness all over the
body, but in the genus _Echiniscus_ the cuticle of the dorsal surface is
arranged in thickened plates, and these plates are finely granulated.
From time to time the cuticle is cast, and this is a lengthy process, so
that it is not unusual to find a Tardigrade ensheathed in two cuticles,
the outer of which is being rubbed off. The Macrobioti lay their eggs in
their cast cuticle (Fig. 250). The end of each of the eight legs bears
forked claws of cuticular origin. The legs are not jointed except in the
genus _Lydella_, where two divisions are apparent.
Within the cuticle is the epidermis, a single layer of cells arranged in
regular longitudinal and transverse rows along the upper and under
surface, where the cells are as uniformly arranged and as rectangular as
bricks. The cells on the sides of the body are polygonal, and not in
such definite rows. The nuclei show the same diagrammatic symmetry as
the cells which contain them, and lie in the same relative position in
neighbouring cells. In a few places, such as the end of each limb and
around the mouth and arms, the cells of the epidermis are heaped up and
form a clump or ridge. In some genera a deposit of pigment in the
epidermis, which increases as the animal grows old, obscures the
internal structures. It is generally brown, black, or red in colour.
[Illustration:
FIG. 252.—_Macrobiotus schultzei_, Gr., × 150. (Modified from Greeff.)
_a_, The six inner papillae of the mouth; _b_, the chitin-lined
oesophagus; _c_, calcareous spicule; _d_, muscle which moves the
spicule; _e_, muscular pharynx with masticating plates; _f_,
salivary glands; _g_, stomach; _h_, ovary; _i_, median dorsal
accessory gland; _k_, diverticula of rectum.
]
The cuticle and epidermis enclose a space in which the various internal
organs lie. This space is traversed by numerous symmetrically disposed
muscle-fibres, and contains a clear fluid—the blood—which everywhere
bathes these organs. This fluid evaporates when desiccation takes place,
and is soon replaced after rain; it forms no coagulum when reagents are
added to it, and it probably differs but little from water. Floating in
it are numerous corpuscles, whose number increases with age. In well-fed
Tardigrades the corpuscles are packed with food-reserves, often of the
same colour—green or brown—as the contents of the stomach, which soon
disappear when the little creatures are starved.
The alimentary canal begins with an oral cavity, which is in many
species surrounded by chitinous rings. The number of these rings and
their general arrangement are of systematic importance. The oral cavity
opens behind into a fine tube lined with chitin, very characteristic of
the Tardigrada, which has been termed the mouth-tube. By its side,
converging anteriorly, lie the two chitinous teeth, which may open
ventrally into the mouth-tube, as in _Macrobiotus hufelandi_ and
_Doyeria simplex_, or may open directly into the oral cavity, as in
_Echiniscus_, _Milnesium_, and some species of _Macrobiotus_. In some of
the last named the tips of the teeth are hardened by a calcareous
deposit. The hinder end of each stylet or tooth is supported by a second
chitinous tooth-bearer,[375] and the movement of each is controlled by
three muscles, one of which, running forwards to the mouth, helps to
protrude the tooth, whilst the other two running upwards and downwards
to the sheath of the pharynx, direct in what plane the tooth shall be
moved.
The mouth-tube passes suddenly into the muscular sucking pharynx, which
is pierced by a continuation of its chitinous tube. Roughly speaking,
the pharynx is spherical; the great thickness of its walls is due to
radially arranged muscles which run from the chitinous tube to a
surrounding membrane. When the muscles contract, the lumen of the tube
is enlarged, and food, for the most part liquid, is sucked in. Two large
glands, composed of cells with conspicuous nuclei, but with ill-defined
cell outlines, pour their contents into the mouth in close proximity to
the exit of the teeth. The secretion of the glands—often termed salivary
glands—is said in many cases to be poisonous.
The pharynx may be followed by a distinct oesophagus, or it may pass
almost immediately into the stomach, which consists of a layer of
six-sided cells arranged in very definite rows. In fully-fed specimens
these cells project into the lumen with a well-rounded contour.
Posteriorly the stomach contracts and passes into the narrow rectum,
which receives anteriorly the products of the excretory canals and the
reproductive organs, and thus forms a cloaca. Its transversely placed
orifice lies between the last pair of legs. The food of Tardigrades is
mainly the sap of mosses and other humble plants, the cell-walls of
which are pierced by the teeth of the little creatures.
The organs to which an excretory function has been attributed are a pair
of lateral caeca, which vary much in size according as the possessor is
well or ill nourished. They recall the Malpighian tubules of such Mites
as _Tyroglyphus_. Nothing comparable in structure to nephridia or to
coxal glands has been found.
The muscles show a beautiful symmetry. There are ventral, dorsal, and
lateral bundles, and others that move the limbs and teeth, but the
reader must be referred to the works of Basse, Doyère,[376] and
Plate[377] for the details of their arrangement. The muscle-fibres are
smooth.
[Illustration:
FIG. 253.—Brain of _Macrobiotus hufelandi_, C. Sch., × about 350.
(From Plate.) Seen from the side. _ap_, Lobe of brain bearing the
eye; _ce_, supra-oesophageal ganglion; _d_, tooth; _Ga_, first
ventral ganglion; _ga’_, sub-oesophageal ganglion; _k_, thickening
of the epidermis round the mouth; _oc_, eye-spot; _oe_, oesophagus;
_op_, nerve running from the ocular lobe of the brain to the first
ventral ganglion; _ph_, pharynx.
]
The nervous system consists of a brain or supra-oesophageal ganglion,
whose structure was first elucidated by Plate, and a ventral chain of
four ganglia. Anteriorly the brain is rounded, and gives off a nerve to
the skin; posteriorly each half divides into two lobes, an inner and an
outer. The latter bears the eye-spot when this is present. Just below
this eye a slender nerve passes straight to the first ventral ganglion.
The brain is continued round the oral cavity as a thick nerve-ring, the
ventral part of which forms the sub-oesophageal ganglion, united by two
longitudinal commissures to the first ventral ganglion. Thus the brain
has two channels of communication between it and the ventral nerve-cord
on each side, one by means of the slender nerve above mentioned, and one
through the sub-oesophageal ganglion. The ventral chain is composed of
four ganglia connected together by widely divaricated commissures. Each
ganglion gives off three pairs of nerves, two to the ventral
musculature, and one to the dorsal. The terminations of these nerves in
the muscles are very clearly seen in these transparent little creatures,
though there is still much dispute as to their exact nature.
The older writers considered the Tardigrada as hermaphrodites, but Plate
and others have conclusively shown that they are bisexual, at any rate
in the genus _Macrobiotus_. The males are, however, much rarer than the
females. The reproductive organs of both sexes are alike. Both ovary and
testis are unpaired structures opening into the intestine, and each is
provided with a dorsal accessory gland placed near its orifice. In the
ovary many of the eggs are not destined to be fertilised, but serve as
nourishment for the more successful ova which survive.
No special circulatory or respiratory organs exist, and, as in many
other simple organisms, there is no connective tissue.
[Illustration:
FIG. 254.—Male reproductive organs of _Macrobiotus hufelandi_, C.
Sch., × about 350. (From Plate.) _a.ep_, Epidermal thickening round
anus; _cl_, cloaca; _gl.d_, accessory gland; _gl.l_, Malpighian
gland; _st_, stomach; _te_, testis; _x_, mother-cells of
spermatozoa.
]
The segmentation of the egg in _M. macronyx_ is total and equal,
according to the observations of von Erlanger.[378] A blastula, followed
by a gastrula, is formed. The blastopore closes, but later the anus
appears at the same spot. There are four pairs of mesodermic diverticula
which give rise to the coelom and the chief muscles. The reproductive
organs arise as an unpaired diverticulum of the alimentary canal, which
also gives origin to the Malpighian tubules. The development is thus
very primitive and simple, and affords no evidence of degeneration.
With regard to their position in the animal kingdom, writers on the
Tardigrada are by no means agreed. O. F. Müller placed them with the
Mites; Schultze and Ehrenberg near the Crustacea; Dujardin and Doyère
with the Rotifers near the Annelids; and von Graff with the Myzostomidae
and the Pentastomida. Plate regards them as the lowest of all
air-breathing Arthropods, but he carefully guards himself against the
view that they retain the structure of the original Tracheates from
which later forms have been derived. He looks upon Tardigrades as a side
twig of the great Tracheate branch, but a twig which arises nearer the
base of the branch than any other existing forms. These animals seem
certainly to belong to the Arthropod phylum, inasmuch as they are
segmented, have feet ending in claws, Malpighian tubules, and an entire
absence of cilia. The second and third of these features indicate a
relationship with the Tracheate groups; on the other hand there is an
absence of paired sensory appendages, and of mouth-parts. Von Erlanger
has pointed out that the Malpighian tubules, arising as they do from the
mid-gut, are not homologous with the Malpighian tubules of most
Tracheates, and he is inclined to place this group at the base or near
the base of the whole Arthropod phylum. They, however, show little
resemblance to any of the more primitive Crustacea. The matter must
remain to a large extent a matter of opinion, but there can be no doubt
that the Tardigrades show more marked affinities to the Arthropods than
to any other group of the animal kingdom.
=Biology.=—Spallanzani, who published in the year 1776 his _Opuscules de
physique animale et végétale_, was the first satisfactorily to describe
the phenomena of the desiccation of Tardigrades, though the subject of
the desiccation of Rotifers, Nematodes, and Infusoria had attracted much
notice, since Leeuwenhoek had first drawn attention to it at the very
beginning of the century. In its natural state and in a damp atmosphere
Tardigrades live and move and have their being like other animals, but
if the surroundings dry up, or if one be isolated on a microscopic slide
and slowly allowed to dry, its movements cease, its body shrinks, its
skin becomes wrinkled, and at length it takes on the appearance of a
much weathered grain of sand in which no parts are distinguishable. In
this state, in which it may remain for years, its only vital action must
be respiration, and this must be reduced to a minimum. When water is
added it slowly revives, the body swells, fills out, the legs project,
and gradually it assumes its former plump appearance. For a time it
remains still, and is then in a very favourable condition for
observation, but soon it begins to move and resumes its ordinary life
which has been so curiously interrupted.
All Tardigrades have not this peculiar power of revivification—
anabiosis, Preyer calls it—it is confined to those species which live
amongst moss, and the process of desiccation must be slow and, according
to Lance,[379] the animal must be protected as much as possible from
direct contact with the air.
According to Plate, the Tardigrada are free from parasitic Metazoa,
which indeed could hardly find room in their minute bodies. They are,
however, freely attacked by Bacteria and other lowly vegetable
organisms, and these seem to flourish in the blood without apparently
producing any deleterious effects on the host. Plate also records the
occurrence of certain enigmatical spherical bodies which were found in
the blood or more usually in the cells of the stomach. These bodies
generally appeared when the Tardigrades were kept in the same unchanged
water for some weeks. Nothing certain is known as to their nature or
origin.
=Systematic.=—A good deal of work has recently been done by Mr. James
Murray on the Polar Tardigrades and on the Tardigrades of Scotland, many
of which have been collected by the staff of the Lake Survey.[380] Over
forty species have been described from North Britain.
The following table of Classification is based on that drawn up by
Plate:—
=Table of Genera.=
I. The claws of the legs are simple, without a second hook. If there
are several on the same foot they are alike in structure and size.
A. The legs are short and broad, each with at least two claws.
2–4 claws Gen. 1. _ECHINISCUS_, C. SCH. (Fig. 249).
7–9 claws Sub-gen. 1_a_. _ECHINISCOIDES_, PLATE.
B. The legs are long and slender; each bears only one small claw.
Gen. 2. _LYDELLA_, DOY.
II. The claws of the legs are all or partly two- or three-hooked.
Frequently they are of different lengths.
A. There are no processes or palps around the mouth.
I. The muscular sucking pharynx follows closely on the
mouth-tube.
α. The oral armature consists on each side of a stout
tooth and a transversely placed support.
Gen. 3. _MACROBIOTUS_, C. SCH. (Fig. 252).
β. The oral armature consists on each side of a
stylet-like tooth without support.
Gen. 4. _DOYERIA_, PLATE.
II. The mouth-tube is separated from the muscular sucking
pharynx by a short oesophagus.
Gen. 5. _DIPHASCON_, PLATE (Fig. 255).
B. Six short processes or palps surround the mouth, and two
others are placed a little farther back.
Gen. 6. _MILNESIUM_, DOY.
1. Genus _ECHINISCUS_ (= _EURYDIUM_, DOY.).—The dorsal cuticle is thick,
and divided into a varying number of shields, which bear thread- or
spike-like projections. The anterior end forms a proboscis-like
extension of the body. Two red eye-spots. There are many species, and
the number has increased so rapidly in the last few years that
specialists are talking of splitting up the genus. _E. arctomys_, Ehrb.;
_E. mutabilis_, Murray; _E. islandicus_, Richters; _E. gladiator_,
Murray; _E. wendti_, Richters; _E. reticulatus_, Murray; _E. oihonnae_,
Richters; _E. granulatus_, Doy.; _E. spitzbergensis_, Scourfield;[381]
_E. quadrispinosus_, Richters; and _E. muscicola_, Plate, are all
British. More than one-half of these species are also Arctic, and _E.
arctomys_ is in addition Antarctic. In fact, the group is a very
cosmopolitan one. The genus is also widely distributed vertically,
specimens being found in cities on the sea level and on mountains up to
a height of over 11,000 feet.
[Illustration:
FIG. 255.—_Diphascon chilenense_, Plate, × about 100. (From Plate.)
_ce_, Brain; _k_, thickening of the epidermis above the mouth; _o_,
egg; _oe_, oesophagus; _p_,?salivary glands; _ph_, pharynx; _sa_,
blood corpuscles; _st_, stomach.
]
1a. Sub-genus _ECHINISCOIDES_ differs from the preceding in the number
of the claws, the want of definition in the dorsal plates, and in being
marine. The single species _E. sigismundi_, M. Sch., is found amongst
algae in the North Sea (Ostend and Heligoland).
2. Genus _LYDELLA_.[382]—The long, thin legs of this genus have two
segments, and in other respects approach the Arthropod limb. Marine.
Plate suggests the name _L. dujardini_ for the single species known.
3. Genus _MACROBIOTUS_ has a pigmented epidermis, but eye-spots may be
present or absent. The eggs are laid one at a time, or many leave the
body at once. They are either quite free or enclosed in a cast-off
cuticle. The genus is divided into many species and shows signs of
disruption. They mostly live amongst moss; but _M. macronyx_, Doy., is
said to live in fresh water. The following species are recorded from
North Britain: _M. oberhäuseri_, Doy.; _M. hufelandi_, Schultze; _M.
zetlandicus_, Murray; _M. intermedius_, Plate; _M. angusti_, Murray; _M.
annulatus_, Murray; _M. tuberculatus_, Plate; _M. sattleri_, Richters;
_M. papillifer_, Murray; _M. coronifer_, Richters; _M. crenulatus_,
Richters; _M. harmsworthi_, Murray; _M. orcadensis_, Murray; _M.
islandicus_, Richters; _M. dispar_, Murray; _M. ambiguus_, Murray; _M.
pullari_, Murray; _M. hastatus_, Murray; _M. dubius_, Murray; _M.
echinogenitus_, Richters; _M. ornatus_, Richters; _M. macronyx_?, Doy.
4. Genus _DOYERIA_.—The teeth of this genus have no support, and the
large salivary glands of the foregoing genus are absent; in other
respects _Doyeria_, with the single species _Doyeria simplex_, Plate,
resembles _Macrobiotus_, and is usually to be found in consort with _M.
hufelandi_, C. Sch.
5. Genus _DIPHASCON_ resembles _M. oberhäuseri_, Doy., but an oesophagus
separates the mouth-tube from the sucking pharynx, and the oral armature
is weak. The following species are British, the first named being very
cosmopolitan, being found at both Poles, in Chili, Europe, and Asia: _D.
chilenense_, Plate; _D. scoticum_, Murray; _D. bullatum_, Murray; _D.
angustatum_, Murray; _D. oculatum_, Murray; _D. alpinum_, Murray; _D.
spitzbergense_, Murray.
6. Genus _MILNESIUM_ has a soft oral armature, and the teeth open
straight into the mouth. A lens can usually be distinguished in the
eyes. Two species have been described, _M. tardigradum_, Doy., British,
and _M. alpigenum_, Ehrb. Bruce and Richters consider that these two
species are identical.