1911 Encyclopædia Britannica/Tardigrada
TARDIGRADA, apparently Arthropodous animals whose
relationship to the great classes of this sub-kingdom is masked
by degenerative modification. They are microscopical in size
and live in damp moss or water. The body is elongated and
furnished with four pairs of short, unjointed, stump-like legs,
each terminated by a pair of claws. The legs of the posterior
Milnesium tardigradum,
Schrank. a, ovary;
b, oval stylite (?); c,
mouth; d., alimentary
canal; e...e, legs.
pair project from the hinder extremity
of the body and the anus opens between
them. The mouth, situated at the opposite
end and armed with a pair of
stylets, leads into an oesophagus, into
which the ducts of a pair of so-called
salivary glands open. Behind this
point there is a muscular pharynx or
gizzard, which communicates with the
wide intestinal tract. No organs of
circulation or respiration are known;
but the nervous system is well developed,
and consists of a pair of
ganglia corresponding with the limbs
and connected by longitudinal commissural
chords. Anteriorly these chords
embrace the oesophagus and unite with
the cerebral mass which innervates the
pair of eyes when present. The sexes
are not distinct, the sexual organs being
represented by a pair of testes and a
single ovary, which open together into
the posterior end of the alimentary canal. The Tardigrada have
been regarded as degenerate Acari largely on account of their
possessing four pairs of ambulatory limbs, which is considered to be an Arachnidan characteristic. But they cannot be affiliated
with this order on account of the total suppression of the
abdomen, of their hermaphroditism and of the communication that
exists between the generative organs and the alimentary tract.
These last characteristics also separate them essentially from
the Pycnogonida, some members of which resemble them to a
certain extent in having only four pairs of limbs, no gnathites,
no respiratory organs, a ganglionated ventral nervous system,
and the abdomen reduced to a mere rudiment projecting
between the last pair of legs.
Several genera and species of Tardigrada have been described, perhaps the best known being Macrobiotus schultzii and Milnesium tardigradum. (R. I. P.)