1893. THE CLASSIFICATION OF ARACHNIDS. 449 
higher Arachnids, he shows that they want the greater number of 
the abdominal segments, while they possess, in a fair state of develop- 
ment, organs homologous with those in the anterior part of the 
body of the higher Arachnids. He speaks of the Mites as ‘larval 
Araneids,” but few naturalists will be inclined to regard them as a 
direct offshoot from the spiders. Extreme smallness—one of the 
supposed advantages gained by fixation at the larval stage—has been 
acquired by many true spiders. Moreover, spiders, like most other 
Arachnids, have no larval stage, but leave the egg in a form similar 
to that of the adult animal. Whether degraded or arrested, the 
affinities of the mites seem to be with the harvestmen, from which 
group they are not readily defined. 
Trouessart (4), however, supports the view that, on account of 
their development with a metamorphosis, the mites should form a 
special sub-class of the Arachnida, and he separates the vermiform 
mites, such as the Phytopti, from the rest as a separate order; but 
the larval stage of mites (in which they have but three pairs of limbs) 
seems quite a secondary adaptation, for, according to Wagner (5) the 
fourth pair appear in the embryo, to be afterwards concealed beneath 
the skin of the larva and then to re-appear during metamorphosis. 
It is remarkable that the theory of fixation at the larval stage 
has also been invoked by Von Kennel (6) to account for the origin of 
the microscopic ‘‘ water bears” (Tardigrada), which have generally 
been regarded as degraded arachnids. He compares them with 
certain midge-larvee, (Cecidomyia) which have the power of partheno- 
genetic reproduction, and suggests that if male organs were also 
precociously developed a species might become permanently larval 
in form. Though he does not assert that the Tardigrada are 
‘“‘arrested ”’ flies, the acceptance of his views—which certainly have 
much to support them—would probably lead to the removal of the 
group from the arachnids to the insects, in which the larval stage is 
often of such supreme importance. 
The replacement of lung-sacs by trachez in several groups of 
arachnids opens up a question of much interest. Reference has been 
made in NatTurRAL SCIENCE (vol i., p. 524) to the supposed origin of 
the lung-sacs from the invagination or enclosure of gill-bearing appen- 
dages. If this view be accepted, we must regard the abdominal 
trachez in the higher spiders, harvestmen, false-scorpions, etc., as 
simplified lung-sacs. Pocock (2) suggests that increased lightness 
would be an advantage derivable from this change; but Bernard 
(3, 7, 8) considers the trachee the more primitive, and the lung-sacs 
elaborated from them. He would derive the whole ventral series of 
arachnid breathing-organs, spinning-glands, coxal glands, and poison- 
glands from the ventral setiparous glands of an annelid ancestor. 
The tracheze of insects, on the other hand, he believes to have arisen 
from the dorsal series of setiparous glands, and he finds the hairy 
area around the stigmata of the chrysalis of the Vapourer Moth 
2G 
