Videnskabelige meddelelser

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Videnskabelige meddelelser

41

ing point; they are found moving about in an Unio the outside

layer of which was frozen (Ha Ide man, 1842).

Scattered in the litterature from the last years (Thon 1901,

Soar 1906, Musselius 1914) we find a few rather insignificant

remarks; Koenike's paper (1915, p. 308) relating to Atax acii-

leatiis (Kæn.) (^ Unionicola) is the only more thorough study from

that period. He shows that it was the nymphs of this species

which Claparéde found in the mussels and which he supposed

to be the nymphs of Atax crassipes. Koenike shows that they

belong to A. aciileatus. In this species the female and perhaps

also the male is free living; the firstnamed looks out for the mus-

sels and lays its eggs in them. The larvæ live in the mussels

until the nymph stage; this is also free living for a certain time

but sooner or later it seeks back to the mussels to complete the

metamorphosis. Also A. tricuspis Koen. is said to be free swim-

ming in the nymph- and in the ripe stage. Also A. Bonzi has been

taken free swimming by Koenike.

With regard to all these species I cannot add any new facts.

The Atax species are very common in the mussels which I

have taken with the dredge in Furesø, Esromsø and besides in

many other lakes.

With regard to the biology of Atax crassipes (O. F. M.) the

reports diverge highly from each other. I have often had occasion

to study this interesting mite at close quarters. It occurs as a plank-

ton organism in all our large lakes; it is a decided pelagic organ-

ism living over the greatest dephts of our lakes and mostly in the

deeper layers of water. Spreading its extremely long legs radially

in all directions it is able to augment its crossectional resistance

to a very high degree ;

it therefore sinks extremely slowly through

the layers of water; it is able to keep itself in the same layer

with extremely slow locomotions. Swimming hairs are not wanting

but not developed to such high degree as we, according to its

mode of life, at a first glance should have expected. It is pro-

vided with powerful movable thorns which can be laid parallel to

the legs when these are moved, but placed at a right angle out

from the legs when these are spread out and used as outriggers.

In accordance with its life as plancton organism it is constructed

much more as a floating than as an active-motive organism. This

has hitherto not been understood by the Hydrachnologists.

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