40 stage in the spaces between the piates of which the gill is com- posed. This stage is short and the nymph is soon formed either in the spaces in the gills or outside them in the mucus over their surface. In the latter case the nymphs accumulate in the mass of mucus at the exhalent aperture. During the second larval stage the mite is very active and moves freely about between mantle and gills. During this stage the mites often voluntarily mi- grate from the mussels, swim here and there or clamber over the bottom ; they probably try to find another mussel as lodgment; how usual the life outside the mussels is we do not know; the author has never dredged them free in the bottom deposits. There seems to be no particular time of the year when eggs are deposited; apparently most numerous in the summer, while during early autumn the adult mites are most abundant. Kelly (1899, p. 414) found, when the water grew colder, relatively fewer adult AtcLv and more abundant eggs. Lodgment is effected pro- bably with different degrees of facility in different species of mus- sels. Those which are most sensitive, most active in closing their Shells and whose shells close most tightly seem to be more immune from the presence of parasites. Neither size of the mussel nor sex seems to affect in any degree the extent of parasitism. With regard to the dephts to which the Hydrachnids go down and the relation to bottom deposits of different kind, vegetation etc. we do not know anything. The maximum number of mites found in a single shell can be extremely high ; in one mussel 15 A. ifpsilophoriis, 406 A. intennediiis were taken The presence of the mites seems to entail few if any ill consequences on the host. Garner (1864, p. 114) claims that their presence causes the growth of pearly prominences on the inside of the shell. Upon the mites themselves the effect is more pronounced. The tracheal system is less perfectly developed, in some cases becoming quite rudimentary ; the body of the parasitic form is large in relation to the length of the legs; the locomoiive power especially that of swimming is but feeble especially in the ripe stage. Species living nearest to the edge of the mantle retain the length of the legs, small size of body and activity of movement almost unimpaired. They can live for weeks outside the mussels and do not suffer from cold ; they are active in water a few degrees above the freez-
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.