Videnskabelige meddelelser

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Videnskabelige meddelelser

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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-