368 THE FUESH-WATER LOCHS OF SCOTLAND 
and water-plants, and the young emerge in practically the adult 
condition. In Paludina^ a stage further has been reached, for the 
ova are retained within the body of the parent, and the young are 
born alive. This is similarly the case in the fresh-water bivalves 
Cyclas and Pisidium, which are provided with brood-pouches in 
which the eggs develop. 
Turning to consider passive migration from the sea, we realise 
that, if this has taken place, it must have been mainly by the trans- 
port of sessile or feebly-swimming forms, through the agency of those 
which are actively locomotive. We have already seen how tidal 
influence may carry certain marine organisms for some miles inland, 
but this process could not effect the colonising of more than an estuary, 
and that only under exceptional circumstances. While it is likely 
that a considerable number of small organisms, both animal and 
vegetable, are passively carried from the shores of the ocean into 
rivers and lakes, it is improbable that many survive the sudden change 
in environment. It is conceivable, for instance, that ova or encysted 
animals might be left dry upon the beach, and transported by winds 
to fresh-water surroundings, but there is not much likelihood that 
they would successfully accommodate themselves to the altered 
conditions. Again, quite a number of diverse organisms might be 
carried from the sea- shore to fresh water sticking to the feet of 
wading birds, and some forms might adhere to active immigrants 
such as fishes and perhaps Crustaceans. 
A case which seems fully proved, in which animals have been con- 
veyed by fish directly from the sea to fresh water, is that of the 
parasitic fish-louse Argulus. Species inhabiting both fresh and salt 
water have long been known to occur, but it was reserved for Wilson ^ 
to prove by experiment that, in certain instances at any rate, the 
change of medium produced little effect, even if suddenly made. 
Other parasitic forms which are probably direct but passive immi- 
grants from the sea are Lernccocera^ Achtheres^ and a Bopyrid.^ 
A truly remarkable example of sessile forms which take advantage 
of the locomotory power of fishes may find fitting mention here. 
We refer to the interesting reproductive habits of the fresh- water 
mussels Anodon and Unio. The ova undergo partial development 
within the parent, but, arriving at the larval stage known as the 
glochidium, are expelled into the water, provided with a long 
adhesive filament. If the latter comes into contact with a passing 
fish, the little larva becomes attached, and by means of the sharp 
spines on its shell secures its hold. The epithelial layers of the fish 
soon grow to enclose the embryo in a definite cyst, and within this 
1 Proc. U.S. Mus., vol. XXV., 1903, p. 648. 
Semper, op. cit, p. 147. 
