NATURE AND ORIGIN OF FRESH-WATER ORGANISMS 367 
once of the fishes, most of which are probably capable of directly 
colonising our rivers and streams, and some of which (salmon, eel, 
sturgeon, lamprey) are still in the habit of migrating from salt water 
to fresh. Then there are certain Crustaceans which may very well 
have actively invaded fresh water. These are the crabs, prawns, and 
crayfishes, which by swimming or crawling would be capable of making 
headwav ao-ainst the current of a river. 
An examination into detail shows us that these forms have 
acquired characteristics which have fitted them for colonising fresh 
water in the way suggested. Most fresh-water crabs, unlike their 
marine allies, which are liberated from the egg as free-swimming larvae 
(Zoea), remain in the shelter of the female's abdomen until they have 
reached their adult form, while the young of the crayfish remain 
attached to the swimmerets of the female until able to lead an inde- 
pendent existence. In the case of fresh- water prawns, we appear to 
have merely an increase in the amount of food-volk, which at least 
ensures that the larvae are set free at a more advanced stage than the 
corresponding marine types. This is actually to be seen within the 
limits of a single species, in the case of the prawn Pakemoncetes 
vulgaris^ which is known to inhabit both the sea and fresh water. 
The eggs of the individuals living in the latter are larger, and hatch 
out at a later stage, than the eggs of marine specimens. All the 
modifications just pointed out have, of course, the one object — that 
of enabling the young to retain the hold upon fresh water which their 
parents have acquired, by the more or less complete suppression of a 
free-swimming larval form, which would be at the mercy of every 
current. 
We may perhaps be justified in including the genera Asellus and 
Gammarus among the types which have actively migrated from the 
sea ; in both cases the eggs are retained within the brood-pouch until 
the adult form is approximately reached. The leeches too we can 
consider as forms which may have actively colonised fresh water, for 
they are powerful swimmers, can attach themselves firmly to rocks or 
stones, and either deposit the eggs in a horny cocoon or carry them 
upon the ventral surface of the parent. 
That a number of the Mollusca which we find in fresh water 
migrated directly from the ocean there can be little doubt. Both 
Lamellibranchs and Gasteropods could actively accomplish this, for, 
though slowly creeping forms, they would in time reach great distances 
from the mouth of a river or stream. Here again we have striking 
examples of how — to avoid the danger of being swept out to sea — the 
free-swimming larval forms characteristic of their marine relations 
have become suppressed. In the well-known genus Bytliinia^ for 
example, eggs well provided with food-yolk are attached to stones 
