360 THE FRESH-WATER LOCHS OF SCOTLAND 
ing in the sea and others in fresh water. This is in itself an indica- 
tion that no very wide gulf is actually fixed between fresh-water and 
marine organisms, and that in point of fact, given suitable conditions, 
representatives of the most diverse classes have been able to accom- 
modate themselves to life in a medium of greater or less density. 
But we have direct evidence on this head in certain cases. The 
probable existence of relict seas has already been referred to, and they 
presuppose the survival of ocean forms in fresher water, although the 
gradual modification may have taken place in past geological time. 
Yet there are instances known which seem clearly to show that 
the process of accommodation to a different medium still proceeds, 
and that quite a number of forms are capable of withstanding 
important changes in salinity. The hydroid polype Cordylophora 
lacustris was orginally discovered in brackish water ; it is common in 
the Norfolk Broads, where there is a considerable admixture of sea- 
water, and is known elsewhere as an estuarine form. Still, it has been 
able to migrate into entirely fresh water, for it has been found in the 
Seine near Paris, in the fresh-water tanks of the Jardin des Plantes, 
and has actually invaded the water-mains of the city of Hamburg.^ 
Another case which indicates the possibilities for an even more 
sensitive type, is that of Crambessa tagi, a large Discomedusan which 
commonly ascends the river Tagiis until it reaches comparatively fresh 
water. 
A more extreme example, embracing animals from several groups, 
is afforded by the fauna of certain artificial ponds at Port Canning, 
Lower Bengal.^ Situated in the neighbourhood of the Ganges delta, 
these ponds are sometimes in communication with the estuary, from 
which they have undoubtedly derived the marine forms which interest 
us. At other times, however, they are completely isolated, and may 
become even more strongly saline than the sea through continued 
evaporation, or during the rainy season may become nearly fresh. 
The most striking of the marine types referred to, which are capable 
of withstanding such profound changes in the nature of the water, 
are a sea-anemone {Metridnim)^ a Hydromedusan with hydroid 
stage {Irene)^ a Cirripede {Balanus), a cheilostomatous Polyzoan 
(Membranipo?'a), and a Polychaste worm. 
Finally, there is an interesting account given by von Kennel* of 
the inhabitants of a lagoon on the east coast of Trinidad, which at 
times is flooded by the sea, and at other times becomes almost 
1 Semper, The Natural Conditions of Existence as they affect Animal Life, 5th 
ed., London, 1906, p. 152. 
2 Haeckel, Zeitschr. f. wiss. Zool, Bd. xix., 1869, p. 509. 
^ Annandale, Records Indian Museum, vol. i., 1907, p. 35. 
4 Arh. Zool. Inst. Wiirzhurg, Bd. vi., 1883, p. 276. 
