Hydrozoa and Ctenophora. log 
be endemic in the Far East and, indeed, may have originated from a genus that 
lives in comparatively deep and therefore cold water in Japan. Browne' has shown 
that Microhydra ryderi and Craspedacustes sowerbyi can hardly be specifically identi- 
cal, unless the organism has been profoundly modified by unnatural conditions of 
life ; but it is improbable that any student of the hydroids would place the polyp 
from Kew in a different genus from the one originally described from Philadelphia 
and since rediscovered at Strassburg. There can be no doubt of the Holarctic origin 
of the latter species at any rate. 
Limnocnida on the other hand has only been found in the tropics. The fact 
that it occurs on both sides of the Indian Ocean is noteworthy.^ It is curious, further- 
more, that in the Oriental Region the medusa has only been found in tributaries of 
the River Kistna or Krishna, in which also occurs the only Oriental representative of 
the moUuscan family Aetheriidae,' a family that flourishes in the tropical parts of 
Africa and America. 
What has been said will be sufficient to prove that the Hydrozoa of fresh water 
have no very close relationships either among themselves or with forms that have 
become established in brackish water. Limnocnida is perhaps remotely related to 
Craspedacustes and the latter probably belongs to the same subfamily as Asenathia ; 
but the different genera undoubtedly represent different attempts made, sometimes 
successfully, at different places and at different epochs to colonize fresh water on the 
part of marine organisms, all of which have either become modified in the course of 
their progress inland, or else have chanced to become modified in the sea in such a 
way as to have been rendered fitter thereby for life in rivers or lakes and thus have 
been enabled to migrate inland. No common line of evolution has been followed, 
and the structural modifications that have been brought about do not seem to be 
correlated with changes in the specific gravity or chemical constitution of the me- 
dium in which the animals live ; adaptation for this purpose has been physiological 
rather than structural. Hargitt^ has shown that the pulsation of the bell of Cras- 
pedacustes, though active in ordinary fresh water, cease in distilled water. I have 
myself shown that a precisely similar result follows in the case of the Schizostomous 
medusa Acromitus if the salinity is reduced below a certain limit.* Thus two me- 
dusae not in any way related to one another, both of which have become adapted 
to live in water of lower salinity and specific gravity than that of the open sea, are 
affected in a similar manner by changes in the composition of the medium in which 
they live, but the one that has estabhshed the fluviatile habit completely is no longer 
affected by a change that produces complete paralysis in the other, which inhabits 
both the open sea and lagoons of brackish water. Neither of these species is 
obviously adapted anatomically, so far as our present knowledge goes, to withstand 
permanent or temporary changes in the chemistry of its environment. In all fresh- 
1 Quart. Journ. Micr. Sci. h, p. 638 ( 1906). 
2 Aniiandale, IV Congrds Internal. ZooL, Monaco 1913, Sec. VI, p. 581 (1914). 
3 Mulleria daiyi, Smith, Proc. Mai. Soc. London III, p. 13 (1898). See also Woodward, ibid., p. 87. 
* Biol. Bull. Washington XIV, p. 304 (fide Mayer). 
^ Mem. Ind. Mus. V, p. loi (1915). 
