Hydrozoa and Ctenophora. 105 
sea, is the only representative of its genus that has become a true inhabitant of 
brackish water. In suitable localities on the shores of the Bay of Bengal and the 
Gulf of Siam it seems to have occupied the position in the estuarine fauna that is 
occupied in many temperate localities by the unrelated Cordylophora lacustris. 
Though capable, however, of Hving for some time in fresh water, it does not seem to 
have adopted the fluviatile and lacustrine habits of that species. 
We do not know as yet anything definite about the life-history of Campanularia 
senulata, which belongs to a universally distributed marine genus ; I include it among 
the inhabitants of brackish water because I have found it reproducing its gonosome 
in an apparently healthy condition in water of low specific gravity. The species has 
evidently a wide range in the Pacific and Indian Oceans, but particulars are still 
lacking and there has been considerable confusion with species of allied genera in 
which the hydrophyton is very similar. 
Campanulina is another marine genus of universal distribution. It probably 
includes many minute species that have up to the present escaped notice. The 
medusa of C. ceylonensis was first discovered in the open sea off Ceylon, but not far 
from the coast. The hydroid (which is very small, inconspicuous and difficult to 
preserve in a recognizable condition) has only been found as yet in brackish water, 
in the Gangetic delta and probably in the Tale Sap. The species is able to complete 
its full life-cycle in water of a specific gravity (corrected) of 1*0085, but apparently 
perishes if the specific gravity sinks much below 1006. 
Our knowledge of the fauna of tropical estuaries and marine lakes is still by far 
too incomplete to permit a detailed statement of the geographical affinities of any 
element in it. All that can be said in reference to the Hydrozoa is that in this 
group, as in the Polyzoa,' some widely distributed marine species are remarkably 
tolerant of low and even variable salinity, while others appear to have a strictly 
limited range in estuaries and marine lakes and to belong to small genera peculiar to 
definite areas. 
From a biological point of view it is already possible to separate the Coelen- 
terates of brackish water into two series, one merely tolerant, the other specialized 
to live in estuarine creeks and similar situations. Among the Oriental forms Campa- 
nularia senulata, Campanulina ceylonensis and Acromitus rahanchatu belong to the 
former category; to the latter Bimeria fluminalis, the species and genera of Actin- 
iaria referred to in a footnote on p. 104, and the abnormal hydrozoan genera Annulella, 
Asenathia and Dicyclocoryne. All the merely tolerant species, and a large pro- 
portion of those that have established themselves permanently in brackish water, 
represent cosmopolitan or at any rate widely distributed marine genera, and though 
specialized physiologically do not appear to be modified structurally. The three 
(or possibly only two) peculiar hydrozoan genera and the Actinian genera Pelocoetes 
and Phytocoetes are all highly modified structurally, and no one of them is known 
to have very near allies among true marine forms. They must, therefore, be 
' Annandale Mem. Ind. Miis. V, pp. 119-133 (1915). 
