Hydrozoa and Ctenophora. 107 
tropical estuaries and rivers, visual organs are degenerate or absent. A character of 
the hydrophyton that has much less systematic importance is thus described in D. 
fUameniata,' "the colonies of this species often have a peculiarly lax appearance owing 
to the fact that the rhizome is adherent only in places and is sometimes produced into 
long filamentous free processes that bear terminal polyps. These, or rather the stalks 
from which they arise, may again become attached at their base to the object on 
which the colony is growing, so that loops of free rhizome are formed." A similar 
feature occurs in two distantly related Ctenostomatous Polyzoa' that live in the 
Gangetic delta, and is evidently a protection against overwhelming mud. The 
Polyzoa are Victorella hengalensis and Bowerhankia caudata. 
We may now compare the Oriental Hydrozoa of brackish water with those that 
have established themselves completely in fresh water. Not more than five genera 
are yet known from any part of the world. They are Hydra, Microhydra, Craspeda- 
custes {==Limnocodium), Limnocnida and Polypodimn. The marine genus Cordylophora 
of the normal family Clavidae has at least one ^ species (C. lacustris, Allman) that is 
able to inhabit pure fresh water and has been found far inland, but this species seems 
to be essentially an inhabitant of estuaries and salt lakes and has probably been 
carried from sea to sea on the bottom of ships. 
Hydra is a cosmopolitan genus consisting probably of three species,* H. vulgaris, 
Pallas,'' H. oligactis, Pallas and H. viridis, Linne. These species are inhabitants of 
fresh water but are found rarely in brackish water. The only form really at home 
in the tropics seems to be H. vulgaris. There is no doubt that the genus is an 
extremely primitive one, and it seems improbable that it ever had a medusoid genera- 
tion. The fertilized egg develops a horny coat which enables it in temperate regions 
to lie dormant through the winter, and through the summer in the tropics. 
Only the polyp and the young medusa of Microhydra are known, and it is not 
absolutely certain that the adult medusa commonly assigned to Limnocodiwn is 
conspecific with the ver}^ Microhydra-\\ke polyp from which it is believed, on cir- 
cumstantial evidence, to originate. In any case it is doubtful whether Microhydra 
and Craspedacustes are generically distinct." In both forms, if the commonly ac- 
cepted identifications are correct, the medusa originates from a polyp without ten- 
tacles and only to a limited extent colonial and is remarkable chiefly for its very 
peculiar sense organs. It has a long and well developed manubrium and is not 
modified in any very obvious adaptive manner. Microhydra ryderi, Potts, has been 
found in rivers both in North America and in Europe, while Craspedacustes sowerbyi,^ 
Lankester (the medusa and supposed polyp) were originally described from a lily-pond 
I Annandale, Mem. Ind. Mas. \, p. 110(1915). 
■2 Annandale, Mem. As. Soc. Bengal VI, p. 25 (1916), aad Mem. Ind. Miis. V, p. 1J5 (1915). 
3 I think it probable that C. whiteleggei, v. Lendenfeld, from Australia, is merely a dwarfed form of this species. See 
von Lendenfeld, Zool. Jahrb. II, p. 97, pi. vi, figs, n-12 {18.S7). 
♦ Annandale, Faun. Brit. Ind. Freshw. Sponges, etc., p. 147 (1911). 
6 The only species I saw in Japan was this one. There is a specimen from Lake Biwa in the Otsu Laboratory. 
6 See Potts, Quart. Journ. Micr. Sci. I, p. 623, pis. xxxv-xxxvi, and Browne, ibid., p. 635, pi. xxxvii (1906). 
7 For literature previous to 1910 see Mayer's Medusae of the World II, pp. 363, 364 ; Douglas (Zeits. wiss. Zool. CII 
(i), pp. 92-110, pi. vi ; 1912) has discussed origin and relationships more recently. 
