Polyzoa of Brackish Water.
With the exception of Victorella, which occurs more commonly in brackish than in fresh water and has been found in the sea, the genera that occur in fresh water are confined or practically confined to that medium; but certain marine ctenostomes and cheilostomes not uncommonly make their way, both in Europe and in India, into brackish water, and in the delta of the Ganges an entoproctous genus also does so. The ctenostomatous genera that are found occasionally in brackish water belong to two divisions of the suborder, the Vesicularina and the Alcyonellea. To the former division belongs Bowerbankia, a form of which (B. caudata subsp. bengalensis, p. 187) is often found in the Ganges delta with Victorella bengalensis. No species of Alcyonellea has, however, as yet been found in Indian brackish waters. The two Indian cheilostomes of brackish water belong to a genus (Membranipora) also found in similar situations in Europe. One of them (M. lacroixii[[AZ]]) is, indeed, identical with a European form that occurs in England both in the sea and in ditches of brackish water. I have found it in the Cochin backwaters, in ponds of brackish water at the south end of the Chilka Lake (Ganjam, Madras), on the shore at Puri in Orissa, and in the Mutlah River at Port Canning. The second species (M. bengalensis, Stoliczka) is peculiar to the delta of the Ganges[[BA]] and has not as yet been found in the open sea. The two species are easily recognized from one another, for whereas the lip of M. bengalensis (fig. 33) bears a pair of long forked spines, there are no such structures on that of M. lacroixii, the dorsal surface of which is remarkably transparent. M. lacroixii forms a flat zoarium, the only part visible to the naked eye being often the beaded margin of the zoœcia, which appears as a delicate reticulation on bricks, logs of wood, the stems of rushes and of hydroids, etc.; but the zoarium of M. bengalensis is as a rule distinctly foliaceous and has a peculiar silvery lustre.
Fig. 33.—Outline of four zoœcia of Membranipora bengalensis, Stoliczka (from type specimen, after Thornely). In the left upper zoœcium the lip is shown open.
Loxosomatoides[[BB]] (fig. 34), the Indian entoproctous genus found in brackish water, has not as yet been obtained from the open sea, but has recently been introduced, apparently from a tidal creek, into isolated ponds of brackish water at Port Canning. It is easily recognized by the chitinous shield attached to the ventral (posterior) surface.
Fig. 34.—Loxosomatoides colonialis, Annandale.
A and B, a single individual of form A, as seen (A) in lateral, and (B) in ventral view; C, outline of a similar individual with the tentacles retracted, as seen from in front (dorsal view); D, ventral view of an individual and bud of form B. All the figures are from the type specimens and are multiplied by about 70.
[AW] Q. J. Micr. Sci. xxxiii, p. 123 (1892).