Geographical Distribution.—The typical form is common in northern India and occurs also in Lower Burma; the subspecies moniliformis appears to be confined to Lower Bengal, while the varieties cambodgiensis and sinensis both occur in China, the former having been found also in Cambodia and Siam. Indian and Burmese localities are:—Bengal, Calcutta (subsp. moniliformis); Berhampur, Murshidabad district (J. Robertson Milne): Central Provinces, Nagpur (Carter): United Provinces, Bulandshahr (H. J. Walton): Burma, Pegu-Sittang Canal (Kirkpatrick).
Biology.—Regarding the typical form of the species Major Walton writes (Rec. Ind. Mus. iii, p. 296):—"In volume i (page 177) of the Records of the Indian Museum, I described the two forms of colonies of Hislopia that I had found in the United Provinces (Bulandshahr). Of these, one was a more or less linear arrangement of the zoœcia on leaves and twigs, and the other, and more common, form was an encrusting sheath on the outer surface of the shells of Paludina. During the present 'rains' (July 1908) I have found many examples of what may be considered a much exaggerated extension of the latter form. These colonies have been on bricks, tiles, and other submerged objects. The largest colony that I have seen so far was on a tile; one side of the tile was exposed above the mud of the bottom of the tank, and its area measured about 120 square inches; the entire surface was almost completely covered by a continuous growth of Hislopia. Another large colony was on a piece of bark which measured 7 inches by 3 inches; both sides were practically everywhere covered by Hislopia."
Major Walton also notes that in the United Provinces the growth of Hislopia is at its maximum during "rains," and that at that time of year almost every adult Paludina in a certain tank at Bulandshahr had its shell covered with the zoœcia. The Calcutta race flourishes all the year round but never forms large or closely compacted zoaria, those on shells of Vivipara exactly resembling those on leaves of Vallisneria.
In Calcutta both eggs and spermatozoa are produced at all times of the year simultaneously in the same zoœcia, but the eggs in one zoœcium often vary greatly in size. When mature they reach relatively considerable dimensions and contain a large amount of food material; but they are set free from the zoœcium as eggs. They lie loose in the zoœcium at a comparatively small size and grow in this position. Nothing is known as regards the development of Hislopia.
Both forms of the species appear to be confined to water that is free from all traces of contamination with brine.
The polypide in this order possesses a leaf-like ciliated organ (the epistome) which arises within the lophophore between the mouth and the anus and projects upwards and forwards over the mouth, which it can be used to close. The zoœcia are never distinct from one another, but in dendritic forms such as Plumatella the zoarium is divided at irregular intervals by chitinous partitions. The lophophore in most genera is horseshoe-shaped instead of circular, the part opposite the anus being deeply indented. There are no parietal muscles. The orifice of the zoœcium is always circular, and there is no trace of any structure corresponding to the collar of the ctenostomes. The tentacles are always webbed at the base.
All the phylactolæmata produce the peculiar reproductive bodies known as statoblasts.
The phylactolæmata, which are probably descended from ctenostomatous ancestors, are confined to fresh or slightly brackish water. Most of the genera have a wide geographical distribution, but (with the exception of a few statoblasts of almost recent date) only one fossil form (Plumatellites, Fric. from the chalk of Bohemia) has been referred to the order, and that with some doubt.
It is convenient to recognize two main divisions of the phylactolæmata, but these divisions hardly merit the distinction of being regarded as suborders. They may be called Cristatellina and Plumatellina and distinguished as follows:—