A=single zoœcium without adventitious buds but with a young resting bud (b), × 70 (dorsal view); B=lateral view of a smaller zoœcium without buds, × 70; C=upper part of a zoœcium with a single adventitious bud, × 70; D=outline of the upper part of a zoœcium with adventitious buds of several generations, × 35; E=remains of a zoœcium with two resting buds (b) attached. All the specimens figured are from Port Canning and, except D, are represented as they appear when stained with borax carmine and mounted in canada balsam.
Resting buds. The resting buds (fig. 31, p. 170) are somewhat variable in shape but are always flat with irregular cylindrical or subcylindrical projections round the margin, on which the horny coat is thinner than it is on the upper surface. This surface is either smooth or longitudinally ridged.
Type in the Indian Museum.
This species differs from the European V. pavida in very much the same way as, but to a greater extent than, the Indian race of Bowerbankia caudata does from the typical English one (see p. 189). The growth of the zoarium is much more luxuriant, and the form of the resting buds is different.
Geographical Distribution.—V. bengalensis is abundant in pools of brackish water in the Ganges delta and in the Salt Lakes near Calcutta; it also occurs in ponds of fresh water near the latter. I have received specimens from Madras from Dr. J. R. Henderson, and it is probable that the form from Bombay referred by Carter to Paludicella belonged to this species.
Biology.—In the Ganges delta V. bengalensis is usually found coating the roots and stems of a species of grass that grows in and near brackish water, and on sticks that have fallen into the water. It also spreads over the surface of bricks, and I have found a specimen on a living shell of the common mollusc Melania tuberculata. Dr. Henderson obtained specimens at Madras from the surface of a freshwater shrimp, Palæmon malcolmsonii. In the ponds at Port Canning the zoaria grow side by side with, and even entangled with those of Bowerbankia caudata subsp. bengalensis, to the zoœcia of which their zoœcia bear a very strong external resemblance so far as their distal extremity is concerned. This resemblance, however, disappears in the case of zoœcia that bear terminal buds, for no such buds are borne by B. caudata; and the yellowish tint of the zoaria of V. bengalensis is characteristic. Zoaria of the entoproct Loxosomatoides colonialis and colonies of the hydroid Irene ceylonensis are also found entangled with the zoaria of V. bengalensis, the zoœcia of which are often covered with various species of Vorticellid protozoa and small rotifers. The growth of V. bengalensis is more vigorous than that of the other polyzoa found with it, and patches of B. caudata are frequently surrounded by large areas of V. bengalensis.
The food of V. bengalensis consists largely of diatoms, the siliceous shells of which often form the greater part of its excreta. Minute particles of silt are sometimes retained in the gizzard, being apparently swallowed by accident.
There are still many points to be elucidated as regards the production and development of the resting buds in V. bengalensis, but two facts are now quite clear as regards them: firstly, that these buds are produced at the approach of the hot weather and germinate in November or December; and secondly, that the whole zoarium may be transformed at the former season into a layer of resting buds closely pressed together but sometimes exhibiting in their arrangement the typical cruciform formation. Resting buds may often be found in vigorous colonies as late as the beginning of December; these buds have not been recently formed but have persisted since the previous spring and have not yet germinated. Sometimes only one or two buds are formed at the base of an existing zoœcium (fig. 37 a), but apparently it is possible not only for a zoœcium to be transformed into a resting bud but for it to produce four other buds round its base before undergoing the change. Young polypides are formed inside the buds and a single zoœcium sprouts out of each, as a rule by the growth of one of the basal projections, when conditions are favourable.
Polypides of V. bengalensis are often transformed into brown bodies. When this occurs the orifice closes together, with the collar expanded outside the zoœcium. I have occasionally noticed that the ectocyst of such zoœcia was distinctly thicker and darker in colour than that of normal zoœcia.
Eggs and spermatozoa are produced in great numbers, as a rule simultaneously in the same zoœcia, but individuals kept in captivity often produce spermatozoa only. The eggs are small and are set free as eggs. Nothing is known as regards their development.