Some polyzoa are oviparous, while in others a larva is formed within the zoœcium and does not escape until it has attained some complexity of structure. Both the ctenostomatous genera that are found in fresh water in India are oviparous, but whereas in Victorella the egg is small and appears to be extruded soon after its fertilization, in Hislopia it remains in the zoœcium for a considerable time, increases to a relatively large size, and in some unknown manner accumulates a considerable amount of food-material before escaping. Unfortunately the development is unknown in both genera.
In the phylactolæmata the life-history is much better known, having been studied by several authors, notably by Allman, by Kraepelin, and by Braem (1908). The egg is contained in a thin membrane, and while still enclosed in the zoœcium, forms by regular division a hollow sphere composed of similar cells. This sphere then assumes an ovoid form, becomes covered with cilia externally, and breaks its way through the egg-membrane into the cavity of the zoœcium. Inside the embryo, by a process analogous to budding, a polypide or a pair of polypides is formed. Meanwhile the embryo has become distinctly pear-shaped, the polypide or polypides being situated at its narrow end, in which a pore makes its appearance. The walls are hollow in the region occupied by the polypide, the cavity contained in them being bridged by slender threads of tissue. The larva thus composed makes its way out of the zoœcium, according to Kraepelin through the orifice of a degenerate bud formed for its reception, and swims about for a short time by means of the cilia with which it is covered. Its broad end then affixes itself to some solid object, the polypide is everted through the pore at the narrow end and the whole of that part of the larva which formerly enclosed it is turned completely inside out. A zoarium with its included polypides is finally produced from the young polypide by the rapid development of buds.
(b) From the Statoblast and Resting Buds.
There is little information available as regards the development of the young polyzoon in the resting buds of the freshwater ctenostomes. In Paludicella and Pottsiella the capsule of the bud splits longitudinally into two valves and the polypide emerges between them; but in Victorella bengalensis one of the projections on the margin of the bud appears to be transformed directly into the tip of a new zoœcium and the capsule is gradually absorbed.
Contradictory statements have been made as regards several important points in the development of the statoblast and it is probable that considerable differences exist in different species. The following facts appear to be of general application. The cellular contents of the capsule consist mainly of a mass of cells packed with food-material in a granular form, the whole enclosed in a delicate membrane formed of flat cells. When conditions become favourable for development a cavity appears near one end of the mass and the cells that form its walls assume a columnar form in vertical section. The cavity increases rapidly in size, and, as it does so, a young polypide is budded off from its walls. Another bud may then appear in a similar fashion, and the zoœcium of the first bud assumes its characteristic features. The capsule then splits longitudinally into two disk-like valves and the young polypide, in some cases already possessing a daughter bud, emerges in its zoœcium, adheres by its base to some external object and produces a new polyparium by budding. The two valves of the statoblast often remain attached to the zoarium that has emerged from between them until it attains considerable dimensions (see [Plate IV], fig. 3 a).
What conditions favour development is a question that cannot yet be answered in a satisfactory manner. Statoblasts can lie dormant for months and even for years without losing their power of germinating, and it is known that in Europe they germinate more readily after being subjected to a low temperature. In tropical India this is, of course, an impossible condition, but perhaps an abnormally high temperature has the same effect. At any rate it is an established fact that whereas the gemmules of most species germinate in Europe in spring, in Bengal they germinate either at the beginning of the "rains" or at that of our mild Indian winter.
Movements.
Fig. 32.—Zoarium of Lophopodella carteri moving along the stem of a water plant, × 4. (From Igatpuri Lake.)
In the vast majority of the polyzoa, marine as well as freshwater, movement is practically confined to the polypide, the external walls of the zoœcium being rigid, the zoœcia being closely linked together and the whole zoarium permanently fixed to some extraneous object. In a few freshwater species belonging to the genera Cristatella, Lophopus, Lophopodella and Pectinatella, the whole zoarium has the power of progression. This power is best developed in Cristatella, which glides along with considerable rapidity on a highly specialized "sole" provided with abundant mucus and representing all that remains of the ectocyst. It is by no means clear how the zoaria of the other genera move from one place to another, for the base is not modified, so far as can be seen, for the purpose, and the motion is extremely slow. It is probable, however, that progression is effected by alternate expansions and contractions of the base, and in Lophopodella (fig. 32), which moves rather less slowly than its allies, the anterior part of the base is raised at times from the surface along which it is moving. The whole zoarium can be released in this way and occasionally drops through the water, and is perhaps carried by currents from one place to another in so doing.