In Pedicellina the plane of the lophophore is at right angles to the stalk, which is separated from its calyx by a marked constriction. In Loxosoma the lophophore is set obliquely,[[542]] and there is no constriction at the base of the calyx. In Urnatella we find an intermediate condition, the lophophore resembling that of Loxosoma, while the constriction at the base of the calyx is similar to that of Pedicellina. Since the latter is known to pass in its development[[543]] through a stage with an oblique lophophore, it may be presumed that Loxosoma is a more archaic form than Pedicellina. In other respects, the structure of the Entoprocta is very constant, whatever the genus.

A pair of ciliated excretory tubes open into the vestibule. These are similar in structure to the "head-kidneys" of the larvae of Polychaet worms, or to the excretory organs of adult Rotifers. Flame-cells have been described by Davenport in the stalk of Urnatella, but it is not known whether they are connected with the excretory tubes of the calyx. The animals are either hermaphrodite or have separate sexes, and the generative organs open by ducts of their own into the vestibule. The nervous system consists of a ganglion placed between the mouth and the anus, giving off a set of nerves, many of which end in delicate tactile hairs placed on the tentacles or other parts of the body.[[544]]

CHAPTER XVIII

POLYZOA (continued)

FRESH-WATER POLYZOA—PHYLACTOLAEMATA—OCCURRENCE—STRUCTURE OF CRISTATELLA—DIVISION OF COLONY—MOVEMENTS OF COLONY—RETRACTION AND PROTRUSION OF POLYPIDES IN POLYZOA—STATOBLASTS—TABLE FOR DETERMINATION OF GENERA OF FRESH-WATER POLYZOA—REPRODUCTIVE PROCESSES OF POLYZOA—DEVELOPMENT—AFFINITIES—METAMORPHOSIS—BUDDING.

Fresh-water Polyzoa.—Although the Gymnolaemata are ordinarily marine animals, fresh-water examples from this Order are not altogether wanting. The Ctenostomata among the typically marine groups show the most tendency to stray into fresh-water.

Alcyonidium and Bowerbankia (Fig. 238) flourish in estuaries, while Victorella and Paludicella (Fig. 250) are only known as fresh or brackish water forms. Victorella was named after the Victoria Docks in London, where it was first found; more recently it has also been discovered in other parts of England and on the Continent.[[545]]

The systematic position of the genera Hislopia and Norodonia,[[546]] which have been described from fresh water of India and China respectively, is at present uncertain. The undoubted Cheilostome Membranipora has, however, a British representative (M. monostachys), which occurs in brackish water, in ditches on the coast of East Anglia. It is there known to form "friable, irregularly-shaped, sponge-like masses," which grow on water-plants.[[547]]

The Entoprocta, as we have seen, are represented in fresh water by the genus Urnatella.

The Phylactolaemata are an exclusively fresh-water group, and they are believed by Kraepelin[[548]] to have been derived from the Ctenostomata. Many of their special peculiarities can, with great probability, be regarded as adaptations to a fresh-water existence. This is particularly clear in the all but universal habit of dying down in the winter, and in the occurrence of the so-called statoblasts (Fig. 251), which are hard-shelled reproductive bodies, absolutely restricted to the Phylactolaemata, and capable of resisting the winter's cold and even a certain amount of drying up. Phylactolaemata have indeed been recorded from the tropics; but it is not yet sufficiently clear how they there behave in these respects. F. Müller[[549]] has found these animals in Brazil, where they are said to be more common at certain periods of the year than at others. Stuhlmann has found them in Tropical Africa (Victoria Nyanza, etc.);[[550]] and Meissner[[551]] has discovered the sessile statoblasts of Plumatella on the shells preserved in the Berlin Museum, of species of the Mollusc Aetheria from various localities in Africa. Fresh-water representatives of a considerable number of other groups of animals agree with the Phylactolaemata in the possession of reproductive bodies which are protected by hard coats. Such, for instance, are the ephippian ova of Daphnia—bodies which have an extraordinary external similarity to statoblasts—the gemmules of Spongillidae, the winter-eggs of Rhabdocoels and Rotifers, and the cysts of Protozoa. The evolution of these bodies in so many widely different cases may have been due to the selection of variations calculated to minimise the dangers attendant on the drying up of the water in summer, or on its freezing in winter.