The Phylactolaemata are by no means uncommon, although they can seldom be found without a careful search. Their presence may often be detected by taking advantage of the property of the free statoblasts of rising to the top of the water, where they can be discovered by skimming the surface with a fine hand-net.
The colonies themselves are usually found attached to water-plants, roots of trees or stones. Most of them flourish best in a zone not more than two feet below the surface. Certain species show a preference for floating leaves, such as those of water-lilies, where they are not liable to be dried up by alterations in the level of the water. Some forms (e.g. Plumatella, Fig. 246) are, however, able to withstand being dried for some time. Most species prefer shady places, and accordingly settle on the lower sides of leaves and sticks. Others (e.g. Cristatella, Fig. 247) have no objection to the direct rays of the sun. Most forms prefer still water, but one or two are found in running water.
Fredericella is a common constituent of the deep-water fauna of Swiss Lakes (down to over forty fathoms); and reaches there a size considerably larger than the shallow-water form of the same species. Paludicella is common at thirteen fathoms. These two genera, with Plumatella, have been found in absolute darkness, under a pressure of 2½-5½ atmospheres, in the Hamburg aqueduct. The Polyzoa and other organisms growing in the water-supply of Hamburg were accused of being concerned in the spreading of cholera, during the recent epidemic, by choking up the water-pipes, and creating obstructions which formed a favourable nidus for the development of cholera-germs.
The colony may take the form of a series of delicate, branching tubes (Plumatella, Fredericella), of more massive aggregations of parallel tubes (as in the Alcyonelloid forms of Plumatella), or of gelatinous masses of varying size (Lophopus, Cristatella).
Fig. 246.—A, Plumatella (Alcyonella) fungosa Pall., Naples (fresh water), small part of a mass, natural size; B, Plumatella repens L., R. Yare, on the leaf of a water-lily, natural size.
Cristatella mucedo (Fig. 247) is remarkable for its power of moving from place to place; it consists of an elongated mass of greenish, gelatinous substance, which, in its fully developed state, may reach a length of eight inches or more, with a transverse diameter of three-eighths of an inch. It has a flattened sole on which it crawls, while the graceful plumes of its numerous polypides protrude as a delicate fringe from its upper side.
The tentacles are about eighty to ninety in number, and they are, as in other Phylactolaemata, united at their bases by a delicate web. The lophophore is horse-shoe-shaped (Fig. 236, 3) throughout the group, with the exception of Fredericella, in which genus it is circular.
In some Phylactolaemata the polypide has been observed to interlace its tentacles, so that the plume becomes a kind of cage, in which the more active Infusoria are imprisoned until their struggles have so far weakened them that they are swept into the mouth by the action of the cilia of the tentacles.[[552]]
