Biology.—The zoaria of the species of Plumatella are found firmly attached to stones, bricks, logs of wood, sticks, floating seeds, the stems and roots of water-plants, and occasionally to the shells of molluscs such as Vivipara and Unio. Some species shun the light, but all are apparently confined to shallow water.

Various small oligochæte worms (e. g., Chætogaster spongillæ,[[BG]] Nais obtusa, Nais elinguis, Slavina appendiculata and Pristina longiseta[[BH]]), take shelter amongst them; dipterous larvæ of the genus Chironomus often build their protective tubes at the base of the zoaria, and the surface of the zoœcia commonly bears a more or less profuse growth of such protozoa as Vorticella and Epistylis. I have seen a worm of the genus Chætogaster devouring the tentacles of a polypide that had been accidentally injured, but as a rule the movements of the lophophore are too quick to permit attacks of the kind, and I know of no active enemy of the genus. The growth of sponges at the base of the zoaria probably chokes some species, but one form (F. fruticosa) is able to surmount this difficulty by elongating its zoœcia (p. 219). A small worm (Aulophorus tonkinensis) which is common in ponds in Burma and the east of India as far west as Lucknow, often builds the tube in which it lives mainly of the free statoblasts of this genus. It apparently makes no selection in so doing but merely gathers the commonest and lightest objects it can find, for small seeds and minute fragments of wood as well as sponge gemmules and statoblasts of other genera are also collected by it. I know of no better way of obtaining a general idea as to what sponges and phylactolæmata are present in a pond than to examine the tubes of Aulophorus tonkinensis.

I am indebted to Mr. F. H. Gravely, Assistant Superintendent in the Indian Museum, for an interesting note regarding the food of Plumatella. His observations, which were made in Northamptonshire, were unfortunately interrupted at a critical moment, but I have reproduced them with his consent in order that other observers may investigate the phenomena he saw. Mr. Gravely noted that a small green flagellate which was abundant in water in which Plumatella repens was growing luxuriantly, was swallowed by the polypides, and that if the polyparium was kept in a shallow dish of water, living flagellata of the same species congregated in a little pile under the anus of each polypide. His preparations show very clearly that the flagellates were passing through the alimentary canal without apparent change, but the method of preservation does not permit the retractile granules, which were present in large numbers in the cell-substance of the flagellates, to be displayed and it is possible that these granules had disappeared from those flagellates which are present in the recta of his specimens. It is clear, therefore, either that certain flagellates must pass through the alimentary canal of Plumatella unchanged, or that the polyzoon must have the power of absorbing the stored food material the flagellates contain without doing them any other injury.

The free statoblasts of Plumatella are as a rule set free before the cells they contain become differentiated, and float on the surface of the water for some time before they germinate; but occasionally a small polypide is formed inside the capsule while it is still in its parent zoœcium. I have, however, seen only one instance of this premature development, in a single statoblast contained in a small zoarium of P. fruticosa found in Lower Burma in March. The fixed statoblasts usually remain fixed to the support of the zoarium, even when their parent-zoœcium decays, and germinate in situ.

The larva (fig. 40 C, p. 207) that originates from the egg of Plumatella is a minute pear-shaped, bladder-like body covered externally with fine vibratile threads (cilia) and having a pore at the narrow end. At the period at which it is set free from the parent zoœcium it already contains a fully formed polypide or pair of polypides with the tentacles directed towards the narrow end. After a brief period of active life, during which it moves through the water by means of its cilia, it settles down on its broad end, which becomes adhesive; the polypide or pair of polypides is everted through the pore at the narrow end, the whole of this end is turned inside out, and a fresh polyparium is rapidly formed by budding.

29. Plumatella fruticosa, Allman. ([Plate III], fig. 1; [plate IV], fig. 4; [plate V], fig. 1.)

Plumatella fruticosa, Allman, Ann. Nat. Hist. xiii, p. 331 (1844). Plumatella repens, van Beneden (? nec Linné), Mém. Acad. Roy. Belg. 1847, p. 21, pl. i, figs. 1-4. Plumatella fruticosa, Johnston, Brit. Zooph. (ed. 2), p. 404 (1847). Plumatella coralloides, Allman, Rep. Brit. Assoc. 1850, p. 335. Plumatella stricta, id., Mon. Fresh-Water Polyzoa, p. 99, fig. 14 (1857). Plumatella fruticosa, id., ibid. p. 102, pl. vi, figs. 3-5. Plumatella coralloides, id., ibid. p. 103, pl. vii, figs. 1-4. Plumatella repens and P. stricta, Carter, Ann. Nat. Hist. (3) iii, p. 341 (1859). Plumatella lucifuga, Jullien (partim), Bull. Soc. zool. France, x, p. 114 (1885). Plumatella princeps var. fruticosa, Kraepelin, Deutsch. Süsswasserbryozoen, i, p. 120, pl. vii, fig. 148 (1887). Plumatella fruticosa, Braem, Unter. ii. Bryozoen des süssen Wassers, p. 9, pl. i, fig. 15 (Bibl. Zool. ii) (1890). Plumatella repens, Annandale, J. As. Soc. Bengal (new series) iii, 1907, p. 88. Plumatella emarginata, Loppens (partim), Ann. Biol. lacustre, iii, p. 161 (1908). Plumatella fruticosa, Annandale, Rec. Ind. Mus. v, p. 45 (1910).

Zoarium. The zoarium in the typical form has a loose appearance due to the fact that the branches are far apart and the ectocyst by no means rigid. When young the zoarium is adherent, but in well-grown polyparia vertical branches, often an inch or more in length, are freely produced. As a rule they have not the strength to stand upright if removed from the water. Branching is ordinarily lateral and as a rule occurs chiefly on one side of a main branch or trunk. In certain circumstances upright zoœcia are pressed together and reach a great length without branching, and in this form (P. coralloides, Allman) daughter-zoœcia are often produced at the tip of an elongated mother-zoœcium in fan-like formation. A depauperated form (P. stricta, Allman), occurs in which the vertical branches are absent or very short. In all forms internal partitions are numerous and stout.

Zoœcia. The zoœcia are cylindrical and bear a simple keel on their dorsal surface. They are never emarginate or furrowed. In the typical form their diameter is more than half a millimetre, and they are always of considerable length. The ectocyst is thin and never very rigid or deeply pigmented, the colour usually being an almost uniform pale pinkish brown and fading little towards the tip of the zoœcium.

Statoblasts. Both free and stationary statoblasts are formed, but the latter are rare and do not always adhere. They resemble the free statoblasts in general form but have a solid margin instead of a swim-ring and are often minutely serrated round the edge. The free statoblasts are at least considerably, sometimes very elongate; in all zoaria it is possible to find specimens that are more than twice as long as broad. The capsule is relatively large and resembles the swim-ring in outline, so that the free portion of the latter is not much narrower at the sides than at the ends. The sides are distinctly convex and the ends rounded; the swim-ring encroaches little on the surface of the capsule.