Plumatella, Lamarck, Animaux sans Vert. (ed. 1re) ii, p. 106 (1816). Alcyonella, id., ibid. p. 100. Plumatella, Allman, Mon. Fresh-Water Polyzoa, p. 92 (1857). Alcyonella, id., ibid. p. 86. Plumatella, Hyatt, Comm. Essex Inst. iv, p. 207, pl. viii (1866). Plumatella, Jullien (partim), Bull. Soc. zool. France, x, p. 100 (1885). Hyalinella, id., ibid. p. 133. Plumatella, Kraepelin, Deutsch. Süsswass. Bryozoen, i, p. 104 (1887). Plumatella, Braem, Unter. ü. Bryozoen des süssen Wassers, p. 2 (Bibliotheca Zoologica, ii, 1890).
Zoarium dendritic, recumbent, erect, or partly recumbent and partly erect.
Zoœcia tubular, not confined in a gelatinous synœcium; the ectocyst usually horny.
Statoblasts often of two kinds, free and stationary, the latter without air-cells and as a rule adherent by one surface, the former provided with a well-developed ring of air-cells but without marginal processes, oval in form, never more than about 0.6 mm. in length.
Polypide with less than 65 tentacles.
Fig. 42.—Outlines of free statoblasts of Plumatella (enlarged).
A, of P. fruticosa (Calcutta); B, of P. emarginata (Calcutta); C, of P. javanica (Travancore); D, of P. diffusa (Sikhim); E, of P. allmani (Bhim Tal); F, of P. diffusa (Rajshahi, Bengal); G, G', of P. punctata (Calcutta); H, of P. diffusa (Sikhim), statoblast further enlarged: A=outline of capsule; B=limit of swim-ring on ventral surface; C=limit of swim-ring on dorsal surface. [The dark area represents the capsule of the statoblast.]
Certain forms of this genus are liable to become compacted together in such a way as to constitute solid masses consisting of elongate vertical zoœcia closely parallel to one another and sometimes agglutinated by means of a gummy substance. These forms were given by Lamarck in 1816 the name Alcyonella, and there has been much dispute as to whether they represent a distinct genus, distinct species, or merely varieties or phases of more typical forms. It appears to be the case that all species which produce vertical branches are liable to have these branches closely packed together and the individual zoœcia of which they are composed more or less greatly elongated. It is in this way that the form known to Allman as Alcyonella benedeni is produced from the typical Plumatella emarginata. Other forms go further and secrete a gummy substance that glues the upright zoœcia together and forces them to elongate themselves without branching. In these conditions the zoœcia become polygonal in cross-section. It is probable that such forms (e. g., Plumatella fungosa (Pallas)) should rank as distinct species, for the gummy secretion is present in great profusion even in young zoaria in which the zoœcia have not yet assumed a vertical position. No such form, however, has as yet been found in India, and in any case it is impossible to regard Alcyonella as a distinct genus.