_Plumatella_, Allman, Mon. Fresh-Water Polyzoa, p. 92 (1857).
_Plumatella_, Hyatt, Comm. Essex Inst. iv, p. 207, pl. viii (1866).
_Plumatella_, Jullien (_partim_), Bull. Soc. zool. France, x, p. 100 (1885).
_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.
_Zooecia_ tubular, not confined in a gelatinous synoecium; 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.
[Illustration: 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 zooecia 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 zooecia 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 zooecia together and forces them to elongate themselves without branching. In these conditions the zooecia 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 zooecia 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.