_Zooecia_ flattened, adherent; the orifice dorsal, either surrounded by a chitinous rim or situated at the tip of an erect chitinous tubule; no parietal muscles.

_Polypide_ with an ample gizzard which possesses a uniform chitinous lining and does not close together when the polypide is retracted.

_Resting bud_, not produced.

Only two genera can be recognized in this family, _Arachnoidea_, Moore, from Central Africa, and _Hislopia_, Carter, which is widely distributed in Eastern Asia. The former genus possesses an upright orificial tubule and has zooecia separated by basal tubules. Its anatomy is imperfectly known, but it certainly possesses a gizzard of similar structure to that of _Hislopia_, between which and _Victorella_ its zooecium is intermediate in form.

Genus HISLOPIA, _Carter_.

_Hislopia_, Carter, Ann. Nat. Hist. (3) i, p. 169 (1858).

_Hislopia_, Stolickza, J. As. Soc. Bengal, x.x.xviii (2), p.

61 (1869).

_Norodonia_, Jullien, Bull. Soc. zool. France, v, p. 77 (1880).

_Hislopia_, _id._, _ibid._ x, p. 183 (1885).

_Norodonia_, _id._, _ibid._ p. 180.

_Echinella_, Korotneff, Biol. Centrbl. xxi, p. 311 (1901).

_Hislopia_, Annandale, J. As. Soc. Bengal (new series) ii, p. 59 (1906).

_Hislopia_, Loppens, Ann. Biol. lacustre, iii, p. 175 (1908).

TYPE, _Hislopia lacustris_, Carter.

_Zoarium._ The zoarium consists primarily of a main axis running in a straight line, with lateral branches that point forwards and outwards.

Further proliferation, however, often compacts the structure into an almost uniform flat area.

_Zooecia._ The zooecia (fig. 35 B, p. 190) are flat and have the orifice surrounded by a chitinous rim but not much raised above the dorsal surface. They arise directly one from another.

_Polypide._ The polypide possesses from 12 to 20 tentacles. Its funiculus is rudimentary or absent. Neither the ovaries nor the testes have any fixed position on the lateral walls of the zooecium to which they are confined.

The position of this genus has been misunderstood by several zoologists.

Carter originally described _Hislopia_ as a cheilostome allied to _Fl.u.s.tra_; in 1880 Jullien perpetuated the error in describing his _Norodonia_, which was founded on dried specimens of Carter's genus; while Loppens in 1908 still regarded the two "genera" as distinct and placed them both among the cheilostomes. In 1885, however, Jullien retracted his statement that _Norodonia_ was a cheilostome and placed it, together with _Hislopia_, in a family of which he recognized the latter as the eponymic genus. Carter's mistake arose from the fact that he had only examined preserved specimens, in which the thickened rim of the orifice is strongly reminiscent of the "peristome" of certain cheilostomes, while the posterior of the four folds into which the tentacle sheath naturally falls (as in all ctenostomes, _cf._ the diagram on p. 191) is in certain conditions rather larger than the other three and suggests the "lip" characteristic of the cheilostomes. If living specimens are examined, however, it is seen at once that the posterior fold, like the two lateral folds and the anterior one, changes its form and size from time to time and has no real resemblance to a "lip."

That there is a remarkable, if superficial, resemblance both as regards the form of the zooecium and as regards the method of growth between _Hislopia_ and certain cheilostomes cannot be denied, but the structure of the orifice and indeed of the whole organism is that of a ctenostome and the resemblance must be regarded as an instance of convergence rather than of genetic relationship.

The most striking feature of the polypide of _Hislopia_ is its gizzard (fig. 38, p. 201) which is perhaps unique (except for that of _Arachnoidea_) both in structure and function. In structure its peculiarities reside mainly in three particulars: (i), it is not constricted off directly from the thin-walled oesophageal tube, but possesses at its upper extremity a thick-walled tubular portion which can be entirely closed from the oesophagus at its upper end but always remains in communication with the spherical part of the gizzard; (ii), this spherical part of the gizzard is uniformly lined with a thick chitinous or h.o.r.n.y layer which in optical section has the appearance of a pair of ridges; and (iii), there is a ring of long and very powerful cilia round the pa.s.sage from the gizzard to the stomach. The cardiac limb of the stomach, which is large and heart-shaped, is obsolete. The wall of the spherical part of the gizzard consists of two layers of cells, an outer muscular layer consisting of powerful circular muscles and an inner glandular layer, which secretes the chitinous lining. The inner walls of the tubular part consist of non-ciliated columnar cells, and when the polypide is retracted it lies almost at right angles to the main axis of the zooecium.

The spherical part of the gizzard invariably contains a number of green cells, which lie free in the liquid it holds and are kept in motion by the cilia at its lower aperture. The majority of these cells can be seen with the aid of a high power of the microscope to consist of a hard spherical coat or cyst containing green protoplasm in which a spherical ma.s.s of denser substance (the nucleus) and a number of minute transparent granules can sometimes be detected. The external surface of many of the cysts is covered with similar granules, but some are quite clean.

There can be no doubt that these cysts represent a stage in the life-history of some minute unicellular plant or animal. Indeed, although it has not yet been found possible to work out this life-history in detail, I have been able to obtain much evidence that they are the resting stage of a flagellate organism allied to _Euglena_ which is swallowed by the polyzoon and becomes encysted in its gizzard, extruding in so doing from its external surface a large proportion of the food-material that it has stored up within itself in the form of transparent granules. It may also be stated that some of the organisms die and disintegrate on being received into the gizzard, instead of encysting themselves.

So long as the gizzard retains its spherical form the green cells and its other contents are prevented from entering the stomach by the movements of the cilia that surround its lower aperture, but every now and then, at irregular intervals, the muscles that form its outer wall contract. The chitinous lining although resilient and not inflexible is too stiff to prevent the lumen of the gizzard being obliterated, but the action of the muscles changes its contents from a spherical to an ovoid form and in so doing presses a considerable part of them down into the stomach, through the ring of the cilia.

[Ill.u.s.tration: Fig. 38.--Optical section of gizzard of _Hislopia lacustris_, with contained green cysts, 240.]

The contraction of the gizzard is momentary, and on its re-expansion some of the green cysts that have entered the stomach are often regurgitated into it. Some, however, remain in the stomach, in which they are turned round and round by the action of the cilia at both apertures. They are apparently able to retain their form for some hours in these circ.u.mstances but finally disintegrate and disappear, being doubtless digested by the juices poured out upon them by the glandular lining of the stomach. In polypides kept under observation in clean tap-water all the cysts finally disappear, and the faeces a.s.sume a green colour. In preserved specimens apparently unaltered cysts are sometimes found in the r.e.c.t.u.m, but this is exceptional: I have observed nothing of the kind in living polypides. Cysts often remain for several days unaltered in the gizzard.

Imperfect as these observations are, they throw considerable light on the functions of the gizzard in _Hislopia_. Primarily it appears to act as a food-reservoir in which the green cysts and other minute organisms can be kept until they are required for digestion. When in the gizzard certain organisms surrender a large proportion of the food-material stored up for their own uses, and this food-material doubtless aids in nourishing the polyzoon. Although the cysts in the gizzard are frequently accompanied by diatoms, the latter are not invariably present. The cysts, moreover, are to be found in the zooecia of polypides that have formed brown bodies, often being actually enclosed in the substance of the brown body. The gizzards of the specimens of _Arachnoidea_ I have examined contain cysts that resemble those found in the same position in _Hislopia_.

_Hislopia_ is widely distributed in the southern part of the Oriental Region, and, if I am right in regarding _Echinella_, Korotneff as a synonym, extends its range northwards to Lake Baikal. It appears to be a highly specialized form but is perhaps related, through _Arachnoidea_, to _Victorella_.

27. Hislopia lacustris, _Carter_.

_Hislopia lacustris_, Carter, Ann. Nat. Hist. (3) i, p. 170, pl. vii, figs. 1-3 (1858).

_Norodonia cambodgiensis_, Jullien, Bull. Soc. zool. France, v, p. 77, figs. 1-3 (1880).

_Norodonia sinensis_, _id._, _ibid._ p. 78, figs. 1-3.

_Norodonia cambodgiensis_, _id._, _ibid._ x, p. 181, figs.

244, 245 (1885).

_Norodonia sinensis_, _id._, _ibid._ p. 182, figs. 246, 247.

_Hislopia lacustris_, Annandale, J. As. Soc. Bengal (new series) iii, p. 85 (1907).

_Hislopia lacustris_, Walton, Rec. Ind. Mus. i, p. 177 (1907).

_Hislopia lacustris_, Kirkpatrick, _ibid._ ii, p. 98 (1908).

_Hislopia lacustris_, Walton, _ibid._ iii, p. 295 (1909).

_Zoarium._ The zoarium forms a flat, more or less solid layer and is closely adherent to foreign objects. As a rule it covers a considerable area, with radiating branches at the edges; but when growing on slender twigs or the stems of water-plants it forms narrow, closely compressed ma.s.ses. One zooecium, however, never grows over another.

_Zooecia._ The zooecia are variable in shape. In zoaria which have s.p.a.ce for free expansion they are as a rule irregularly oval, the posterior extremity being often narrower than the anterior; but small triangular zooecia and others that are almost square may often be found. When growing on a support of limited area the zooecia are smaller and as a rule more elongate. The orifice is situated on a slight eminence nearer the anterior than the posterior margin of the dorsal surface. It is surrounded by a strong chitinous rim, which is usually square or subquadrate but not infrequently circular or subcircular. Sometimes a prominent spine is borne at each corner of the rim, but these spines are often vestigial or absent; they are rarely as long as the transverse diameter of the orifice. The zooecium is usually surrounded by a chitinous margin, and outside this margin there is often a greater or less extent of adherent membrane. In some zooecia the margin is obsolete or obsolescent. The dorsal surface is of a gla.s.sy transparency but by no means soft.

[Ill.u.s.tration: Fig. 39.--_Hislopia lacustris._

A=part of a zoarium of the subspecies _moniliformis_ (type specimen, from Calcutta), 15; A=green cysts in gizzard; E=eggs.

B=outline of part of a zoarium of the typical form of the species from the United Provinces, showing variation in the form of the zooecia and of the orifice, 15.]

_Polypide._ The polypide has from 12 to 20 tentacles, 16 being a common number.

TYPE probably not in existence. It is not in the British Museum and Prof. Dendy, who has been kind enough to examine the specimens from Carter's collection now in his possession, tells me that there are none of _Hislopia_ among them.

27 _a._ Subsp. moniliformis, nov.

_Hislopia lacustris_, Annandale, J. As. Soc. Bengal (new series) ii, p. 59, fig. 1 (1906).