Hislopia, Carter, Ann. Nat. Hist. (3) i, p. 169 (1858). Hislopia, Stolickza, J. As. Soc. Bengal, xxxviii (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.
Zoœcia. The zoœcia (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 zoœcium 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 Flustra; 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 zoœcium 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 œsophageal tube, but possesses at its upper extremity a thick-walled tubular portion which can be entirely closed from the œsophagus 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 horny 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 passage 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 zoœcium.
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 mass 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.