Vesicularidæ, Hincks, Brit. Marine Polyzoa, p. 512 (1880).
Zoœcia constricted at the base, deciduous, attached to a stem that is either recumbent or vertical.
GenusBOWERBANKIA,Farre.
Bowerbankia, Farre, Phil. Trans. Roy. Soc. cxxvii, p. 391 (1837).Bowerbankia, Hincks,op. cit.p. 518.
Zoariumvertical or recumbent.Zoœciaovate or almost cylindrical, arranged on the stem singly, in clusters or in a subspiral line.Polypidewith 8 or 10 tentacles.
Bowerbankia caudata,Hincks.
Bowerbankia caudata, Hincks,op. cit.p. 521, pl. lxxv, figs. 7, 8.
This species is easily distinguished from all others by the fact that mature zoœcia have always the appearance of being fixed to the sides of a creeping, adherent stem and are produced, below the point at which they are thus fixed, into a pointed "tail."
Subsp.bengalensis,Annandale.
Bowerbankia caudata, Thornely, Rec. Ind. Mus. i, p. 196 (1907).Bowerbankia caudata, Annandale,ibid.p. 203.Bowerbankia caudataracebengalensis,id.,ibid.ii. p. 13 (1908).
The Indian race is only distinguished from the typical form by its greater luxuriance of growth and by the fact that the "tail" of the zoœcia is often of relatively great length, sometimes equaling or exceeding the rest of the zoœcium. The stem, which is divided at irregular intervals by partitions, often crosses and recrosses its own course and even anastomoses, and a fur-like structure is formed in which the zoœcia representing the hairs become much elongated; but upright branches are never formed. The zoarium has a greenish or greyish tinge.
Typein the Indian Museum.
Geographical Distribution.—B. caudatasubsp.bengalensisis common in brackish water in the Ganges delta, where it often occurs in close association withVictorella bengalensis, and also at the south end of the Chilka Lake in the north-east of the Madras Presidency. Although it has not yet been found elsewhere, it probably occurs all round the Indian coasts.]
Division PALUDICELLINA, nov.
This division consists of two very distinct families, the species of which are easily distinguished at a glance by the fact that in one (the Paludicellidæ) the zoœcia are tubular, while in the other (the Hislopiidæ) they are broad and flattened. The anatomical and physiological differences between the two families are important, and they are associated together mainly on account of the method of budding by means of which their zoaria are produced.
Illustration: Fig. 35.—Single zoœcia of Victorella and Hislopia (magnified).Fig. 35.—Single zoœcia ofVictorellaandHislopia(magnified).
Fig. 35.—Single zoœcia ofVictorellaandHislopia(magnified).
A, zoœcium ofVictorella pavida, Kent, with the polypide retracted (after Kraepelin).B, zoœcium ofHislopia lacustris, Carter (typical form from the United Provinces), with the collar completely and the tentacles partly protruded.A=collar; B=orifice; C=tentacles; D=pharynx; E=œsophagus proper; F=gizzard; G=stomach; G'=cardiac portion of stomach; H=intestine; J=rectum; K=anus; L=young egg; M=green cysts in gizzard; N=testes; O=ovary; O'=funiculus.The muscles are omitted except in fig. B.
Family PALUDICELLIDÆ.
Paludicellidæ, Allman, Mon. Fresh-Water Polyzoa, p. 113 (1857).Homodiætidæ, Kent, Q. J. Micr. Sci. x, p. 35 (1870).Victorellidæ, Hincks, Brit. Marine Polyzoa, p. 558 (1880).Paludicellidées, Jullien, Bull. Soc. zool. France, x, p. 174 (1885).Paludicellides, Loppens, Ann. Biol. lacustre, iii, p. 170 (1908).Victorellides,id.,ibid.p. 171.
Zoarium.The zoarium is recumbent or erect, and is formed typically either of zoœcia arising directly in cruciform formation from one another, or of zoœcia joined together in similar formation with the intervention of tubules arising from their own bases. Complications often arise, however, either on account of the suppression of the lateral buds of a zoœcium, so that the formation becomes linear instead of cruciform, or by the production in an irregular manner of additional tubules and buds from the upper part of the zoœcia. A confused and tangled zoarium may thus be formed, the true nature of which can only be recognized by the examination of its terminal parts.
Zoœcia.The zoœcia are tubular and have a terminal or subterminal orifice, which is angulate or subangulate as seen from above. Owing to this fact, to the stiff nature of the external ectocyst, to the action of circular muscles that surround the tentacular sheath, and to the cylindrical form of the soft inverted part, the orifice, as seen from above, appears to form four flaps or valves, thusIllustration: Valve design.
Polypide.The alimentary canal is elongate and slender as a whole, the œsophagus (including the pharynx) being of considerable length. InPaludicellaandPottsiellathe œsophagus opens directly into the cardiac limb of the stomach, which is distinctly constricted at its base; but inVictorellathe base of the œsophagus is constricted off from the remainder to form an elongate oval sac the walls of which are lined with a delicate structureless membrane.Victorellamay therefore be said to possess a gizzard, but the structure that must be so designated has not the function (that of crushing food) commonly associated with the name, acting merely as a chamber for the retention of solid particles. In this genus the cardiac limb of the stomach is produced and vertical but not constricted at the base. The tentacles in most species number 8, but inPaludicellathere are 16.
Resting buds.The peculiar structures known in Europe as "hibernacula" are only found in this family. The name hibernacula, however, is inappropriate to the only known Indian speciesas they are formed in this country at the approach of summer instead of, as in Europe and N. America, at that of winter. It is best, therefore, to call them "resting buds." They consist of masses of cells congregated at the base of the zoœcia, gorged with food material and covered with a resistant horny covering.
The family Paludicellidæ consists of three genera which may be distinguished as follows:—
Of these three genera,Pottsiellahas not yet been found in India and is only known to occur in N. America. It consists of one species,P. erecta(Potts) from the neighbourhood of Philadelphia in the United States.
Victorellaincludes four species,V. pavidaknown from England and Germany and said to occur in Australia,V. müllerifrom Germany (distinguished by possessing parietal muscles at the tip of the zoœcia),V. symbioticafrom African lakes andV. bengalensisfrom India. These species are closely related.
Paludicellais stated by Carter to have been found in Bombay, but probably what he really found was the young stage ofV. bengalensis. A single species is known in Europe and N. America, namelyP. ehrenbergi, van Beneden (=Alcyonella articulata, Ehrenberg).
I have examined specimens of all the species of this family as yet known.
Genus 1.PALUDICELLA,Gervais.
Paludicella, Gervais, Compt. Rend. iii, p. 797 (1836).Paludicella, Allman, Mon. Fresh-Water Polyzoa, p. 113 (1857).?Paludicella, Carter, Ann. Nat. Hist. (3) iii, p. 333 (1859).Paludicella, Jullien, Bull. Soc. zool. France, x, p. 174 (1885).Paludicella, Kraepelin, Deutsch. Süsswasserbryozoen, i, p. 96 (1887).Paludicella, Loppens, Ann. Biol. lacustre, iv, p. 14 (1910).
Zoarium.The nature of the zoarium in this genus is well expressed by Ehrenberg's specific name "articulata," although the name was given under a false impression. The zoœcia arise directly from one another in linear series with occasional side-branches. The side-branches are, however, often suppressed. The zoarium as a whole is either recumbent and adherent or at least partly vertical.
Zoœcia.Although the zoœcia are distinctly tubular as a whole, two longitudinal axes may be distinguished in each, for the tip is bent upwards in a slanting direction, bearing the orifice at its extremity. The main axis is, however, at right angles to the dorso-ventral axis, and the dorsal surface, owing to the position of the aperture, can always be readily distinguished from the ventral, even when the position of the zoœcium is vertical. Each zoœcium tapers towards the posterior extremity. Parietal muscles are always present.
Illustration: Fig. 36.—Structure of Paludicella ehrenbergi (A and B after Allman).Fig. 36.—Structure ofPaludicella ehrenbergi(A and B after Allman).
Fig. 36.—Structure ofPaludicella ehrenbergi(A and B after Allman).
A=a single zoœcium with the polypide retracted. B=the base of the lophophore as seen from above with the tentacles removed. C=the orifice of a polypide with the collar expanded and the tentacles partly retracted.a=tentacles;c=collar;d=mouth;e=œsophagus;f=stomach;g=intestine;k=parieto-vaginal muscles;p=parietal muscles;o=cardiac part of the stomach;r=retractor muscle;s=funiculus.
Polypide.The most striking features of the polypide are the absence of any trace of a gizzard and the highly specialized form assumed by the cardiac part of the stomach. There are two funiculi, both connecting the pyloric part of the stomach with the endocyst. The ovary develops at the end of the upper, the testis at that of the lower funiculus.
Resting buds.The resting buds are spindle-shaped.
Kraepelin recognized two species in the genus mainly by their method of growth and the number of tentacles. In hisP. müllerithe zoarium is always recumbent and the polypide has 8 tentacles, whereas inP. articulataorehrenbergithe tentacles number 16 and upright branches are usually developed. It is probable,however, that the former species should be assigned toVictorella, for it is often difficult to distinguishPaludicellafrom young specimens ofVictorellaunless the latter bear adventitious terminal buds. The gizzard ofVictorellacan be detected in well-preserved material even under a fairly low power of the microscope, and I have examined specimens of what I believe to be the adult ofmülleriwhich certainly belong to that genus.
It is always difficult to see the collar ofPaludicella, because of its transparency and because of the fact that its pleats are apparently not strengthened by chitinous rods as is usually the case. Allman neither mentions it in his description of the genus nor shows it in his figures, and Loppens denies its existence, but it is figured by Kraepelin and can always be detected in well-preserved specimens, if they are examined carefully. If the collar were actually absent, its absence would separatePaludicellanot only fromVictorellaandPottsiella, but also from all other ctenostomes. In any case,Victorellais distinguished fromPaludicellaandPottsiellaby anatomical peculiarities (e. g., the possession of a gizzard and the absence of a second funiculus) that may ultimately be considered sufficiently great to justify its recognition as the type and only genus of a separate family or subfamily.
The description ofPaludicellais included here on account of Carter's identification of the specimens he found at Bombay; but its occurrence in India is very doubtful.
Genus 2.VICTORELLA,Kent.
Victorella, Kent, Q. J. Micr. Sci. x, p. 34 (1870).Victorella, Hincks, Brit. Marine Polyzoa, p. 559 (1880).Victorella, Kraepelin, Deutsch. Süsswasserbryozoen, i, p. 93 (1887).
Type,Victorella pavida, Kent.
Zoarium.The zoarium consists primarily of a number of erect or semi-erect tubular zoœcia joined together at the base in a cruciform manner by slender tubules, but complications are introduced by the fact that adventitious buds and tubules are produced, often in large numbers, round the terminal region of the zoœcia, and that these buds are often separated from their parent zoœcium by a tubule of considerable length, and take root among other zoœcia at a distance from their point of origin. A tangled mass may thus be formed in which it is difficult to recognize the regular arrangement of the zoœcia that can be readily detached at the growing points of the zoarium.
Zoœcia.The zoœcia when young closely resemble those ofPaludicella, but as they grow the terminal upturned part increases rapidly, while the horizontal basal part remains almost stationary and finally appears as a mere swelling at the base of an almost vertical tube, in which by far the greater part, if not the whole, of the polypide is contained. Round the terminal part of thistube adventitious buds and tubules are arranged more or less regularly. There are no parietal muscles.
Polypide.The polypide has 8 slender tentacles, which are thickly covered with short hairs. The basal part of the œsophagus forms a thin-walled sac (the "gizzard") constricted off from the upper portion and bearing internally a thin structureless membrane. Circular muscles exist in its wall but are not strongly developed on its upper part. There is a single funiculus, which connects the posterior end of the stomach with the base of the zoœcium. The ovaries and testes are borne on the endocyst, not in connection with the funiculus.
Resting buds.The resting buds are flattened or resemble young zoœcia in external form.
Victorella, although found in fresh water, occurs more commonly in brackish water and is known to exist in the littoral zone of the sea.
26.Victorella bengalensis,Annandale.
Victorella pavida, Annandale (necKent), Rec. Ind. Mus. i, p. 200, figs. 1-4 (1907).Victorella bengalensis,id.,ibid.ii, p. 12, fig. 1 (1908).
Zoarium.The mature zoarium resembles a thick fur, the hairs of which are represented by elongate, erect, slender tubules (the zoœcia), the arrangement of the whole being very complicated and irregular. The base of the zoarium often consists of an irregular membrane formed of matted tubules, which are sometimes agglutinated together by a gummy secretion. The zoarium as a whole has a faint yellowish tinge.
Zoœcia.The zoœcia when young are practically recumbent, each being of an ovoid form and having a stout, distinctly quadrate orificial tubule projecting upwards and slightly forwards near the anterior margin of the dorsal surface. At this stage a single tubule, often of great relative length, is often given off near the orifice, bearing a bud at its free extremity. As the zoœcium grows the tubular part becomes much elongated as compared with the basal part and assumes a vertical position. Its quadrate form sometimes persists but more often disappears, so that it becomes almost circular in cross-section throughout its length. Buds are produced near the tip in considerable profusion. As a rule, if they appear at this stage, the tubule connecting them with the parent zoœcium is short or obsolete; sometimes they are produced only on one side of the zoœcium, sometimes on two. The buds themselves produce granddaughter and great-granddaughter buds, often connected together by short tubules, while still small and imperfectly developed. The swelling at the base of the zoœcium, when the latter is fully formed, is small.
Polypide.The polypide has the features characteristic of the genus. The base of the gizzard is surrounded by a strong circular muscle.
Illustration: Fig. 37.—Victorella bengalensis (type specimens).Fig. 37.—Victorella bengalensis(type specimens).
Fig. 37.—Victorella bengalensis(type specimens).
A=single zoœcium without adventitious buds but with a young resting bud (b), × 70 (dorsal view); B=lateral view of a smaller zoœcium without buds, × 70; C=upper part of a zoœcium with a single adventitious bud, × 70; D=outline of the upper part of a zoœcium with adventitious buds of several generations, × 35; E=remains of a zoœcium with two resting buds (b) attached. All the specimens figured are from Port Canning and, except D, are represented as they appear when stained with borax carmine and mounted in canada balsam.
Resting buds.The resting buds (fig. 31, p. 170) are somewhat variable in shape but are always flat with irregular cylindrical or subcylindrical projections round the margin, on which the horny coat is thinner than it is on the upper surface. This surface is either smooth or longitudinally ridged.
Typein the Indian Museum.
This species differs from the EuropeanV. pavidain very much the same way as, but to a greater extent than, the Indian race ofBowerbankia caudatadoes from the typical English one (see p. 189). The growth of the zoarium is much more luxuriant, and the form of the resting buds is different.
Geographical Distribution.—V. bengalensisis abundant in pools of brackish water in the Ganges delta and in the Salt Lakes near Calcutta; it also occurs in ponds of fresh water near the latter. I have received specimens from Madras from Dr. J. R. Henderson, and it is probable that the form from Bombay referred by Carter toPaludicellabelonged to this species.
Biology.—In the Ganges deltaV. bengalensisis usually found coating the roots and stems of a species of grass that grows in and near brackish water, and on sticks that have fallen into the water. It also spreads over the surface of bricks, and I have found a specimen on a living shell of the common molluscMelania tuberculata. Dr. Henderson obtained specimens at Madras from the surface of a freshwater shrimp,Palæmon malcolmsonii. In the ponds at Port Canning the zoaria grow side by side with, and even entangled with those ofBowerbankia caudatasubsp.bengalensis, to the zoœcia of which their zoœcia bear a very strong external resemblance so far as their distal extremity is concerned. This resemblance, however, disappears in the case of zoœcia that bear terminal buds, for no such buds are borne byB. caudata; and the yellowish tint of the zoaria ofV. bengalensisis characteristic. Zoaria of the entoproctLoxosomatoides colonialisand colonies of the hydroidIrene ceylonensisare also found entangled with the zoaria ofV. bengalensis, the zoœcia of which are often covered with various species of Vorticellid protozoa and small rotifers. The growth ofV. bengalensisis more vigorous than that of the other polyzoa found with it, and patches ofB. caudataare frequently surrounded by large areas ofV. bengalensis.
The food ofV. bengalensisconsists largely of diatoms, the siliceous shells of which often form the greater part of its excreta. Minute particles of silt are sometimes retained in the gizzard, being apparently swallowed by accident.
There are still many points to be elucidated as regards the production and development of the resting buds inV. bengalensis, but two facts are now quite clear as regards them: firstly, that these buds are produced at the approach of the hot weather and germinate in November or December; and secondly, that the whole zoarium may be transformed at the former season into a layer of resting buds closely pressed together but sometimes exhibiting in their arrangement the typical cruciform formation. Resting buds may often be found in vigorous colonies as late asthe beginning of December; these buds have not been recently formed but have persisted since the previous spring and have not yet germinated. Sometimes only one or two buds are formed at the base of an existing zoœcium (fig. 37a), but apparently it is possible not only for a zoœcium to be transformed into a resting bud but for it to produce four other buds round its base before undergoing the change. Young polypides are formed inside the buds and a single zoœcium sprouts out of each, as a rule by the growth of one of the basal projections, when conditions are favourable.
Polypides ofV. bengalensisare often transformed into brown bodies. When this occurs the orifice closes together, with the collar expanded outside the zoœcium. I have occasionally noticed that the ectocyst of such zoœcia was distinctly thicker and darker in colour than that of normal zoœcia.
Eggs and spermatozoa are produced in great numbers, as a rule simultaneously in the same zoœcia, but individuals kept in captivity often produce spermatozoa only. The eggs are small and are set free as eggs. Nothing is known as regards their development.
Polypides are as a rule found in an active condition only in the cold weather, but I have on one occasion seen them in this condition in August, in a small zoarium attached to a shell ofMelania tuberculatataken in a canal of brackish water near Calcutta.
Family HISLOPIIDÆ.
Hislopidées, Jullien, Bull. Soc. zool. France, x, p. 180 (1885).Hislopiidæ, Annandale, Rec. Ind. Mus. i, p. 200 (1907).
Zoariumrecumbent, often forming an almost uniform layer on solid subjects.
Zoœciaflattened, adherent; the orifice dorsal, either surrounded by a chitinous rim or situated at the tip of an erect chitinous tubule; no parietal muscles.
Polypidewith 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, andHislopia, Carter, which is widely distributed in Eastern Asia. The former genus possesses an upright orificial tubule and has zoœcia separated by basal tubules. Its anatomy is imperfectly known, but it certainly possesses a gizzard of similar structure to that ofHislopia, between which andVictorellaits zoœcium is intermediate in form.
GenusHISLOPIA,Carter.
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 describedHislopiaas a cheilostome allied toFlustra; in 1880 Jullien perpetuated the error indescribing hisNorodonia, 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 thatNorodoniawas a cheilostome and placed it, together withHislopia, 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 betweenHislopiaand 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 ofHislopiais its gizzard (fig. 38, p. 201) which is perhaps unique (except for that ofArachnoidea) 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 cansometimes 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 toEuglenawhich 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.
Illustration: Fig. 38.—Optical section of gizzard of Hislopia lacustris, with contained green cysts, × 240.Fig. 38.—Optical section of gizzard ofHislopia lacustris, with contained green cysts, × 240.
Fig. 38.—Optical section of gizzard ofHislopia 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 circumstances 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 fæces assume a green colour. In preserved specimens apparently unaltered cysts are sometimes found in the rectum, 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 inHislopia. 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 zoœcia of polypides that have formed brown bodies, often being actually enclosed in the substance of the brown body. The gizzards of the specimens ofArachnoideaI have examined contain cysts that resemble those found in the same position inHislopia.
Hislopiais widely distributed in the southern part of the Oriental Region, and, if I am right in regardingEchinella, Korotneff as a synonym, extends its range northwards to Lake Baikal. It appears to be a highly specialized form but is perhaps related, throughArachnoidea, toVictorella.
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 formsnarrow, closely compressed masses. One zoœcium, however, never grows over another.
Zoœcia.The zoœcia are variable in shape. In zoaria which have space for free expansion they are as a rule irregularly oval, the posterior extremity being often narrower than the anterior; but small triangular zoœcia and others that are almost square may often be found. When growing on a support of limited area the zoœcia 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 zoœcium is usually surrounded by a chitinous margin, and outside this margin there is often a greater or less extent of adherent membrane. In some zoœcia the margin is obsolete or obsolescent. The dorsal surface is of a glassy transparency but by no means soft.
Illustration: Fig. 39.—Hislopia lacustris.Fig. 39.—Hislopia lacustris.
Fig. 39.—Hislopia lacustris.
A=part of a zoarium of the subspeciesmoniliformis(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 zoœcia and of the orifice, × 15.
Polypide.The polypide has from 12 to 20 tentacles, 16 being a common number.
Typeprobably 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 ofHislopiaamong them.
27a.Subsp.moniliformis, nov.
Hislopia lacustris, Annandale, J. As. Soc. Bengal (new series) ii, p. 59, fig. 1 (1906).
In this race, which is common in Calcutta, the zoœcia are almost circular but truncate or concave anteriorly and posteriorly. They form linear series with few lateral branches. I have found specimens occasionally on the shell ofVivipara bengalensis, but they are much more common on the leaves ofVallisneria spiralis.
Typein the Indian Museum.
The exact status of the forms described by Jullien asNorodonia cambodgiensisandN. sinensisis doubtful, but I see no reason to regard them as specifically distinct fromH. lacustris, Carter, of which they may be provisionally regarded as varieties. The varietycambodgiensisis very like my subspeciesmoniliformisbut has the zoœcia constricted posteriorly, while var.sinensis, although the types were found onAnodontashells on which there was plenty of room for growth, resemble the confined phase ofH. lacustrisso far as the form of their zoœcia and of the orifice is concerned.
Geographical Distribution.—The typical form is common in northern India and occurs also in Lower Burma; the subspeciesmoniliformisappears to be confined to Lower Bengal, while the varietiescambodgiensisandsinensisboth occur in China, the former having been found also in Cambodia and Siam. Indian and Burmese localities are:—Bengal, Calcutta (subsp.moniliformis); Berhampur, Murshidabad district (J. Robertson Milne):Central Provinces, Nagpur (Carter):United Provinces, Bulandshahr (H. J. Walton):Burma, Pegu-Sittang Canal (Kirkpatrick).
Biology.—Regarding the typical form of the species Major Walton writes (Rec. Ind. Mus. iii, p. 296):—"In volume i (page 177) of the Records of the Indian Museum, I described the two forms of colonies ofHislopiathat I had found in the United Provinces (Bulandshahr). Of these, one was a more or less linear arrangement of the zoœcia on leaves and twigs, and the other, and more common, form was an encrusting sheath on the outer surface of the shells ofPaludina. During the present 'rains' (July 1908) I have found many examples of what may be considered a much exaggerated extension of the latter form. These colonies have been on bricks, tiles, and other submerged objects. The largest colony that I have seen so far was on a tile; one side of the tile was exposed above the mud of the bottom of the tank, and its area measured about 120 square inches; the entire surface was almost completely covered by a continuous growth ofHislopia. Another large colony was on a piece of bark which measured 7 inches by 3 inches; both sides were practically everywhere covered byHislopia."
Major Walton also notes that in the United Provinces the growth ofHislopiais at its maximum during "rains," and that at that time of year almost every adultPaludinain a certaintank at Bulandshahr had its shell covered with the zoœcia. The Calcutta race flourishes all the year round but never forms large or closely compacted zoaria, those on shells ofViviparaexactly resembling those on leaves ofVallisneria.
In Calcutta both eggs and spermatozoa are produced at all times of the year simultaneously in the same zoœcia, but the eggs in one zoœcium often vary greatly in size. When mature they reach relatively considerable dimensions and contain a large amount of food material; but they are set free from the zoœcium as eggs. They lie loose in the zoœcium at a comparatively small size and grow in this position. Nothing is known as regards the development ofHislopia.
Both forms of the species appear to be confined to water that is free from all traces of contamination with brine.
OrderPHYLACTOLÆMATA.
The polypide in this order possesses a leaf-like ciliated organ (the epistome) which arises within the lophophore between the mouth and the anus and projects upwards and forwards over the mouth, which it can be used to close. The zoœcia are never distinct from one another, but in dendritic forms such asPlumatellathe zoarium is divided at irregular intervals by chitinous partitions. The lophophore in most genera is horseshoe-shaped instead of circular, the part opposite the anus being deeply indented. There are no parietal muscles. The orifice of the zoœcium is always circular, and there is no trace of any structure corresponding to the collar of the ctenostomes. The tentacles are always webbed at the base.
All the phylactolæmata produce the peculiar reproductive bodies known as statoblasts.
The phylactolæmata, which are probably descended from ctenostomatous ancestors, are confined to fresh or slightly brackish water. Most of the genera have a wide geographical distribution, but (with the exception of a few statoblasts of almost recent date) only one fossil form (Plumatellites, Fric. from the chalk of Bohemia) has been referred to the order, and that with some doubt.
It is convenient to recognize two main divisions of the phylactolæmata, but these divisions hardly merit the distinction of being regarded as suborders. They may be called Cristatellina and Plumatellina and distinguished as follows:—
Division I,Plumatellina, nov.—Ectocyst well developed; zoaria without a special organ of progression; polypides contained in tubes.
Division II,Cristatellina, nov.—Ectocyst absent except at the base of the zoarium which is modified to form a creeping "sole"; polypides embedded in a common synœcium of reticulate structure.
The Cristatellina consist of a single genus and probably of a single species (Cristatella mucedo, Cuvier), which is widely distributed in Europe and N. America, but has not been found in the Oriental Region. Eight genera of Plumatellina are known, and five (possibly six) of these genera occur in India.
Division PLUMATELLINA, nov.
The structure of the species included in this division is very uniform as regards the internal organs (see fig. 40 opposite and fig. 47a, p. 236). The alimentary canal is simpler than that of the Paludicellidæ. A short œsophagus leads directly into the stomach,the cardiac portion of which is produced as a vertical limb almost cylindrical in form and not constricted at the base. This limb is as a rule of greater length than the œsophagus. The pyloric part of the stomach is elongated and narrow, and the intestine short, straight, and of ovoid form. There are no cilia at the pyloric opening. A single funiculus joins the posterior end of the stomach to the wall of the zoœcium, bearing the statoblasts. Sexual organs are often absent.