MEDUSA OF MICEOHYDBA RYDERI. 635

On the Freshwater Medusa liberated by Micro-hydra ryderi, Potts, and a Comparison

with Limnocodium.

ByEtlwartl T. Browne, B.A.,

Zoological Research Laboratory, University College, London.

Witli Plate 37.

THROUGH the kindness of Professor Bay Lankester, Ihave had the pleasure of examining a specimen of theMedusa liberated from the freshwater Hydroid Microhydraryderi .

The veteran American naturalist, Mr. Potts, of Philadelphia,recently sent to Professor Lankester a manuscript on " KnownForms of Medusas Inhabiting Fresh Water," for publicationin the ' Quarterly Journal/ and this communication of mineforms a kind of appendix to it; it should be regarded as such,since I have before me an advanced proof of Mr. Potts'paper. The title given by Mr. Potts to his communicationdid not adequately convey the importance of its contents, andhas been modified accordingly by the editor. Mr. Potts hasat last given us a description with excellent figures of theHydroid phase of Microhydra and the first figure of theMedusa.

When Professor Lankester showed me the original draw-ings of Microhydra I noticed the remarkable resemblancebetween the hydroid phase and that of Limnocodium, butwe were doubtful about the Medusa, as Mr. Potts had notgiven a detailed description of it. Since we were not sureabout the presence of sense organs, Professor Lankester asked

636 EDWARD T. BROWNE.

Mr. Potts if he could spare a specimen for further examination.A specimen was very kindly sent over from America by Mr.Potts, and I sincerely thank Professor Ray Lankester for hisgenerosity in handing it to me for examination.

The figures of Medusa illustrating Mr. Potts' paper werenot drawn from a living Medusa, but from a specimen whichhad been in weak formalin for several years. To obtain areally satisfactory drawing of a Medusa it must be made whilstthe animal is alive. In drawing a preserved specimen allow-ances have frequently to be made for contraction and dis-tortion, and therein lies a source for error. After studyingliving Medusse for a few years the allowance for defects canbe fairly well estimated, though occasionally one may gobadly astray.

The specimen which I examined was in formalin and in goodcondition, but the umbrella was badly crumpled on one side.It was a difficult object to examine owing to its minuteness,being less than half a millimetre in diameter, and not easy tofix in a definite position. The drawings which I have madewere finished before I received the proofs of Mr. Potts' plates.I then noticed that my drawing of the Medusa did not quiteagree with that made by Dr. Moore, so I again examined thespecimen, but found that it was not necessary to make anyalterations.

THE DESCRIPTION OF THE MEDUSA OF MICEOHYDEA (PL 37, fig. 1).

Umbrella.—The umbrella is campanulate, a little broaderthan high (0'4 mm. in width and 0'3 mm. in height), withthin walls. No nematocysts could be found on the ex-umbrella, though the ectoderm cells were plainly visible.The velum is broad, and here also the ectoderm cells with arounded nucleus could be easily seen.

Stomach.—The stomach is large for the size of theMedusa, being about three quarters the length of the cavityof the umbrella. It appears to be more cylindrical thanquadrangular in transverse section, and tapers slightlytowards the mouth. In this specimen the mouth is fairly

MEDUSA OF MICROHYDRA RYDERr. 637

well expanded, but there are indications of four small lips,which are simply infolds of the margin.

Canal system.—There are four radial canals, which arenot at all conspicuous, and have the appearance of thin linesrunning from the base of the stomach to the margin of theumbrella. In fact the canals are rendered visible by thebrownish colouration of their endoderm cells. A circularcanal is probably present, but the thick layer of ectodermround the margin of the umbrella prevented me from findinga definite canal. To demonstrate the existence of a circularcanal would have necessitated the cutting of sections.

Absence of gonads.—The specimen does not show theslightest trace oE gonads. The figures of the Medusa illus-trating Potts' paper (PL 36, figs. 13 and 14) would lead oneto believe that gonads were present along the whole length ofthe radial canals and that they also extended round thestomach. The Hydromedusee either have their gonads uponthe stomach and its lobes, or else upon the radial canals, butnone are known to possess gonads in both positions. If aMedusa was found with gonads upon the stomach and radialcanals it would have to be placed in a special order. Theseshaded thickenings are undoubtedly misleading, and I fail tosee what they are intended to represent.

Ten tac le s (PL 37, fig. 2).—There are eight tentacles (fourper-radial and four inter-radial) similar in size and shape.They are probably in a semi-contracted condition, which givesthem a rather stunted appearance. The base of the tentaclesis apparently attached for a very short distance, on its upperside, to the margin of the umbrella. The manner in whichthe tentacles curl at their base and hang down points to anattachment, though I could not clearly see it. There is noindication of a definite basal bulb at the base of the tentacles.Nematocysts are very scarce in the tentacles, and it was onlyafter a long search that I found any, as they were not on thesurface but underneath the ectoderm cells.

Around the margin of the umbrella there is a thick layerof ectoderm cells, similar in structure to those of the tentacles,

638 EDWARD T. BROWNE.

and between these cells there are a few nematocysts, likethose found in the tentacles.

Absence of sense organs.—When I saw the originaldrawing of Plate 36, fig. 14, before I received the specimen,I thought that the little circles at the bases of the tentaclesrepresented marginal sensory vesicles situated just above theroot of the tentacle. They certainly have the appearance ofsense organs with a single otolith. In diagrams, and oftenin good drawings, sense organs are drawn in a similar manner.I thoroughly searched the margin of the umbrella for senseorgans, finally using an oil immersion lens, but failed to findany indications of a seuse organ either at the base of thetentacles or in between the tentacles. The margin of theumbrella has a slight brownish colour, as if the Medusa hadbeen killed with Flemming's solution. All the nuclei are ofa faint brownish colour, easily seen with an oil immersionlens, and the cell walls are also well defined, so that if anysense organs had been present they ought to have beenvisible. The little circles in the figure are intended foroptical sections of the bases of the tentacles. They representthe attachment of the tentacle to the margin of the umbrella,and have nothing whatever to do with sense organs.

A COMPARISON BETWEEN LIMNOCODIUM AND MICROHTDEA.

When I first saw the drawings of Microhydra and com-pared them with the figures of Limnocodium, it seemedquite possible that M i c r o h y d r a might turn out to be thewell-known Limnocodium, but after examining the Medusaof Mic rohydra the idea of such a possibility soon vanished.

Hydroid.—In the Hydroid phase the resemblance betweenLimnocodium and Mic rohydra is very close, as will beseen on comparing PI. 35, fig. 10, with PI. 36, fig. 17. In bothforms the hydranth has degenerated to its simplest condition—i.e. to merely a body without tentacles, and when the nativeplace of Limnocodium has been discovered we may obtaina clue to the cause of degeneration.

MEDUSA OF MICBOHTDBA. RYDBRI. 639

In the case of Microhydra the Hydroid appears to attachitself to rocks and stones in swift-running streams.

Under such conditions long flexible tentacles, like thosepossessed by the common freshwater Hydra, would streamout with the current, and be of little use for the catching offood. One would rather expect to find in such a situation aHydroid with very short and fairly stiff tentacles, like those ofOoryne, which lives between tide-marks. But Microhydrahas, perhaps, conquered its new habitat at the expense ofits tentacles, as it may be reasonably assumed that thisHydroid is descended from one which formerly inhabitedthe sea.

There is, however, a difference between the Hydroid phaseof Limnocodium and that of Microhydra. TheHydroidofLimnoc odium secretes from its body a glutinous mucus, towhich adhere particles of mud and other debris, so that aprotecting case is formed round the body, leaving only theoral end free, and this end is capable of contracting withinthe tube.

The Hydroid of Microhydra, so far as I can judge fromthe figures and description, forms no protecting case to itsbody. Potts' figure 25 shows the Hydroid attached to theglass of the aquarium with the "base and adjacent partsshowing adherent threads of Nostoc and adventitiousparticles." Any one who has kept a freshwater aquariumknows that the glass becomes thickly coated with unicellularAlgas. Mr. Potts figures this coating surrounding the base ofthe Hydi'oid, the body of the Hydroid being shown by himquite naked.

M e d u s a.—The comparison between the Medusa of L i m n o-codium and of Microhydra is not so simple as that of theirHydroids. In the first place a stage exactly similar to thatof Microhydra has not been described and figured forLimnocodium.

Fowler has described the Medusa-bud on the Hydroid ofLimnocodium at a very early stage, whilst still attached to thepolyp (PI. 35, fig. 1), but as his supply of material failed he was

I640 EDWARD T. BROWNE.

not able to proceed any further. Next we have some very earlyfree-floating stages described by Lankester (PL 35, figs. 7and 8). Although there is still no absolute proof that theMedusa-buds found upon the Hydroid do develop into theMedusa known as Limnocodium, still the circumstantialevidence is very strong.

I have seen many species of marine Hydroids bud offMedusae, but have never seen Medusas liberated at such anearly stage as those of Limnocodium, which look as if theywere developing direct from eggs. On the other hand, theMedusa of Microhydra looks like a Medusa which has justbeen liberated from its Hydroid, and Mr. Potts states thatprobably none of the specimens seen were more than two orthree days old. The Medusa of Microhydra on liberationis at a far more advanced stage than the earliest floatingembryos of Limnocodium. (Compare PL 35, fig. 8, withPL 36, fig. 18).

The embryo of L imnocod ium has already got one senseorgan developed, so one would expect to find sense organs inthe Medusa of Microhydra if it had any. If an adultMedusa has sense organs one always finds (I cannot rememberan exception) a certain number (generally about four or eight)of sense organs present in the young Medusa when ready forliberation. To pin one's faith on the absence of sense organsin preserved specimens is not a safe proceeding, becausesensory vesicles have at times a wonderful way of becominginvisible after preservation, especially when alcohol is used.Their disappearance is genei-ally due to excessive shrinkageof the tissues when the specimens are too rapidly transferredfrom sea water to strong alcohol. The great advantage ofdilute formalin is that it does not produce a shrinkage of thejelly, and that the sense organs can be more easily found.

The great difference between the Medusa of Microhydraand that of Limnocodium lies in the structure of thetentacles. A few years ago I found out that the shape andstructure of the tentacles, and particularly the shape of thebasal bulb, were an exceedingly useful and reliable aid in

MEDUSA OF MIOBOHYDRA BYDEEI. 641

the determination of species. The tentacles of L i m n o -codium are quite unlike those of M i c r o h y d r a . I have afew specimens of L imuocod ium which came from Regent'sPark in my collection, and they show the tentacles in allstages of development. The tentacles even at their earlieststage, when as mere buds upon the margin of the umbrella,show a chai'acter which is not found in the tentacles ofM i c r o h y d r a , nor have I found or yet met with it in any otherMedusa. The nematocysts are definitely arranged at the endsof little papillas. At first there are one or two nematocystsin each papilla, but later on the number increases to aboutfive or more. For the purpose of comparison I selected avery small tentacle of L i m n o c o d i u m , a little over onemillimetre in length, and made a drawing (PI. 37, fig. 3)from the central portion of the tentacle to the same scale asthe drawing of the tentacle of M i c r o h y d r a (PI. 37, fig. 2).I t will at once be seen that there is a marked difference betweenthe tentacles of these two Medusas. The nematocysts havealso a different shape (PI. 37, figs. 4 and 5).

THE REPRODUCTION OF MICROHYDRA.

The hydroid has two methods of reproduction; one isasexual, the other is sexual. Mr. Potts considers thebudding of new hydranths, which are not set free, to be asecond asexual method. The Hydroid is at first a singlepolyp, later on from its base another polyp is developed,but as the second polyp is not detached a colony of twoindividuals is formed. The Hydroid phase of L i m n o c o d i u min the same manner is also colonial, but has from two to fourpolyps. This is not a case of reproduction, as there is noincrease in the number of independent individuals, but simplyone of branching to form a colony.

The asexual method of reproduction of M i c r o h y d r a seemsto me, from the appearance of the figures given by Potts(PI. 36, figs. 17, 21, 24), to be reproduction by fission, whichoccurs in certain marine Hydroids.

642 EDWARD T. BROWNE.

AlltDan, in 1871, gave an account, with figures, of repro-duction by spontaneous fission in a Hydroid which he namedSchizocladium ramosum. Although Allman had workedfor many years upon British Hydroids, yet he had neverbefore met with a case of fission amongst them. Hincks, in1872, found Campanular ia neg lec ta reproducing byfission in a similar manner, and refused to accept Allman'snew genus Schizocladium, "which seems to rest on a singlecharacter, the development of fission-frustules in a certainway—a character which, there is reason to believe, mayhave a wide range amongst the Hydroida." Hincks suggeststhat Allman's Schizocladium is probablyan Obelia. Allmanfound the colony in Loch Long (Firth of Clyde), and states thatit bore a considerable resemblance to that of Obelia di cho-toma. It was without gonosomes, and it was the absence ofthe gonosomes that led Allman to establish a new genus for aHydroid that was reproducing by fission. Allman states thatthe frustule on liberation has a distinct endoderm and ecto-derm, but no perisarc. Ib has no means of locomotion, andattaches itself by a mucous excretion from its surface to theglass of an aquarium. Soon after attachment the mucousexcretion forms round the fission-frustule a very thin tube,which is the perisarc. Once attached a hydrauth developsfrom it, then later on other hydranths are formed and a littlecolony arises by branching.

Fission is not merely the nippiug off a small portion of thecoenosarc, as the fission-frustule contains all the elements neces-sary for the formation of a new colony. It does not usuallytake place at the same time as sexual reproduction. In Micro-hydra the method of fission is different from that inmarine Hydroids, as there are no branches. An outgrowthtakes place from the side of the hydranth (PI. 36, fig. 21),and this is nipped off and develops into a hydranth.

It is very probable that the Hydroid of Limnocodiumalso reproduces asexually by fission. Parsons, who kept theHydroids in. an aquarium, states " that the polyps made theirappearance on the side of a sponge which had been in contact

rr

MEDUSA OF MICRO BYDRA RYDERI. 643

with a pipe (the hot-water pipe in the Victoria Regia tank).This fact leads me to the inference that the polyps weredeveloped from germs contained in the water which I broughtaway with me, for I do not see how they could have got therewhile the sponge was alive; moreover, they were in differentstages of development, the earliest stage seen by me beinga little mound of fuscous coloured sarcode." Fowler hasfigured a section of a bud, which " may either remain attachedto the parent, or may be nipped off and settle close by, itstissues in either case gradually undergoing the differentiationswhich characterise the adult."

The sexual method of reproduction of Microhydra is nodoubt by means of Medusse. Up to the present time only theearliest stage of the Medusa is known. The young Medusahas the appearance of an Anthomedusa, but it is impossibleto assign it to a definite family until the later stages havebeen seen. It would be most interesting to know whatbecame of the Medusa after leaving the Hydroid. The Medusais set free in a stream or river, so that it must be carried alongwith the current in the direction of the sea.

The exact method of the sexual reproduction of Limno-codium still remains a mystery. During the period it livedin England only the male Medusa was found, and not theslightest evidence on the presence of the female sex wasobtainable. The Medusa has not been seen in England since1893, so it is evident that our stock has completely died out.

In 1901 Limnocodium suddenly appeared in the VictoriaRegia tank at Lyons, and an account of it is given by Vaneyand Conte. The Hydroid phase was searched for, but was notfound, and the Medusas were all males.

In 1905 Boecker recorded the appearance of L imno-codium in the Victoria Regia tank at Munich, and againonly males were seen. The author does not mention theoccurrence of the Hydroid phase.

644 EDWARD T. BROWNE.

EEFBRENCES.

On Microhydra.

1885, POTTS, E.—"Microhydra ryderi," Science Bulletin, p.v. (a supple-ment in 'Science,1 vol. v, No. 123).

1897. POTTS, E.—"A North American Freshwater Jelly Pish (Microhydraryderi)," 'American Naturalist,' vol. xxxi, pp. 1032-1035.

On Limnocodium.

1890. FOWLEE, G. H.—"Notes on the Hydroid Phase of Limnocodiumsowerbii," 'Quart. Journ. Micr. Sci.,' vol. xxx, pp. 507-513,pi. xxxii. (This paper contains the bibliography of Limnocodiumup to 1885.)

1893. LANKESTER, E. R.—" Reappearance of the Freshwater Medusa(Limnocodium sowerbii)," 'Nature,' vol. xlix, p. 127.(Records the occurrence of Limnocodium at Sheffield, andgives an excellent summary of it3 life-history.)

1901. VANEY, C, and CONTE, A.—"Sur le Limnocodium sowerbii,"'Zool. Anzeiger,' Bd. xxiv, pp. 533-534; two figures in text.(Records the occurrence of the Medusa at Lyons.)

1905. BOECK.EE, E.—" Ueber das Vorkommen von Limnocodium imMiinclmer botanischen Garten," ' Biol. Centralbl,' Bd. xxv, pp. 605-606.

On Reproduction in Hydroids by Fission.

1871. ALLMAN, G. J.—" On a Mode of Reproduction by Spontaneous Fission inthe Hydroida," ' Quart. Journ. Micr. Sci.,' vol. xi, pp. 18-21, pi. ii.

1871. ALLMAN, G. J. 'Monograph of the Gymnoblastic Hydroids,' p. 151,6g. 61.

1872. HINCKS, T.—" Contributions to the History of the Hydroida," 'Ann.Mag. Nat. Hist,,' Series 4, vol. ix, p. 390.

1901. BILLAED, A.—"De la Scissiparite Chez lcs Hydroides,'1 'Comptes-rendus. Acad. Sci. Paris,' torn. 133, pp. 441-443.

E X P L A N A T I O N OF P L A T E 37.

Il lustrating Mr. E. T. Browne's paper " On the FreshwaterMedusa liberated by M i c r o h y d r a r y d e r i , Pot ts , anda Comparison with L i m n o c o d i u m . "

FIG.1.—Lateral view of the medusa of Microhydra ryderi. x 150.

FIG. 2.—A tentacle of the medusa of Microhydra. Outer side, x 500.

MEDUSA OF MI0EOHTDE4A RYDERI. 645

f- FIG. 3.—The central portion of a very young tentacle of the medusa ofLimnocodium. Drawn for comparison with fig. 2. X 500.

FIG. 4.—A. group of three nematocysts in the tentacle of the medusa ofMicroliydra. x 1000.

PIG. 5.—Nematocysts from the tentacle of Limnocodium. x 1000.

P o s t s c r i p t . — T h e wri t ing of this paper has led me to•/ commence investigations on the methods of asexual repro-v daction amongst Hydroids, and the work is now being carriedij on in the Marine Laboratory at Plymouth. I have found* Allman's "Schizocladium ramosam" and have observed•> the formation of fission-frustules, their liberation, and subse-^. quent development. My observations completely confirmL those made by All man.

The frustnle when nipped off consists of a thin, transparentlayer of ectoderm with nematocysts, a thick layer of endodermloaded with granules, and a hollow central cavity. The bud

.'. which is detached from the hydroid phase of Mic rohydrai (Potts, PI. 36, figs. 17 and 24) is exactly like the fission-

frustule of Schizocladium, both in shape and structure.jf Hincks' suggestion that Sch izoc lad ium is probably an

Obelia has turned out to be correct. Some of the colonies^ have liberated Medusae which belong to the genus Obelia.jk I have also found aClava-like hydroid detaching numerous

fission-frustules from its hydrorhiza. These buds drop to theI bottom of the aquarium and lightly attach themselves by one|k- end to the glass. They are now a week old and have not yet', begun to develop.

t Nov. 4th, 1906.

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