THE MICROSCOPE

Evening Star, Issue 19328, 14 August 1926, Page 18

THE MICROSCOPE

SOME OF ITS WONDERS HYDRA: A MIATURE MBNSTER [Written bv G. A. Rawson, for tlie Star.’] No. Yin. There is a large group of lowly organisms known to scientists as the Hydrozca, representatives of which live in both fresh and salt water. A typical example of this group is the freshwater Polype, Hydra, a tiny creature which is a source of unfailing interest to the microscopist; lb is easily procured, being found in almost every pond, where it clings to the leaves and rootlets of duckweed, twigs, and various kinds of aquatic vegetation. Its average length is about one-eighth of an inch, so that it is quite a giant compared with many of the minute denizens of tho ponds wherein it is found. There are throe varieties of hydra—tho common (H. vulgaris), usually of an orange-brown hue, possessing from seven to twelve tentacles longer than tho body; another form of a browner hue, known as Hydra fusca, with from six to eight tentacles several times longer than its body; and green form, or Hydra viridis, which has from six to ten tentacles shorter than tho body. This last is tho best known of all, and wo shall confine our attention mainly to it. Tho creature is called Hydra, after that famous water-serpent of Grecian mythology, which is tabled to have had many heads, which grew again as fast as they were cut off by Hercules. As we proceed we shall soon see tho applicability of tho name to this animal. It is an extremely fragile creature, and must b© handled with tho greatest caution if wc wish to preserve its beauty. At first sight it appears rather unattractive, and might bo mistaken for a mere lump of greenish jelly. As we watch one m a drop of water under tho microscope it soon begins to move. In a_vcry short spaco of time a number of feelers or tentacles are protruded from what may bo called tho head. When these are all extended and are waving about in the water they remind one somewhat of the feathery fronds 6f a palm tree. Look! now they sro swaying about in the water. 1 gently tap the vessel containing tho Hydra, so as to cause a slight vibration, and instantly the tentacles are withdrawn back into the body of tho creature, and it assumes the appearance of an inert lump of jelly. Suddenly a new whim appears to seize it, for it detaches itself from the pond weed to which it was anchored, and swims _ about for a while, at length securing itself to a new piece of weed. Tho following description of the Hydra is based mainly upon that given by Dr Dallinger in tho latest edition of Carpenter’s ‘ Tho Microscope and Its Revelations,’ but with the omission of various technical details which would not prove very interesting to the general reader: —The body of the Hydra consists of a simple tubular bag, or sac, which answers the purpose of a stomach, and is capable of varying its shape and dimensions in a remarkable manner. The u mouth * is a central opening at the upper end ot this sac, and is surrounded by a circle of tentacles, or “ arms,” varying from six to ten in number, and arranged with great regularity- around the orifice. ■ At its lower end the body is prolonged into a narrow base, and on this is situated a suctorial disc, by means of which the Hydra usually; attaches itself to any submerged object, while its tondril-lik© tentacles are permitted to float freely in the surrounding water. . ~ Two layers compose the wall or tno body, the outer of which is known as the ectoderm and the inner as the endoderm. The tentacles are made ot the same materials as the body, but their surfaces are covered with little wart-like prominences, which, upon careful examination, are found to be composed of clusters of ‘ thread cells, with a single large cell in the centre of each, to which is attached, a long, slender dart. . ~ , , Tho curious and ingenious method oy which the Hydra captures its prey by means of its* tentacles must now engage our attention. To see this dearly we°shall require a much greater magnification on our microscope than wo have previously been usm::, say, diameters. Then we shall discover that the Hydra is the possessor ot a really terrible armory of weapons nibn which it attacks its foes and procures its food. When tho tentacles are examined under a magnification of 500 diameters vast numbers of the atovementioned curious thread cells distinctly discernible. short, stings, and whole battel ics of them can bo discharged at once from the cells in which they are embedded.” Should any tiny creatures such as small rotifers or infusorians, be unfortunate enough to get within icach of these weapons they appear tobo come electrified or para yrecL and in a few moments are dead. When the victim is quite benumbed tho tentacles will be seen gradually to wrap round 5+ nnd draw it into the inoutn. m “Hread-cclls” which thickly stud the tentacles of onr little Hydra are responsible for this dead y e 1 its victim. Mach coll is composed of a little sac filled with a poisonous Quid, and both dart and fluid are in a state of considerable tension, so that the least external pressure niptum the little sac containing them, the contents are then forcibly ejected, and the end of the dart enters the victim and pierces it ready for the poisonous fluid to follow. Anyone who has the proper appliances and possesses a lair amount of patience may easily watch tho whole process of the capturing, paralysing, and devouring of such tiny creatures as the Hydra subsists upon. The elasticity of tho tentacles is very remarkable. These organs enable the Hydra to capture animals whose activity, as compared with its own slight powers of movement, is vastly greater, and might well be supposed to render them safe from its clutches. However, if a minute worm or other tiny creature, when moving through the water, happens' bo touch one or these waving tentacles it is immediately seized, and other arms are quickly coiled round it, after the manner of an octopus, and it is soon conveyed to the stomach, within which it may sometimes be seen moving for a little while. But soon its struggles cease. The soft parts of its body dissolve away completely, and the hard, indigestible portions are ejected by the Hydra through the month. These stinging “ thread cells ” are common to tho group of Hydrozoa,- of which Hydra is tho lowest representative. One microscopist introduced some water fleas (daphnia), which usually abound in every pond, into a tank containing a few Hydras. The experiment proved interesting, for, although water fleas are extremely agile creatures, yet a most exciting scene soon took place. The water fleas evidently knew their foes, for they kept at a respectful distance from them. But all their efforts to save themselves were unavailing; the darts flew thick and fast, ana one by one the creatures were captured and quickly succumbed and were drawn into the voracious maws of the Hydras by the encircling tentadefy

Mr Lewis Wright relates an amusing anecdote concerning a fight between a water flea and a Hydra. A gentleman friend of his was once giving a lecture on ‘Pond Life’ to an audience of soldiers. He was illustrating his remarks with a lantern projection microscope, and a living Hydra (of course, enormously magnified) _ was thrown upon the screen, when it was seen to seize a passing water flea. Instead of being paralysed, the latter made the most violent ettorts to escape amid excited murmurs from the assembled soldiers, who were keenly interested in the contest, as the habits of the Hydra had just been previously explained to them. At last, concludes Mr Wright, the Daphma (water flea) hurled itself free, when suddenly every man m _ the room sprang to his feet and joined in a deafening cheer—the queerest experience, the lecturer said, had ever had !” I have exhibited living Hydras through the projection microscope on various occasions to juvenile, as well as to adult audiences, but never with such an exciting termination as that described by Mr Wright. . . jQCbi. ho oen&medJg.