Almost every miscroscopic observer is familiar with the extreme beauty of the horny polypidoms of the Anthozoa, and the calcareous structure of the Polyzoa, when examined as transparent objects in their recent state. There are few persons who have not regretted the extent to which these become disfigured by drying, so as to afford hardly an idea of the elegance which had previously rendered them so attractive. The failure of all attempts to preserve them in balsam and restore them to their original transparency and sharpness of outline induced me, during a recent visit to the coast of Pembrokeshire, to try some experiments in the hope of overcoming this difficulty, which have yielded some interesting results.
The great obstacle to preserving these structures in balsam arises from their retaining, when dried, air in their tubes and cells so obstinately that it is hardly practicable to get rid of it, as well as from their shrivelling up in the process of drying. By the following plan I find the polypidoms may be preserved as permanent preparations, retaining the appearance of the most beautiful recent specimens, wanting only the expanded tentacula of the former inhabitants of their cells to complete the appearance they present when living in their native seas.
The specimens should, if possible, be preserved in weak spirit until leisure is afforded for their preparation: if, however, they have been dried, they should be soaked in cold water for a day or two before being submitted to the following processes.
Select perfect specimens of the proper size for the microscope, which in the larger zoophytes should not exceed two inches in length. Immerse them in water, heated to 120°, in a glass cylinder, and place them under an air-pump receiver. Slowly exhaust the air; torrents of bubbles are given off from the surface of the tubes and cells, and very soon the water will appear to be in a state of active ebullition. In a few minutes re-admit air into the receiver, and after a short time again exhaust; repeat this three or four times. By this process the air is removed from the cells and tubes, watery vapour taking its place; at the same time, by the repeated admission of water into them, and its removal during the process of exhaustion, the internal structure of the poly-86 pidoms becomes freed from the dead polypes and other animal matter. With the exception of a few of the cellular Polyzoa, especially Flustra foliacea and Gemellaria loricata, I have never found any difficulty in thus removing every airbubble.
The polypidoms should now be removed and allowed to drain for a few seconds on a piece of bibulous paper, and then placed in an earthen vessel fitted with a cover and previously heated to about 200°. The best thing for this purpose’is one of the common, thick, white pots, with its cover, used used by druggists to hold ointment. These are so thick that they retain their temperature sufficiently long for the purpose required. They are most conveniently heated by boiling them for a few minutes in water, lifting them out with a pair of forceps, and hastily wiping them with a thick cloth. The specimens, being dropped into one of these vessels, and covered with the loosely-fitting lid, are then to be placed under the receiver of an air-pump, and the air rapidly exhausted. By this process the specimens are very quickly and completely dried, the water being evaporated from the cells and tubes so rapidly that they hardly collapse or wrinkle.
The specimens are to be removed in an hour or two from the air-pump, and dropped into a glass cylinder containing perfectly transparent camphine. This may be quite cold when the homy, tubular polypidoms, as those of the Sertulariee, are used, but should be previously heated to 100°, when the calcareous, cellular Polyzoa are the objects to be preserved. The vessel, being covered with a large watch-glass, must be placed on the air-pump, and the air exhausted and re-admitted two or three times. After this the vessel may be set aside until it is convenient to place the specimens in balsam in the following manner:—
One of the slips of glass intended for each specimen should have a narrow piece of card-board fastened by a little glue to each end so as to prevent the subsequent injury of the structure from pressure. The slip thus prepared should then be carefully cleaned from any dust, and be held over a spiritlamp to warm it sufficiently to allow the balsam to flow freely over it. This should be applied by means of a thick glass rod, so as to cover the glass with a large body of balsam. All air-bubbles must be carefully removed by a needle point in the usual way. Whilst still warm, the polypidoms (previously removed from the camphine and drained for a minute in a watch-glass) should be grasped by a pair of forceps and carefully immersed in the balsam. A second plate of glass, without the pieces of card, should be quickly warmed on the’spirit-lamp, and a thin layer of balsam spread over its surface.It must then be carefully placed over the specimen, by allowing one end to rest on one piece of card-board fixed to the slip of glass, and then gradually lowered. If this be adroitly done, not a bubble of air will be entangled in the preparation. The plates should then be gently grasped in the middle by the wooden forceps or fingers, and fastened together by means of the smallest quantity of sealing wax at each end. Slips of paper are to be carefully pasted round the sides and ends, and the preparation may then be preserved without injury.
Thus prepared, such specimens become the most beautiful of transparent objects for the miscroscope. Their translucency is as complete as in the fresh zoophyte. The structure of the cells and vesicles is most beautifully exhibited. Scarcely any more beautiful objects for the microscope can be thus obtained than those of the common Sertularia abietina and operculata. The vesicles in each are most interesting. The curious mouths of the former, and the opercular lids of the latter, are sure to arrest the attention. These objects are finely shown by a two-inch object-glass; the bird’s-head processes of Cellularia avicularia require, however, an inch-glass; a deeper objective being very seldom required, except for making out very minute structures.
But it is when these objects are examined by polarized light that the most interesting results are obtained. For this purpose, let a piece of selenite be placed on the stage of the microscope, and the polarizing prisms arranged so that the ray transmitted is absorbed by the analyzer. Of course in the absence of the selenite, all light would disappear from the instrument, and none would reach the eye. On placing the selenite on the stage it will, if of proper thickness, allow an abundance of green light to be transmitted. Selenite which presents a bluish or violet tint when thus examined, is not so fitted for these observations.
If, then, a specimen of Sertularia operculata be placed on the selenite stage and examined with a two-inch object-glass, a most beautiful spectacle presents itself. The central stem is shown to be a continuous tube, assuming a more or less pink tint throughout its whole extent. The cells assume a bluish or sometimes violet tint, their pointed orifices, and, indeed, their whole structure becoming much more distinct than when examined by common light. The vesicles appear paler than the rest of the object, and their lids, which so remarkably resemble the operculum of the theca of a moss, being composed of a somewhat denser structure, generally assume a yellowish or orange tint, so that they become beautifully distinct. This zoophyte is often covered with very minute bivalve shells, distinguished by the naked eye from the vesicles only by their circular form,and these when present add much to the beauty of the specimen, presenting a striated structure, and becoming illuminated with the most brilliant colours.
Thus, when submitted to polarized light, thé zoophyte becomes not only a most beautiful, but an instructive object, the relation of the cells to the tube which bears them, and the continuity of the latter being so readily seen. Sertularia filícula is also an interesting object, the waved stem or central tube becoming of a deep dusky red, whilst the cells assume but little colour, renders their mutual relation very obvious. Sertularia abietina is also a fine object, especially when loaded with vesicles as it so often is in the autumn. Halecium halicinum, perhaps the least elegant of this class of beings, assumes a very interesting appearance, its cells assuming a moderate amount of colour. The very beautiful Plumularia falcata acquires fresh beauty under polarized light; for although its cells do not become coloured, merely assuming a pale green, yet the tubular stem becomes more or less of a crimson hue, presenting the appearance of a beautiful feather. It is really remarkable how much more distinct every structure appears, and how much greater a charm is thrown over the elegant structure of the polypidoms when examined in the green light of the selenite. They seem almost, to an imaginative eye, to be once more in their native element.
The most splendid tints are exhibited by the calcareous structure of the Polyzoa, and among these the Flustra truncata is perhaps the most interesting. When a preparation of this zoophyte is examined by polarized light with a two-inch glass without the selenite, the structure of the cells, and the shape of their mouths, are well seen; but in several portions of the specimen the walls of the cells present the appearance of a tesselated pavement, several minute, spherical, coloured structures being scattered over it. On replacing the object-glass by one of one-half inch focus, these spherical bodies present the dark cross with beautiful tints in each quadrant, at first sight resembling the carbonate of lime I discovered some years ago in the urine of the horse. On examining them carefully, however, the polarizing structure will, in many of them, be found to be identical with that seen in the crystalline lens of the cod, or in a spheroid of unannealed glass when immersed in oil, and different from that of a slice of calc-spa or circular plate of unannealed glass. The centre of each spherule being occupied by a black cross with the tinted quadrants, the whole being circumscribed by a black circle. Beyond this extends a second set of black arms with more varied tints between them. A more interesting structure I have never had occasion to examine than that presented by these spherules of carbonate of lime. On placing the selenite plate under the specimens, the black cross and circle became green; and a very beautiful result occurs from some tints being raised, and others depressed, in the scale of colours. On digesting a piece of Flustra truncata in diluted hydrochloric acid, and then putting it upon balsam, like the fresh specimen, this beautiful structure disappeared; all appearances of tessellated tints and coloured spheres had vanished. Hence they depended upon the crystallized arrangement of the carbonate of lime.
The more common Flustra Joliacea is an interesting object on the selenite stage, but does not exhibit the peculiar polarizing structure of the other species.
The Cellularia avicularia is a brilliant object with the selenite stage, its cells being covered with plates of carbonate of lime; it presents a fine display of tints, the bird’s head appendages being exceedingly beautiful.
The Gemellaria loricata is one of the most beautiful objects with the selenite, the cells assuming a pale pink, and the obovate orifices of each—provided apparently with a frame of carbonate of lime to keep them patent—assumes a fine and rich orange tint.
I have alluded to some of the most beautiful of the structures which have occurred to me; but I feel sure, that those observers who have more time at their disposal, will add to our knowledge of the diversity existing between the polarizing structure of these polypidoms. I would especially draw attention to the curious spherules of Flustra truncata; they deserve a very careful examination. 1 was disappointed in not detecting a similar structure in the birds’ heads of Cellularia.
I cannot close this little communication without alluding to ad excellent and very simple plan for preserving the zoophytes as wet preparations, so as to retain the polypes and their tentacular arms in situ. Ellis stated nearly a century ago, that if the zoophytes were plunged into brandy so as to kill them speedily, they might be preserved for a long time. I find, however, that it is better to select a very vivacious specimen and plunge it into cold pure water—the polypes are killed almost immediately, and their tentacula often do not retract: proper sized specimens should then be selected, and preserved in weak alcohol. For this purpose little phials* about two inches long should be made, from very thin, flat glass tube, so as to be half an inch wide and about a quarter of an inch, or even less, from back to front. The specimens being fixed to a piece of thin platinum wire, should then be placed in one of these flat phials (previously filled with weak spirit), so as to reach about half-way down. When several of these are thus arranged, they should be placed in a glass cylinder and removed to the air-pump. On pumping out the air, a copious ebullition of bubbles will take place, and many of the tentacula, previously concealed, will emerge from the cells. After being left in vacuo for a few hours the bottles should be filled up, closely corked, and tied over, like common anatomical preparations. I find that, for all examinations with a one or two-inch object-glass, these bottles are most excellent, and afford cheap and easy substitutes for the more expensive and difficultly managed cells. In this manner specimens of the genera Cycloum, Membranipora, Alcyonidium, and Crisia, exhibit their structure most beautifully.
A few dozen of these little bottles hardly occupy any room, and would form a useful accompaniment of the microscopist by the sea-side. Any one who would visit the caverns in St. Catherine’s Island, at Tenby, could reap a harvest which would afford instruction and amusement for weeks. In these caverns, so rich in zoophytes and sponges that they are really roofed with the Laomedece, Grantice, and their allies, whilst the elegant Tubularice afford a garden-like ornament to the shallow pools on the floor, the walls abounding with the pink, yellow, green, and purple Actinice, days may be spent with instruction and amusement of the most interesting kind. I have, indeed, been informed by my friend Mr. Dyster, of Tenby, who has devoted himself to the investigation of the inhabitants of these caverns with great zeal and success, that no locality affords, in the same space, such an abundant treat for the zoophytologist. I cannot too strongly recommend a visit to them, to all who have a few days leisure in the summer.
Mr. Pastorelli, of Cross-street, Hatton Garden, who has taken much pains to manufacture these little flat phials, supplies them at a very low price.
