Imágenes de páginas
PDF
EPUB
[ocr errors][merged small][merged small][merged small][merged small][merged small]

We have already seen that in the year 1741, Trembley discovered his “ Polype à Panache.” This is the first recorded discovery of a fresh-water Polyzoon. Trembley communicated his discovery to Reaumur and Bernard de Jussieu ; and these celebrated naturalists detected the statoblasts, which they took for eggs, and observed the escape from them of the young Polyzoon.

In 1744, Trembley published his famous memoirs on fresh-water polypes.* In these is a full description of the “ Polype à Panache,” accompanied with two figures; the description is wonderfully accurate, and the anatomical details have been in few points surpassed by subse

for examination, and I have been thus enabled to make the annexed drawing, which represents accurately the structure so far as it was possible to determine it from the single specimen examined.

The animal inhabited a fissure in a fragment of rock, from which it was capable of being protruded to the length of about two lines, and into which it would again suddenly withdraw on the least annoyance. When fully protruded, it presented the form of a cylindrical stem, totally destitute of annulation, and bearing upon its summit a crescentic disc, margined with tentacula. This tentaculiferous disc exactly resembled the lophophore and tentacula of a hippocrepian Polyzoon. In the body of the crescentic disc was placed the mouth, over which there arched a valve-like lip, situated exactly as the epistome of a Polyzoon. The mouth led into an esophagus which could be easily traced backwards within the perfectly transparent walls of the stem, until it was lost in the piece of rock which concealed the posterior part of the animal. Parallel to the esophagus, and on the side corresponding to the concavity of the crescent, the rectum was seen passing forwards to open by a distinct anus, in the bottom of this concavity, just beside the mouth. Round the entire margin of the crescent was borne, in uninterrupted succession, a series of ciliated tentacula, surrounded at their base by a membrane resembling the calyx of a phylactolæmatous Polyzoon.

So far the structure is absolutely undistinguishable from that of a hippocrepian Polyzoon, which it resembles even in minute and apparently non-essential details ; but when we come to examine the rest of the organization, we find that the polyzoal type is widely departed from. On the rectal side of the animal may be seen passing from behind forwards, and in close relation with the intestine, a very distinct pulsating vessel, which carries a stream of red corpusculated blood into the cavity of the crescentic disc. From this the blood passes into all the tentacles, and may be seen as a continuous stream flowing towards their extremities. When it arrives there, it takes a retrograde course, flowing back again through the same channel into the cavity of the disc. We now find that the returning blood has entered another great vessel, which lies upon the æsophagus, and is formed by the union of two branches, one from each arm of the crescent, which embrace the esophagus just behind the mouth. Through this vessel the blood flows backwards in a continuous stream. The great afferent and efferent trunks, now described, were traced as far back as the fissure in which the little animal was lodged, where the concealment of the parts rendered it impossible to follow them further. In the afferent

* TREMBLEY,' Mémoires pour servir à l'Histoire d'un genre de Polypes d'eau douce.' Leyde, 1744. quent observers. Trembley has demonstrated a complete alimentary canal, consisting of csophagus, stomach, and intestine; but though he has seen the emission of the excrementitial matter from the intestine, the actual termination of this tube has escaped him. He has observed the retractor muscles, and has noticed the funiculus attached to the fundus of the stomach ; this last organ, however, he supposes to be a muscle destined, like the true retractors, for the withdrawal of the polypide into its cell. He has shown the connection between the polypide and its cell, and demonstrated that the former ought not to be viewed as the mere fabricator of the latter, as the caddis worm is of his case, but that each is an inseparable part of one and the same animal. He witnessed also the circulation of the fluid in the perigastric space. This circulation was rendered evident by the presence of small spherical bodies, which were kept

which at once led him to infer the continuity of the cavities of the different cells. These corpuscles he suspected, but without sufficient grounds, to be the eggs of his polype. We find that, in a letter addressed to Bonnet, and quoted by this philosopher in his. Considérations sur les Corps Organisés,'* Trembley describes with great accuracy the statoblasts of a fresh-water

vessel the current of blood was intermittent. Immediately on being emptied, the whole of the visible portion of the vessel contracted vigorously, and then slowly expanded for the reception of a fresh influx of blood, the pulses being at very regular intervals. No reversed action could be observed in the pulsation of this vessel, though, as already mentioned, the current of blood in the tentacula was alternately direct and inverse. In the efferent vessel there was no pulsation, and the current was here perfectly continuous. The circulation in the interior of the crescentic disc appears to be properly extra-vascular, and the cavity of this disc must be viewed as a great sinus.

Another important point of departure from polyzoal structure is found in the absence of a proper perigastric space, the alimentary canal and vessels being fixed in the stem, and embraced by its walls, so as to leave little or no intervening space, while the perigastric fluid is here replaced by the true blood in its proper vessels. There is, therefore, no power of invagination, and retraction is probably an act entirely similar to that by which the ordinary tubicolous Annelides are withdrawn into their tubes. Well-marked longitudinal, muscular (?) fibres are visible in the stem.

The nervous system was not detected; and, as it was impossible to expose the posterior part of the animal without running the risk of its entire destruction, much of its organization still remains altogether unknown..

As to the true relations of the little animal now described, it appears to me that they are to be found with the Annelides rather than with any other group of the animal kingdom; and, notwithstanding the singular resemblance in certain parts of its structure between it and a hippocrepian Polyzoon, I believe that this resemblance points to no real affinity, and must be viewed only as a remarkable example of representative form,—of homomorphism as distinguished both from homology and analogy,—of which the resemblance between the Polyzoa and the Polypes affords another, and which is paralleled in the vegetable kingdom by such instances as that presented by the repetition of the Cactus form in the African Euphorbiæ.

The above account was drawn up and in the printer's hands before I had read Dr. Wright's description, from which it slightly differs in one or two particulars, but in non nugatory the special point which has caused its introduction here ; namely, the remarkable and highly instructive example afforded by it of the assumption of polyzoal form by a widely different type of organization, without this identity of form in any way indicating a relation of homology.

none

* BONNET, Considérations sur les Corps Organisés. Amst., 1762.

Polyzoon of a different species from the “ Polype à Panache,” and probably a Plumatella. He informs us that he obtained them in England in 1745, and that, after having preserved them for many months in a dry state, they produced young polypes on being placed in water. He naturally considered these statoblasts as eggs.

In the · Transactions of the Royal Academy of Sciences of Stockholm, for the year 1746, is a memoir on fresh-water polypes, by Bäck.* In this memoir two kinds of Polyzoa are described; one is the “Polype à Panache” of Trembley, whose figures he reproduces; the other is probably Alcyonella.

In Baker’s ‘Employment for the Microscope,' published in 1753,7 we have, under the name of “ The Bellflower Animal,” the description of a Polyzoon which would seem to be identical with the “ Polype à Panache.” Baker is an accurate observer and a faithful recorder of what he has witnessed. The structure of his " Bellflower Animal” is described with much fulness ; like Trembley, he has recognised a complete digestive canal, and has even gone further than the celebrated historian of the · Polypes d'Eau douce,' in detecting the termination of the intestine.

In 1754, Schäffer published a memoir on the fresh-water polypes of the neighbourhood of Regensburg. $ In this memoir he describes and figures, under the name of “ Kammpolyp,” a Polyzoon composed of branched tubes, which extend themselves over the surface of submerged stems and leaves. There would seem to be little doubt of Schäffer's animal being the true Plumatella repens of succeeding authors.

In 1755, Rösel published the first volume of the Supplement to his ‘Insecten-Belustigungen.'S We have here a history of the fresh-water Polyzoa which had come under his own observation. Under the name of “ der Federbusch-polyp,” he describes the Plumatella repens; his specimens mostly belonged to the free variation of this Polyzoon, while Schäffer seems to have had in view the attached form. In his account of the internal structure, though he had the advantage of Trembley's and Baker's observations on the “ Polype à Panache,” he falls far behind these naturalists in accuracy. He observed the statoblasts in the interior of the tubes, and, under the impression that he had recognised similar bodies on the under surface of the floating fronds of Lemna, he believes them to be the seeds of thisplant, and supposes what he had seen in the interior of the tube to be some of these seeds which the Polyzoon had swallowed. This belief had so taken possession of him, that, plainly seeing the statoblasts external to what Trembley had already described as the alimentary canal, he is forced to deny the accuracy of Trembley's account of the esophagus, stomach, and intestine of the “Polype à Panache;" and though he has seen, and expressed in his figures the expulsion of the fæcal matter, and even, though imperfectly, represented the alimentary canal, he cannot bring himself to recognise in this tube its proper function. The fæces, moreover, are incorrectly figured as escaping at the side of the plume, opposite to the rectum. His

* Bạck, ‘Berättelse om Watten Polypen, i anledning af dem som äro fundne omkring Stockholm.' Acta Holm., vii.

+ BAKER, 'Employment for the Microscope.' London, 1753.

I J. Chr. SCHAFFER, ' Die Armpolypen in den süssen Wasser um Regensburg entdeckt und beschrieben. Regensburg, 1754.

§ Rösel, 'Insecten-Belustigungen, Nürnb., 1746—1761.

ina

description and figures of the external characters are tolerably good, with the exception of what he says of a kind of collar, which he compares to the iron ferule on the handle of an awl, and represents as surrounding the orifices of the tubes. Rösel has here evidently seen the tentacular sheath in a semi-everted state, in which it presents somewhat the appearance he has described; and believing this to be a permanent character of his polype, has been thus led into error. He has witnessed the currents caused by the tentacular cilia, but has attributed them to water expelled from the mouth.

A discovery by Rösel of much interest was that of certain little roundish bodies, about the size of the head of a pin, and of a yellowish colour, which, in the month of May, 1754, he found in the water of a neighbouring pond, and which, on being allowed to rest, emitted from various parts of their surface little plumes like those of his “ Federbusch-polyp.” Rösel has described these little bodies under the title of “Der Kleinere Federbusch-polyp mit dem ballenförmigen Körper.” They are the young condition of the Polyzoon, for which Cuvier afterwards constituted a new genus, under the name of Cristatella.

Linnæus's grand invention of a binary nomenclature had now been established, and we accordingly find in the tenth edition of the Systema Naturæ,' published in 1758, the freshwater Polyzoa for the first time designated by a generic and specific name.* The “ Federbusch-polyp” of Rösel is here described under the title of Tubipora repens, but with Bạck erroneously referred to among the synonyms. In the “Fauna Suecica,' 1761, we find the same animal mentioned under the same name.t

In 1766, Pallas published his · Elenchus Zoophytorum.'I In this work we have the fresh-water Polyzoa described under generic and specific names, in accordance with the example already set by Linnæus. Under the genus Tubularia, Pallas includes two fresh-water Polyzoa; one is the “ Polype à Panache ” of Trembley, which is now described by Pallas under the name of Tubularia crystallina ; the other, the “ Federbusch-polyp” of Rösel, described under that of Tubularia gelatinosa.

In the year 1768, Pallas presented to the Royal Academy of Sciences of St. Petersburg, a memoir on a peculiar production which he had discovered in a lake connected with the River Kliasma, near Vlademir, in Russia. It was in the form of large fungoid or spongy masses,

ed of multitudes of closely compacted tubes, each of which opened upon the surface of the mass by a pentagonal or hexagonal orifice, which allowed of the exsertion of a polypoid body in all respects resembling those of Trembley and Rösel. To his newly discovered animal, Pallas gave the name of Tubularia fungosa; it is identical with that subsequently named Alcyonella stagnarum by Lamarck. The memoir is illustrated by a plate exhibiting the external characters of the Polyzoon, and a magnified but not very correct view

e tentacular crown, and, though quite destitute of anatomical detail, must yet be viewed as an important contribution to the zoology of the day.

Leendert Bomme, of Flissingen, in 1769, described the tentacular currents in certain

* LINNÆUS, 'Systema Naturæ,' editio decima. Holmiæ, 1758, vol. i, p. 790.
+ LINNÆUS, ‘Fauna Suecica.' Stockholmiæ, 1761, p. 537.

PALLAS, 'Elenchus Zoophytorum.' Hagæ-Comitum, 1766.

Pallas, Descriptio Tubulariæ fungosæ prope Volodemirum observatæ. 'Nov. Comm. Petr., xii, p. 565.

ma

marine Polyzoa, and detected the cilia by whose action these currents were produced ;* he observed similar currents, and the cilia causing them, in some fresh-water Polyzoa which he found in the Isle of Walcheren.

We have seen that Trembley ascribed the currents in question to the motion of the tentacula, while Rösel, on the other hand, attributed them to the water ejected from the mouth. Leendert Bomme has the credit of referring them to the true cause; while in this discovery he has effected an important extension of our acquaintance with vibratile cilia, organs which had previously not been known beyond the Infusoria and Rotifera, and which have since been recognised as so very widely distributed, and so intimately connected with some of the most important functions of living beings.

In the · Vermium terrestrium et fluviatilium Historia, published in 1773,+ Müller institutes the name of Tubularia repens for the “Kammpolyp” of Schäffer, which he considers specifically distinct from the “Federbusch-polyp” of Rösel, chiefly on the grounds of its being deprived of the collar, which, as we have already seen, Rösel has erroneously attributed to his animal.

Müller has distinctly traced with Trembley and Baker an oesophagus, stomach, and intestines, and has pointed out the error of Rösel regarding the statoblasts, which this naturalist mistook for the seeds of Lemna.

In September, 1774, Blumenbach described, at a meeting of the Royal Society of Göttingen, a new fresh-water Polyzoon, which he had discovered in the neighbourhood of that town. I He names it Tubularia sultana, and gives a figure of it in his · Manual of Natural History,' published in 1779.$ It is at once distinguished from all previously described freshwater Polyzoa by the tentacula being disposed in a circle, instead of presenting the form of a crescent, by which the others are all characterised. Blumenbach's Tubularia sultana is the Polyzoon afterwards discovered by Gervais at Plessis-Piquet, near Paris, and for which this naturalist found it necessary to constitute a new genus under the name of Fredericella. Blumenbach characterises it in his manual as “ Tubularia crista infundibuliformi, ad basin ciliata ;" the latter part of this diagnosis can scarcely refer to the true vibratile cilia which clothe the tentacula in their entire length, and is apparently framed from an imperfect observation of the caliciform membrane with which the base of the plume is furnished, and whose deeply festooned margin may have suggested the character “ciliata.”

In 1776, Eichhorn published his · Beiträge zur Naturgeschichte der Kleinsten Wasserthiere.'ll In this we have a description with figures of a fresh-water Polyzoon, which he calls “Der Polyp mit dem Feder-busch.” Eichhorn's animal would seem to be the attached form of Plumatella repens, and identical with Schäffer's. He represents a specimen extending over

* LEENDERT BOMME, ‘Bericht aangaande verscheiden zoonderlinge Zee-Insecten.' Acta Vliss., 1769.

+ Müller, 'Vermium terrestrium et fluviatilium Historia.' Lips., 1773.

| BLUMENBACH, Von den Federbusch-polypen in den Göttingeschen Gewässern. Göttin. Mag.,' i, p. 117.

Ś BLUMENBACH, 'Handbuch der Naturgeschichte. Göttingen, 1779.

|| EICHHORN, ‘Beiträge zur Naturgeschichte der kleinsten Wasserthiere in den Gewässern und um Danzig.' Danz., 1776.

« AnteriorContinuar »