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indicate homological identity, though we should be cautious in concluding as to homological relations from mere resemblance; where functions of the same kind are to be performed, there must necessarily be a certain degree of resemblance in the instruments, and this resemblance may show itself not only in form but in position and relation to surrounding parts, though the organs themselves be fundamentally different; and it needs the light afforded by development before our conclusions as to homological identity can be considered as fully established, while our ignorance of facts connected with the embryology of the Brachiopoda deprives us of this important test in the present instance. Mr. Huxley" has thrown still further light on the relations between the Polyzoa and Brachiopoda. He compares the relative positions and mode of articulation of the valve which closes the cell in the marine cheilostomatous Polyzoa and of the peculiar bodies known as avicularia, which occur in the same group of Polyzoa, with corresponding characters in the shells of the typical Brachiopods. He shows that the smaller valve of the Brachiopod is articulated with the larger, just as the operculum of the Polyzoon is united with its cell, or as the lesser valve of an avicularium is articulated with the larger one; while the anus has the same relative direction in both, being in the one case turned away from the operculum of the Polyzoon, and in the other from the smaller valve of the Brachiopod. He further shows that, leaving the pedicle muscles out of consideration, the arrangement of the other muscles is exactly what occurs in an avicularium ; “the adductors, which pass from the larger valve to be inserted into the smaller, in front of its point of support, corresponding precisely with the occlusor muscles of the avicularium; while the cardinal muscles which arise from the larger valve, and pass to be inserted into the cardinal process of the smaller, behind the point of support, are identical with the divaricator muscles of the avicularium.” The relations involved in the above views are very interesting. In order, however, to admit them, even to the extent which our ignorance of Brachiopodal embryology will admit, we must cease to look upon the avicularium as a mere organ in the ordinary sense of this word, and consider it as a distinct though very peculiar zooid, homologically repeating the structure of the ordinary zooid, a view against which no valid objections can be urged. We thus perceive that the affinities of the Polyzoa pass off in two very evident directions —in one through the Tunicata, and in the other through the Brachiopoda; while the Tunicata and Brachiopoda conduct us by well-marked relations through the Lamellibranchiata into the higher Mollusca. In determining the limits between the Mollusca and the Molluscoida, Huxley draws the boundary line upon the molluscan side of the Brachiopoda, thus including the latter in the Molluscoida. To this view, however, I have not been able to assent. Notwithstanding the obvious affinities just referred to between the Brachiopoda and the Polyzoa, I yet believe that the Brachiopoda are much more intimately allied to the Lamellibranchiata than they are to the Polyzoa. The oral arms of the Brachiopoda, in which the most interesting resemblance between this group and the Polyzoa is to be found, while they are undoubtedly related to the lophophore of the latter, have just as decided a relation to the labial tentacles of the Lamellibranchiata, while not only in the condition of the nervous and circulatory systems, which are both upon
* Article “Mollusca” in English Cyclopaedia,” 1855.
a different plan from anything we find in the Tunicata or Polyzoa, but in that of the organization generally, the Brachiopoda possess characters which will scarcely admit of the association of these bivalves with the Molluscoida.”
* I cannot conclude this division of our subject without referring to a most singular little animal, whose true position is undoubtedly among the Annelida, but which in many points repeats the polyzoal form so exactly as to render it at first uncertain whether it be not really a hippocrepian Polyzoon that the observer has under his eyes.
I am indebted to Dr. Wright, of Edinburgh, for an opportunity of examining the little animal in question, of which he obtained two or three specimens from the coast of Devonshire, in May, 1856, and which he preserved alive for some weeks in a vessel of sea-water. Dr. Wright has published an account of it in the “Edinburgh New Philosophical Journal’ for October, 1856, where he describes it under the name of Phoronis hippocrepia. He also kindly placed one of his specimens at my disposal
Phoronis hippocrepia, Wright; the annelidan homomorph of the hippocrepian Polyzoa.
Fig. 13. View in a plane at right angles to that of fig. 12; the view of the tentaculiferous crescent is semi-ideal, the tentacula being supposed to be removed from part of its margin, in order to expose the mouth and over-arching lip.
Fig. 12. Wiewed from the side of the oesophagus.
a a. Tentaculiferous crescent. b. Mouth. c. Lip. d. CEsophagus. e. Intestine. f. Wessel conveying the blood backwards from the tentacula and crescentic disc. g.g. The two vessels from whose union the vessel fresults. h. Wessel conveying the blood forwards towards the crescentic disc and tentacula. ii. Longitudinal muscular fibres in stem.
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 oesophagus 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 oesophagus, 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 phylactolaematous 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 oesophagus, and is formed by the union of two branches, one from each arm of the crescent, which embrace the oesophagus 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 Oesophagus, 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 in continual motion, and were often propelled from one cell into a neighbouring one; a fact 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 Euphorbiae.
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 none which can render 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,
* Bon NET, ‘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 Baeck.” 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,t 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 oesophagus, stomach, and intestine of the “Polype à Panache;” and though he has seen, and expressed in his figures the expulsion of the focal matter, and even, though imperfectly, represented the alimentary canal, he cannot bring himself to recognise in this tube its proper function. The faeces, moreover, are incorrectly figured as escaping at the side of the plume, opposite to the rectum. His
* BAeck, “Berättelse om Watten Polypen, i anledning af dem som firo fundne omkring Stockholm.” Acta Holm., vii.
+ BAKER, “Employment for the Microscope.” London, 1753.
f J. CHR. Schäffer, Die Armpolypen in den siissen Wasser um Regensburg entdeckt und beschrieben.” Regensburg, 1754.
§ Rösel, “Insecten-Belustigungen.” Nürnb., 1746–1761.