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In the same year, MM. Dumortier and Van Beneden presented, conjointly, to the Royal Academy of Sciences at Brussels, a second and very important memoir on the 'Natural History of the fresh-water Polyzoa.'* This memoir is occupied with the anatomy of the genera Paludicella, Fredericella, Alcyonella, and Lophopus, and accompanied by numerous wellexecuted figures. It contains extensive and careful anatomical details of all the genera treated of in the memoir. It describes and figures with much minuteness the development of the bud in Paludicella, and mentions the occurrence of a peculiar winter bud in this Polyzoon, occupying the position of the ordinary buds, but destined to remain during the winter months in an undeveloped state. The structure of the testicle in Alcyonella is examined, and the spermatozoa, with their vesicles of evolution, demonstrated; but these cells are not sufficiently distinguished from the contained spermatozoon. The ciliated embryos of Alcyonella are also described and figured, but the authors do not pursue their development into much detail, while they consider them as identical with statoblasts in a particular stage of evolution, and deprived of their external shell. The statoblasts themselves in Alcyonella and Fredericella are described, but the essential structure of an ovum is attributed to them, while some confusion has arisen with regard to the statoblast of Lophopus crystallinus, the body described as such being manifestly the statoblast of Cristatella. A statement formerly made by M. Dumortier, that the tentacula of Lophopus crystallinus are deprived of cilia, is repeated here; it is asserted that, instead of ciliary vibrations, the tentacula of this Polyzoon present a moniliform current, which ascends one side and descends the other of each tentacle; the appearance of these currents is compared to that of an endless chain in uninterrupted motion, and attention is drawn to the analogy of this phenomenon with that of the decomposition of water by the galvanic battery. I have no doubt, however, that the phenomenon thus described is truly a case of ciliary vibration, and that the cilia have merely escaped the observer in consequence of some defect in the microscope employed in their investigation. I have repeatedly had under my own observation a species of Lophopus, which I do not hesitate to refer to M. Dumortier's species, and yet I found the cilia in all cases perfectly distinct. An opinion previously expressed by M. Van Beneden, when he thought he had seen apertures (“ bouches aquifères ”) for the admission of water into the perigastric space, is here given up, and the source of the error pointed out. Further, M. Van Beneden, now finding a testicle in the same cell with the statoblasts, modifies his previous views as to the unisexualism of Alcyonella, and, comparing this Polyzoon to the plants belonging to the twenty-third class of Linnæus, he suggests that male, female, and hermaphrodite individuals may all coexist in the same cænæcium. On the whole, this memoir of the learned Belgian naturalists, though in some respects incorrect, must be regarded as the most important, in an anatomical point of view, of any which had as yet appeared.
In the same year (1848), Sir J. G. Dalyell published the second volume of his · Rare and Remarkable Animals of Scotland, '† and described in this work several species of fresh-water Polyzoa, as inhabitants of that part of the British Islands. Dalyell is a truthful observer and a graphical describer of the habits of the lower invertebrate animals, but he is not
* DUMORTIER et Van BENEDEN, Hist. Nat. des Polypes composés d'eau douce, 2° partie. Com. plément au tome xvi des Mém. de l'Acad. Roy. des Sciences et Belles-lettres de Bruxelles,' 1848.
† DalyeLL, ' Rare and Remarkable Animals of Scotland, represented from living subjects.' Loudon, 1817-8.
always acquainted with the labours of others in the same field; and in the case of the fresh-water Polyzoa, it is often extremely difficult to identify his species—a difficulty much enhanced by the want of exactness in his numerous figures. Under Lamarck's name of Cristatella vagans, he describes the C. mucedo (Cuvier), and is the first after Gervais to figure fully-developed specimens of this beautiful Polyzoon. Under the name of Alcyonella gelatinosa he describes the true Alcyonella fungosa, while his Alcyonella stagnarum appears to be the young condition of a Plumatella. His Plumatella repens would seem to include more than one species, but neither from his descriptions nor figures is it possible to determine the exact animal intended.
At the meeting of the British Association for 1849, I noticed the addition to the Irish Fauna, of Lophopus crystallinus, which I had found abundantly in the pond of the Zoological Gardens near Dublin, * and described a new species of Plumatella (P. coralloides of the present monograph); and on the same occasion I described the distribution of the nerves in Plumatella repens.t
In January, 1850, I presented to the Royal Irish Academy, a memoir on the · Natural History of the genus Alcyonella.' This memoir contained a historical introduction to the subject, recorded the addition to the British Fauna of the A. flabellum of Van Beneden, and gave a detailed account of the anatomy of A. fungosa. The muscles of this Polyzoon were divided into eight distinct sets, the distribution of the nervous system was demonstrated, and the structure of the locomotive embryos was described, and certain errors in the description given by Meyen of these bodies were pointed out.
In the same year, Mr. Albany Hancock, already well known by numerous important papers on the anatomy of the Mollusca, but especially by his association with Mr. Alder in their beautiful · Monograph on the Nudibranchiata, published an admirable paper on certain species of fresh-water Polyzoa obtained in a small lake in Northumberland. In this paper the author gives a very full account of the anatomy of Plumatella, Fredericella, and Paludicella, characterised by great accuracy, and illustrated by excellent figures, though in some points, as already noted in the anatomical portion of the present work, I have found reason to differ from his conclusions. He also draws attention to the resemblance between the arms of the lophophore in the hippocrepian Polyzoa and the oral arms of the Brachiopoda, and compares the arrangement and action of certain muscles in the two groups—important points tending to throw light on the affinities of the Polyzoa. The same paper contains descriptions of new species, which the author records under the names of Plumatella punctata, P. Allmani, and Paludicella procumbens. After carefully considering Mr. Hancock's description of his Paludicella procumbens, I cannot satisfy myself that the characters on which the species is founded are sufficient to entitle it to be considered distinct from P. Ehrenbergi ;
* ALLMAN, On Lophopus crystallinus. Reports of British Association,' 1849.
† Allman, On the Nervous System and certain other points in the Anatomy of the Bryozoa. * Reports of British Association,' 1819.
Allman, The Natural History of the genus Alcyonella. 'Proceedings of Royal Irish Academy,' 1850.
$ Hancock, On the Anatomy of the fresh-water Bryozoa, with descriptions of three new species. 'Ann. and Mag. Nat. Hist.,' March, 1850.
the most important difference, consisting in the greater number of tentacula, is founded on a figure of Paludicella Ehrenbergi, given by myself some years before, and which having been incorrectly engraved with too many tentacles, has thus unfortunately become a source of error in Mr. Hancock's determination of the Northumberland species.
In the same year, I presented to the British Association a report on the state of our knowledge of the fresh-water Polyzoa,* in which it was my object to give a detailed account of the anatomy of these animals, and a synopsis, with diagnoses, of all the known species; and in the year 1852, I read, before the Royal Irish Academy, a memoir on the homologies of the organs in the Tunicata and Polyzoa,t in which the fresh-water hippocrepian forms were adduced as affording a means of clearing up some difficult points in the homological relations of the two groups.
In 1851, we find several communications on the subject of the fresh-water Polyzoa of Pennsylvania, presented by Dr. Joseph Leidy to the Academy of Natural Sciences of Philadelphia. I In these the author describes some new species of Plumatella, and two new genera (Pectinatella, Leidy, and Urnatella, Leidy) of fresh-water Polyzoa.
In 1854, Leidy presented to the Academy an additional notice, in which, with an amended diagnosis of Urnatella, he confirms the claim of this animal to rank as a distinct genus of fresh-water Polyzoa; while, at the same time, he describes another new species of Plumatella. Leidy's account of Urnatella is confined to a simple diagnosis, but the author proposes to give, hereafter, a full description of the genus. Leidy's communications on the subject of the fresh-water Polyzoa must be regarded as among the most important contributions in a zoographical point of view which have of late years been made to this department of natural history.
For some years past, I have continued to make the fresh-water Polyzoa the subject of careful study, and the result has been the acquisition of many new facts, and the correction of some errors into which I had previously fallen. The later additions which I have thus succeeded in making to our knowledge of these animals have been hitherto unpublished, and are now, in the present monograph, for the first time made known.
Habits Of The FRESH-WATER PolyzoA.
Besides presenting well-marked differences in form, the fresh-water Polyzoa differ also considerably from one another in their habits. Some delight in the pure clear water of
* Allman, Report on the present state of our knowledge of the fresh-water Polyzoa. “Report of British Association,' 1850.
+ Allman, On the Homology of the Organs of the Tunicata and the Polyzoa. “Transactions of the Royal Irish Academy,' vol. xxii, 1852.
* LEIDY, in ‘ Proceedings of the Academy of Natural Sciences of Philadelphia,' vol. v, pp. 261, 265, 321.
§ Id., vol, vii, p. 191.
subalpine lakes or of rapid rivulets, where, by close adhesion to the under surface of stones, they avoid the danger of being carried away by the current of the stream ; while others prefer slowly running rivers or the sluggish waters of canals and ponds.
They are almost all light-shunning animals, loving the dark and gloomy recesses of the lakes and rivers they frequent, where they may be found beneath the shade of aquatic plants, or attached to the inferior surface of stones, or lurking under the arches of bridges where a ray of direct sunlight never enters ; Cristatella alone delights in exposure to the full influence of the solar beams, and may be seen basking upon the upper side of submerged stones, or creeping over the stems of aquatic plants in the clear waters of lakes and ponds.
With the solitary exception of Cristatella, they are all, in their adult condition, utterly incapable of locomotion, being then permanently attached to some fixed object; Cristatella, however, creeps about on the stones and plants of its native lake, and thus affords the only instance as yet known of a truly locomotive Polyzoon.
Lastly, some are timid creatures, withdrawing into the recesses of their cells on the slightest disturbance, and not again daring to venture forth until a long lapse of time has convinced them that all is once more quiet without; while others must be roughly handled before they will think of retreating; and the light-loving Cristatella rejoices in the constant exposure of its plumy crown, no ordinary disturbance will force it to retire, it seems altogether incapable of existing, except in the midst of the countless vortices which its ciliated tentacula are for ever whirling around it.
GENERAL GEOGRAPHICAL RANGE OF THE FRESH-WATER POLYZOA.
The number of forms of fresh-water Polyzoa which, after careful comparison, I have deemed it right to retain as distinct species, amount to twenty-one in all. Of these, sixteen are British; and of the remaining five, one has only been described as occurring in the fresh waters of Belgium, while four are confined to the United States of America Of these four American species, two constitute respectively the types of two distinct genera.
We do not, however, as yet possess a sufficient number of observations to entitle us to advance any generalisation of much value as to the geographical distribution of the freshwater Polyzoa. The most northern point at which we have any positive information of the discovery of a species is probably the neighbourhood of Stockholm (Bäck, in “ Acta Holm., vii, 1745), while the neighbourhood of Nice (Risso, ' Hist. Nat. de l'Europe Mérid.') would seem as far as is yet known to be the most southern limit of the group in the Eastern Hemisphere, and Philadelphia (Leidy, ' Proceed. Acad. Nat. Sci. of Philadelphia,' vols. v and vii) to be the most southern 1 mit at which we have any record of a fresh-water Polyzoon having been discovered in the Western Hemisphere. Vlademir, in Central Russia, where Alcyonella fungosa was originally found by Pallas (Pallas, in . Nov. Comm. Petr.,' 1768), is the most eastern point, while Philadelphia (Leidy, l. c.) must for the present be viewed as the extreme limit towards the west.
The species, so far as we are as yet acquainted with their distribution, are thus confined to the North Temperate Zone, not one having been recorded as occurring south of the Mediterranean in the Old World, or of Philadelphia in the New. It is by no means improbable that they also exist in warmer latitudes, and we cannot but be surprised that they should have escaped the explorations of the numerous naturalists who have examined the regions lying further towards the tropics. We know that the fresh-water tanks in India have been subjected to examination, both botanically and zoologically; and though we should naturally expect to find the fresh-water Polyzoa luxuriating in such a habitat, none of the naturalists who have described the productions of these reservoirs make any mention of them; and yet the species of Spongilla which abound there, and whose European representatives are so frequently found associated with Polyzoa, have been made the subject of interesting and elaborate investigations.
The United States of America appear to be especially rich in the fresh-water Polyzoa ; and when we bear in mind, that it is but lately that these animals have received the attention of American naturalists, while but a small portion of that part of the world has hitherto been examined with special reference to them, and that nevertheless—though all the European species have not yet been recorded as living there--two entirely new generic types have been brought to light, while many of the species found present a state of luxuriant development of which we know nothing in the Old World,—when we bear in mind all these facts, we can scarcely avoid the belief that North America will yet prove the grand metropolis of the tribe.
Further facts must be obtained, before we can arrive at any generalisations of much importance regarding the altitudinal distribution of the Polyzoa. All the British fresh-water species occur in these islands at the level of the sea, and most of them have also been met with in our alpine and subalpine lakes. I have found Plumatella repens and P. fruticosa in Lac Leculejo in the Pyrenees, at an altitude of 4590 feet; and P. repens in Lac d'Aul, another Pyrenean lake, at an altitude of about 6500 feet, which is the greatest elevation at which any Polyzoon has as yet been recorded.
The depth at which the fresh-water Polyzoa occur in the waters frequented by them is never considerable. I have met with Plumatella jugalis attached to the long petioles of Nymphæa alba, in the waters of a sluggish canal, at about four feet below the surface; but in most instances, the fresh-water Polyzoa will be found at much less depths, and frequently at the very surface, attached to the under side of floating leaves, or upon the stones at the margin of lakes, where they are exposed to the ripple as it breaks upon the shore.