The Zoologist/4th series, vol 6 (1902)/Issue 729/''Lynceus'' and the ''Lynceidæ''
LYNCEUS AND THE LYNCEIDÆ.
By the Rev. Thomas R. R. Stebbing, M.A., F.R.S.,
In recent years a great impulse has been given to the study of Crustacea by the numerous expeditions sent out more or less expressly for the purpose of deep-sea exploration. As the result of voyages carried out in rapid succession through the last forty years, a host of forms distinguished for novelty, queerness, or beauty have contributed interest and animation to the pursuits of the carcinologist. The species popularly attractive have for the most part, though by no means exclusively, belonged to the Malacostraca. But during the same period in which the higher marine Crustacea have been thus decidedly making their mark, it happens that in a quite opposite direction the fresh-water Entomostraca have found their way to a modest celebrity. Though not many of them are of any considerable size, they attain in various ways to economic importance by their astonishing abundance. Of those that can be easily captured in almost any pond or ditch the variety is very considerable, and the number of species obtainable can be largely increased by a little extra exertion, without any appeal to imperial resources. Apart from ordinary methods of fishing for them at the sides or in the centre of pools and watercourses, that which more than anything else stands in antithesis to the costly labour of dredging and trawling in submarine abysses is the process, applicable at least to many fresh-water Entomostraca, of dredging on dry land. Many sheets of water at certain seasons completely evaporate, and expose a moistureless floor. If earth be taken from this and placed in water, under suitable circumstances of temperature, there is a good prospect that a crop of Entomostraca will be raised from it. The secret is that crustacean eggs have been deposited in the soil, and have there been biding their time till conditions appropriate to aquatic creatures should once more revisit their thirsty and solidified home.
Of the three orders—Branchiopoda, Ostracoda, and Copepoda—into which the Entomostraca are commonly divided, the present paper touches only the first, and that in two of its divisions—the "leaf-limbed" Phyllopoda, and Cladocera, with conspicuously "branched antennæ." The former especially excite surprise, when they are successfully grown from dried mud, because of their superior magnitude. Some of them also are remarkable for having no carapace, others for being almost entirely enclosed in what looks like the shell of a bivalved mollusc. The Cladocera are closely related to the Phyllopoda, and are most widely known through the little Water-Flea (Daphnia pulex), which, though little, is much larger than the Chydorus sphæricus, to be presently mentioned. The labours of G.O. Sars in Norway, of W. Lilljeborg in Sweden, of Jules Richard in France, of G.S. Brady, D.J. Scourfield, and Thomas Scott in Great Britain, and, in truth, of quite a multitude of learned writers all over the world, have discussed almost every conceivable detail in the structure, habits, and distribution of these animals. Even as to their classification, a very near approach to agreement has been arrived at. All the more desirable is it that every source of confusion should, if possible, be eliminated from the names in common use. But in regard to the genus and family which form the subject of this paper, there is something a little parallel to "the strange case of Dr. Jekyll and Mr. Hyde." It is the object of the following discussion to unmask them, in the sense of showing what are the animals to which the designations that form the title of this paper should rightfully be applied.
It is said of mud that, if you throw plenty, some of it is sure to stick. With scientific names there is an understanding that, if they are thrown according to the rules of the game, they ought to stick at least to some of the objects at which they have been thrown. In early attempts at classification a generic name is often attached to an incoherent miscellany of species. When these are subsequently assorted under appropriate headings, the original generic titles do not always quite know where they are. They stand a chance of being left out in the cold, as things no longer wanted—a kind of disinherited vagrants, blissfully forgotten, till some meddling bibliographer disturbs the peaceful oblivion in which their claims lie buried.
When Otho Fridrich Müller, in 1785, definitely established the genus Lynceus, which he had already brought forward in his 'Prodromus' of 1776, he assigned to it nine species, the first being L. brachyurus, the second L. sphæricus, a variation from the order adopted in the earlier work. That these nine species have since been distributed among numerous genera is well known; but in this distribution the true position of the genus Lynceus has been lost sight of. Since Müller singled out no species as typical of the genus, it was at the outset open to anyone, in dividing the genus, to allot the original name to which species he pleased. No stress whatever can, in my opinion, be laid on the circumstance that Latreille, in his 'Considérations générales,' pp. 91, 421 (1810), mentions 'Monoculus brachyurus, Fab.," as a typical example of Lynceus. He was not discriminating between species and species, and was pretty evidently without the knowledge requisite for doing so. So far as he is concerned all the nine species remain exactly where Müller placed them. There is no hint of an idea that any ought to be transferred to a separate generic division. It is different with Dr. Leach, for he, in 1816, definitely began that partition of the genus which has since been greatly developed. When describing the Crustacea as a division of the Annulosa in the 'Encyclopædia Britannica,' p. 416 (1816), Leach assigns to Lynceus the single species brachyurus, and to a new genus Chydorus the single species sphæricus. The inference, then, can scarcely be escaped that, whatever else happens to these two genera, neither can be upheld without at least including in it the species assigned to it by Leach. Desmarest, in 1825, rejects Chydorus, upbraiding Leach for having established it merely upon Müller's error in regard to the antennæ. Desmarest himself, whose acquaintance with the subject was not very profound, includes five species under Lynceus, but says never a word about L. brachyurus. His objection to Chydorus has been overruled, and with good reason, since, however weak the distinction drawn by Leach, the application was put beyond doubt by the references which he gives to the species above mentioned. Subsequently Dr. Baird distributed six of Müller's species over the genera Eurycercus, Chydorus, Camptocercus, Alona, Pleuroxus, and Peracantha, these, with the genus Acroperus, forming his family Lynceidæ. Here it is a little startling to observe that the family Lynceidæ is set up, but the genus Lynceus itself is shut out. Also three of the Müllerian species are not accounted for, namely, L. brachyurus, L. longirostris, and L. socors. Of these Baird refers the second to Bosmina, as a genus of the Daphniidæ; the third is perhaps unidentified; the first remains over for consideration.
In 1867 Norman and Brady, in their well-known paper on "The Families Bosminidæ, Macrothricidæ, and Lynceidæ," reassemble, under the title Lynceus, no less than eleven genera, no one of which, however, includes any of the three species omitted by Baird from the family Lynceidæ. The principle on which these two authors acted was that for generic distinction structural characters should be insisted on rather than variations of form. Though they do not make it very clear how the line is to be drawn between form and structure, there was something to be said for their opinion that generic subdivision in the family had been carried beyond the point which was justified by any thoroughness of contemporary knowledge. None the less they recognized the importance of the treatise by G.O. Sars on the 'Cladocera Ctenopoda,' 1865, and they contemplated the possibility that in the future some or all of the rejected names might properly be reinstated. This has come to pass. But authors have still been content to follow Baird in adopting a Cladoceran family Lynceidæ, in which no genus Lynceus was included, although of other generic names within that family the number has risen to at least eighteen. At length, however, in his 'Cladocera Sueciæ' (1900 or 1901), the veteran zoologist, Wilhelm Lilljeborg, raised a protest against this way of treating O.F. Müller's genus, and restored it in favour of Lynceus quadrangularis, Müller, which Baird had transferred in 1845 to a new genus Alona. But, as already explained, the action of Leach has affixed the title Lynceus to the species L. brachyurus. Accordingly, in any settlement of claims it is to that species that attention must first be given. It has nothing to do with Alona, for it is not one of the Cladocera at all. It is a phyllopod, and, as may be seen from the 'Fauna Norvegiæ' of G.O. Sars (vol. i. p. 116, 1896), in which it is fully described and beautifully figured, it belongs to the genus which Lovén called Limnetis, and Lièvin called Hedessa. Both those names, therefore, are superseded by Müller's Lynceus, as limited by Leach. As a further consequence the family Limnetidæ must yield its name to Lynceidæ, which takes rank as a phyllopod family; while in the Cladocera the name Lynceidæ must be discarded in favour of a family Chydoridæ, Chydorus having been detached earlier than any other Cladoceran genus from that fruitful mother of genera, Müller's Lynceus.
Since there is reason to expect that the changes of nomenclature above introduced will not be greeted with any rapturous enthusiasm by all zoologists, it may be good policy to ward off the "precious balms" of rebuke by evidence that the changes are not unsupported by precedent. In 1865 Sars established a family Lyncodaphnidæ, which Lilljeborg still upholds. But already, in 1867, Norman and Brady had substituted for it the name Macrothricidæ, with the explanation: "Sars' name for this Family 'Lyncodaphnidæ,' not being derived from the typical genus of the Family, in accordance with the usually received rules of nomenclature, we have substituted for it that here employed." It may of course be urged that this was only a youthful delinquency on the part of two authors whose acknowledged eminence is of later date. But such an argument will fall very flat in face of the circumstance that G.O. Sars himself, with the openmindness habitual to him, has now accepted the name Macrothricidæ, using it prominently in his papers of 1900 and 1901 on South American Entomostraca. Some may even now prefer the scholarly emendation Macrotrichidæ, long ago proposed by Dr. E. v. Martens in the 'Zoological Record.' The common outcry, that there is no "compensation for disturbance" in these rearrangements of nomenclature, is itself very inconsiderate. There can never be any fixity until a settlement has been carried out on the thoroughgoing application of sound principle. In the present instance the student of Cladocera should feel himself in a very happy position. By obedience to law he is involved in nothing worse than the sacrifice of the name Lynceidæ in favour of Chydoridæ. If he persists in wrongfully retaining Lynceus as a Cladoceran genus, he must sacrifice for it Alona, Baird, instituted in 1843, which has not only been current for nearly sixty years, but has suggested the names of the nearly related genera, Alonella, Alonopsis, Euryalona, Pseudalona, and the specific name of Camptocercus aloniceps, all to be made cryptic by the burial of their parent. Seeing that the species of Alona itself are numerous, convenience and principle are not in conflict over its validity. They are alike interested in upholding it.