Зображення сторінки

sides, where there is a rank vegetation of Sphagnum and water-grasses. Very probably, however, in the more sheltered and weedy portions of the larger lakes they would be found; but be this as it may, the microscopist will be disposed to think that their “room is better than their company,” as they have neither rarity nor beauty to recommend them. G. O. Sars and other naturalists have, in some of the large lakes of Sweden, obtained some curious marine Amphipoda by dredging, thus affording an interesting confirmation of the fact, that the Scandinavian peninsula is slowly rising from the sea ; and though similar discoveries might not reward the naturalist in our English lakes, it would still be worth while to try the fortune of the dredge in some of them.

The Crustacean inhabitants of our lakes belong, then, almost exclusively, to the order Entomostraca ; and of the three divisions of that order which constitute the great bulk of the British fresh-water species (Cladocera, Ostracoda, and Copepoda) the Ostracoda are, on the whole, very poorly represented. One very common species, Cypris ovum (Jurine), exists in alınost every collection of water, from the lowest to the most elevated ; and in company with it, very often, a closely allied species, C. lævis, Müller. Cypris compressa, Baird, is of rather rarer occurrence; while in Loughrigg Tarn, and some of the small lakes of south Northumberland, I have found a very fine species (C. obliqua, Brady), which is closely related to, if not identical with, C. elliptica, Baird. Notodromas monachus (Müller) occurs abundantly in many of the lochs of Selkirkshire and Dumfriesshire, as also a little member of the family Cytheridæ, Limnocythere inopinata, Baird, which is probably much more common than it appears to be, but may very readily be overlooked, owing to its minuteness, and to its living almost entirely amongst mud.

The group Copepoda includes several lacustrine generaDiaptomus, Cyclops, and Canthocamptus—but the various species have not as yet been adequately investigated.

The third great division, Cladocera, is much more numerously represented, and, so far as mountain districts are concerned, forms much the most interesting section. It is not needful or desirable to occupy tłe pages of this magazine with technical descriptions of these species, especially as a monograph of the more important families, comprising descriptions of all the species, has recently been published, anıl is easily accessible to all who are interested in the subject.* My

*“A Monograph of the British Entomostraca, belonging to the families Bosminidæ, Macrothricidæ, and Lynceidæ.” By the Rev. A. M. Norman, M.A., and George S. Brady, M.R.C.S., C.M.Z.s. With six plates. London: Williams and Norgate. 1867.



purpose here extends only to the description of certain forms found since the publication of the monograph referred to, and to the presentation of a few observations relative chietly to distribution. And before proceeding to notice the various species seriatim, a few general remarks will not be out of place.

In considering the question of altitudinal distribution, it is quite possible that we might arrive at erroneous conclusions, owing to the mixing up of other accidental circumstances not connected solely with elevation. We find that Entomostraca are usually most abundant where there is a profusion of vegetation; in little sheltered bays, margined with sedge and rushes, and bearing luxuriant beds of Myriophyllum, Potamogeton, or other water-plants-lovely little inlets, which we may find plentifully on the shores of Windermere and Derwentwater. And when we notice that in elevated tarns the number, both of species and individuals, is comparatively small, it is well to remember that this may partly depend upon the fact that these sheets of water are mostly, owing to their exposure and want of shelter, almost entirely destitute of vegetation, except of a very stunted kind; so that the poverty of the fauna is doubtless dependent partly on these causes, and only indirectly through them on elevation and temperature. For it is not unusual to find in pools close to the margin of some lake a Crustacean fauna totally different from that inhabiting the lake itself. Thus, in some pools on the shores of Ennerdale Water, which had formed, apparently, in the hollows left by. turf-cutters, and were filled with Myriophyllum, Utriculariæ, and Potamogetons, I took an astonishing number of various Daphnia, Lyncei, etc.-seventeen species in all ; while along the whole length of Ennerdale Lake itself I could not succeed in capturing a single specimen of any kind. But the margins of this lake are excessively barren and stony; so much so, that in walking along its northern shore I could not detect a single patch of weed, and the net, when put into the clear water, collected nothing but little masses of spawn-of what animal I do not know. Still, I have no doubt that a prolonged search might have revealed spots more favourable to microscopic life, and that, even where no vegetation existed, Entomostraca might perhaps have been found-if not then, under other conditions of water or atmosphere. For vegetation does not appear to be absolutely essential to the lower forms of animal life. I have taken various Entomostraca in water where no vegetation, at any rate, higher than Diatoms or Desmids, existed; but in such situations they are always scanty, if existent at all, and the number of species capable of living under such conditions seems to be very limited. In Ennerdale Water, however, this paucity of animal life would appear to be distinctly the result of lack of vegetation : it cannot be dependent on elevation, for the level of the lake is only 369 feet above the sea, and, as has been noticed, the pools closo to it swarmed with life. It may be noted also that the members of the family Daphniadæ scarcely ever occur, except where there is abundance of vegetation, while the Lynceidæ seem to thrive well on a stunted cover of Lobelia Dortmunne or Isoetes lacustris.

It may be interesting here to place side by side (as types of the very wide difference existing between the Crustacean faunas of weedy pools of low elevation, and exposed mountain tarns of great elevation) lists of the Entomostraca obtained in two such localities.

Pools IN ENNERDALE: height above the sea, 370 feet.Daphnia reticulata, D. puler, D. mucronata, Acantholeberis curtirostris, Ilyocryptus sordidus, Sida crystallina, Bosmina longispina, Lynceus harpa, L. quadrangularis, L. elongatus, L. trincatus, L. globosus, L. barbatus, Eurycercus lamellatus, Dimptomus castor, Cypris lævis, C. ovum.

ANGLE Tarn, under Bowfell: height, 1553 feet.-Dosmina longjispina, Lynceus elongatus, L. guttatus, L. criquus, L. testudinarius, L. sphericus.

DAPANIA JARDINII, Baird (Figs. 9, 10).--This curious species has, I believe, not been previously figured; and, indeed, I am not aware that it has been noticed by any observer, except Dr. Baird (Edinburgh, New Philosophical Journal, vol. vi., 1857, p. 24). I took three or four specimens in Rydal Water in 1861, and from one of these the drawing given in the accompanying plate was made. Its claim to specific rank may, however, be reasonably doubted. The produced vertex, by which it is chiefly distinguished from Daphnia pulcx, is known in the case of D. mucronata to be a variable character, and the form known as D. cornuta is acknowledged to be merely a variety of the latter species. D. Jardinii is indeed smaller and more slenderly formed than is usually the case with D. pulex, but I am not able to discover from my specimens any specific character more valid than that already referred to, nor does Dr. Baird's description indicate any such. On the other hand, it should be stated that the three specimens preserved in my collection all have the cephalic cornua, though of variable size and shape, and I do not remember that any specimens of the normal D. pulex occurred in company with them. The length of my largest specimen is one-sixteenth of an inch, exclusive of the posterior spine.

Daphnia pulex (Lin.) and D. vetula (Müll.) occur commonly in lowland pools; and in the peaty hollows in Ennerdale, already referred to, I found also D. mucronata and D. reticulata, the latter in very great abundance. But in pools of this kind occurring at a greater elevation, the Daphniæ seem to give place to another member of the same family-- Acantholeberis curvirostris--which will presently be noticed.

SIDA CRYSTALLINA (Müll.) occurs plentifully amongst reeds and rushes round the margin of lakes, but does not, apparently, reach any great elevation.

ACANTHOLEBERIS CURVIROSTRIS (Müll.).—This species is not uncommon throughout the British islands in pools of peaty water, ranging from near the sea-level to considerable eleva. tions. It does not so often occur in clear lakes and tarus. Sprinkling Tarn, and Crag Lake, Northumberland, are the only such localities in which I have any record of its occurrence.

ILYOCRYPTUS SORDIDUS (Liévin).-A rare and curious species, of which I found two specimens in my gathering from the peaty pools in Ennerdale. It had previously been found, though very sparingly, in two situations in Northumberland and Durham.

DREPANOTHRIX HAMATA, G. 0. Sars, is of not unfrequent occurrence in the upland districts of the north of England and south of Scotland, frequenting lakes and clear water. In the Lake district, I have found it in Rydal Water, Blea Tarn (Langdale), Little Langdale Tarn, and Easdale Tarn.

POLYPHEMUS PEDICULUS, Müll., though common in the moorland lochs of Northumberland and southern Scotland, is not so in the Cumberland district, the only lake in which I have found it being Derwentwater.

Bosmina LONGIROSTRIS (Müll.) and B. LONGISPINA, Leydig, occur, one or both of them, in almost all pieces of water in the Lake district. The males of these animals differ remarkably from the females in having the anterior antennæ connected with the body by a sort of ball-and-sucket joint, and in the tapered form of the abdomen. This sex is, however, rarely met with; the only place in which I have found it being a small rushy tarn on some hills, called the Humbles, on the north-western border of Northumberland. This pool contained abundance of the Bosmina, but scarcely any other species; and I found this to be the case also in a somewhat similar situation on the northern slope of Mickle Fell, in Yorkshire; but in this latter case my gathering contained no males.

LYNCEUS HARPÆ (Baird), a very common species in almost all clear pieces of water throughout the kingdom, and on the continent of Europe, seems little affected by elevation, being met with in almost all the pools, lakes, and tarns of the Lake district.

« НазадПродовжити »