« AnteriorContinuar »
tion; and this results, not so much from any greater facility in the exhibition of objects by the last-named method, as from its being one less frequently employed in conjunction with lamps. The reflected illumination obtainable in open daylight, out of the direct sun, is never too strong, and is well adapted to objects of considerable size. To see smaller opaque objects clearly and comfortably several contrivances are advantageous. Lieberkuhns have lately been neglected by many observers to an unreasonable extent. For low powers, a silver-side reflector, mounted on a brass stand with universal motions, is extremely handy. Messrs. Beck make an admirable parabolic reflector to fit to a 1} or objective, and furnished with a moveable arm which brings a flat mirror into such a position that the illumination is instantly changed from slanting to nearly vertical, which enables some, surface markingson metals, etc.,—to be made out with great distinctness. Mr. Crouch has constructed a parabolic silver reflector (like Beck’s), with an universal joint to suit different powers, but without the flat mirror, which for special purposes is invaluable. A lieberkuhn works beautifully with Ross's ;-inch, Beck's small-angled - ths, and similar glasses of Powell and Lealand.
No microscopist should be satisfied without acquiring skill in all the methods of illumination I have mentioned, and where objects admit of being seen in a variety of ways, all should be tried, as each will bring out some special feature. While an object is indistinct, the observer should avoid paying much attention to it. He should simply watch the changes he can effect in attempts to show it properly, and reserve steady examination until all the adjustments are in order. Few persons are aware how much the eye is under control of the mental faculty of attention, and what advantages they may gain by acquiring the habits recommended in the preceding remarks.
THE ANNELIDAN WORMS, OR ANNELIDES (ANNELIDA).
The worms belonging to this third Order of Annelides, the Terricola, are such as live in general on the land, buried in the earth or in dung heaps; or such as live in tubes in the water, or in soft mud. The degradation of all the organs requisite for the ordinary purposes of life, is carried, in the case of these worms, to the highest degree. The species of which the Order consists are usually deprived of what, in the other Orders, are called feet, and have only a certain number of bristles by which their movements are performed. They have no distinct head, and of course do not possess either eyes, antennæ, jaws, or proboscis. The mouth is usually placed at the anterior extremity of the body, and sometimes gives passage to a labial appendage. The body is distinctly segmented, and the setæ or bristles, mentioned above (some of which are called spinets), are partially retractile.* There are no external organs of respiration, or branchiæ. On the body there is a more or less large series of pores, which are believed to be the entrances to oblong vesicles arranged along the sides, and in which the blood is aërated. This fluid is almost always coloured red or yellow.t The Terricolous worms are hermaphroditical and oviparous; the young being like the parent from their very birth.
There are two rather large families belonging to this Order of Annelides ; species of which are found in Great Britain. These are the Lumbricidæ and Naides.
The first family Lumbricidæ is the most conspicuous in species, and the earth-worms (Lumbrici) are the most typical. They are much larger, fleshier, and more opaque than the Naides, and live buried in the ground, or amongst decayed vegetables, and in moist
There are three kinds of bristles visible in this Order of worms, 1, the spine, with a blunt base, tapering to a slightly bent point ; 2, the spinet, slightly bent, somewhat like tho italic letter S; and, 3, the bristle proper, slender and setaceous, only found in some of the lacustrine forms.
+ The nervous system in this Order of worms is very similar to that of the previously described Orders, Errantia and Sedentaria (see Plate I., Fig. 3), showing the brain, medullary cords forming the esophageal ring, the large nerves going to the anterior extremity of the head, and the ventral ganglio.
soil. The bristles with which they are provided for locomotion, are chiefly the spinets, those mentioned as like the italic letter S. In a certain portion of the body, several segments coalesce to form what has been called the clitellus, which is generally smooth and indistinctly annulated.
The common earth-worm, or dew-worm* (Lumbricus terrestris), is the most abundant, and the best known species in this country; and its anatomy, general habits, and mode of life, have been made the subject of many treatises by various authors.
It may be distinguished from the other kinds by its clitellus consisting of six segments, of a smooth, glistening, yellowish brown colour, by a mamillate-formed upper lip, and having on the second segment two impressed furrows behind the snout, extended from margin to margin, either separate (1 I), or connected by a cross bar like our letter H. The tail, instead of being quite round, is flattened and spatulate. The common earth-worm attains a length of from eight to ten inches, though it varies very much in that respect. The habitation which it prefers is a loose rich soil, more especially a vegetable mould. It cannot live in either pure sand or clay, nor in an over wet soil, nor in peat. In pastures, new or old, it is often found under stones, lying in smooth runs which it has made for an easy escape. The surface is smooth and glistening, often iridiscent, which is more or less marked according to the nature of the soil in which the worm is found burrowing. As I have said before, they are hermaphrodite, but the copulation of two individuals is necessary for the impregnation of the eggs. These they deposit in capsules, at a considerable depth in the soil. Laid in spring, the young are hatched in summer. escapes through a tubulous aperture, visible at one end of the capsule, and is then rather more than an inch in length, and exactly resembles the parent, except that we see no clitellus.
The earth-worm is for the most part a nocturnal animal. In the night season, and at early morning hundreds may be seen, though very few are to be found moving about in the day. This insignificant creature is of great use to man. “Earth-worms,” says Gilbert White, in his “Natural History of Selborne," "though in appearance a small and despicable link in the chain of nature, yet, if lost, would make a lamentable chasm. For to say nothing of half the birds, and some quadrupeds, which are almost entirely supported by them, worms seem to be great promoters of vegetation, which
* So called “from the animal coming forth from its concealment, in the dew of the morn,' and before the sun has dried up the moisture of the ground.”—Johnston.
The fætal young
VOL. III. NO. VI.
would proceed but lamely without them, by boring, perforating, and loosening the soil, and rendering it pervious to rains and the fibres of plants, by drawing straws and stalks of leaves and twigs into it; and most of all, by throwing up such infinite numbers of lumps of earth called worm casts, which being their excrement, is a fine manure for grain and grass. Worms probably provide new soil for hills and slopes where the rain washes the earth away; and they affect slopes, probably to avoid being flooded. Gardeners and farmers express
their detestation of worms; the former because they render their walks unsightly, and make them much work; and the latter because, as they think, worms eat their green corn. But these men would find that the earth without worms would soon become cold, hardbound, and void of fermentation, and consequently sterile.” Mr. Darwin maintains a similar theory, and in an elaborate paper read before the Geological Society, he essays to show that the earth-worm is a most active and powerful agent in adding to the depth of the soil, and covering comparatively barren tracts with a superficial layer of vegetable mould. He found upon his attention being called to the subject, that several whole fields, which had been covered with lime and burnt marl and cinders, had these substances buried in a few years to the depth of some inches beneath the turf. The history of one of these fields, as related by him, is very interesting. “Previously to fifteen years since it was waste land; but at that time it was drained, harrowed, ploughed, and well covered with burnt marl and cinders. It has not since been disturbed, and now supports a tolerably good pasture. The section here was turf half an inch, mould two and a half inches, a layer one and a half inch thick, composed of fragments of burnt marl (conspicuous from their bright red colour, and some of considerable size, namely, one inch by half an inch broad, and a quarter inch thick), of cinders, and a few quartz pebbles mingled with earth; lastly about four and a half inches beneath the surface was the original black peaty soil. Thus, beneath a layer (nearly four inches thick) of fine particles of earth, mixed with some vegetable matter, these substances now occurred, which fifteen years before, had been spread on the surface. The appearance in all cases was as if the fragments had, as the farmers believe worked themselves down. It does not, however, appear at all possible that either the powdered lime or the fragments of burnt marl and the pebbles could sink through compact earth to some inches beneath the surface, and still remain in a continuous layer; nor is it probable that the decay of the grass, although adding to the surface some of the constituent parts of the mould, should sepa
rate in so short a time the fine from the coarse earth, and accumulate the former on those objects which so lately were strewed on the surface.” The explanation of this is due, he says, to the digestive process by which the common earth-worm is supported. On carefully examining between the blades of grass in the field above mentioned, there was scarcely a space of two inches square without a little heap of the cylindrical castings of worms. It is well known that worms swallow earthy matter, and that, having separated the serviceable portion, they eject at the mouth of their burrows the remainder in little intestine-shaped heaps. The worm is unable to swallow coarse particles; and as it would naturally avoid pure lime, the fine earth lying beneath either the cinders or burnt marl, or the powdered lime, would, by a slow process, be removed and thrown up to the surface. This supposition is not imaginary, for in a field in which cinders had been spread out only half a year before, Mr. Darwin actually saw the castings of the worms heaped on the smaller fragments. Nor is the agency so trivial as it at first might be thought, the great number of earth-worms (as every one must be aware who has ever dug in a grass field) making up for the insignificant quantity of work which each performs.
On the above hypothesis, the great advantage of old pastureland, which farmers are always particularly unwilling to break up, is explained; for the worms must require a considerable length of time to prepare a thick stratum of mould, by thoroughly mingling the original constituent parts of the soil, as well as the manures added by man. In the peaty field, in fifteen years, about three and a half inches had been well digested. It is probable, however, that the process is continued, though at a slow rate, to a much greater depth ; for as often as a worm is compelled by dry weather or any other cause to descend deep, it must bring to the surface, when it empties the contents of its stomach, a few particles of earth. It is probable that every particle of earth in old pasture-land has passed through the intestines of worms, and hence that in some senses the term "animal mould” would be more appropriate than vegetable mould.”—“English Cyclopædia.”
If worms are really, at the present time, of use to the agriculturist, they were, if we may believe our forefathers, of no less value to the physician in healing all manner of diseases. Though no longer in use in our pharmacopeias, it may amuse the reader to know what value was placed upon them by the ancient practitioners of the healing art.
Topsell, in his history of serpents, thus writes