Essays on the Microscope / Containing a Practical Description of the Most Improved Microscopes, a General History of Insects, etc., etc.

by George Adams

[Plate XI.] Fig. 6, 8, 9, and 10.

Chaos redivivum, Linn. Syst. Nat. 1326. Ledermüller Micros. Ergötzungen, p. 33, tab. 17. Baker Micros. made easy, p. 81. Ibid. Empl. for the Micros. p. 244, pl. 10, no. 8 and 9. Rozier Journal Physique, Mars 1775, Mars 1776. Adams Micrograph. Illustr. 4th edition, p. 125, pl. 38, fig. 179.

The eels in paste have been more distinguished than most other animalcula, as well on account of their many curious properties, as the various speculations and theories to which they have given rise. Four different species of eels may be found in paste; of the first, I shall now give a particular description. The body is filiform or like a thread, round, pellucid, replete with little grains in the middle, both extremities very pellucid and empty, the fore-part a little truncated, the hind-part terminating in a very short bristly point. It is the same of every age and size. To be certain of procuring this species of eels, boil some flower in water, to which you have added a few drops of vinegar; provide an earthen pot which has an hole at the bottom, and fill it with earth; then put the paste in a piece of coarse cloth, and bury it in this earth; the pot is to be exposed to the sun in the summer, or kept in a warm place in the winter; by these means in ten or twelve days you will very seldom fail of finding a large quantity of eels in the paste.

This eel, when at its full growth, is about one-tenth of an inch long, and rather less than one-hundredth of an inch in diameter. Fig. 6 represents one of these eels magnified about one-hundred and twenty times, only compressed so much between two plates, by means of an adjusting screw, as not only to prevent it from moving, but to lengthen and flatten it in a small degree. At the upper part there are two little moveable pieces or nipples, a a, between which an empty space b is formed, that terminates in the mouth; the hinder-part is round, but there projects from it a short setaceous tail w; in the young eels the termination of the tail is not so abrupt as in the present specimen, but it finishes by a gradual diminution. There is probably a vent near z, for the passage of the excrements; because when that part has been gently pressed, two or three jets of a very subtile substance have been observed to issue from it. If the pressure be increased, a small bladder will be forced out, a further compression bursts the bladder, and the intestines are forced through the opening.

A greater degree of magnifying power is necessary to obtain an exact idea of the viscera of these eels. Fig. 10 represents the alimentary duct further magnified, from its origin to the belly. It is shewn here as separated from the animal, which is easily effected; for nature, assisted by very little art, performs the operation. The oesophagus, b c; Fig. 6 and 10, at its origin a a, is very small, but soon grows larger, as at c, and forms a kind of oblong bag, c d; the diameter of this increases till it comes to d, where it swells out as at d e f; it then grows smaller till it comes to g, when it again swells out at g k l. The part k l is the stomach. M. Becli has shewn, that the alimentary duct of many species of worms is formed of two bags, one of which is inclosed within the other. It is the same with this animalculum; the little vessel b c, that we have called the oesophagus, which is the origin of the bag c d, enters into the same bag, and preserves its form within it till it comes to m, from whence it is prolonged in the form of a black line m n, which passes by the axis of the duct e, and apparently terminates itself at the beginning of the abdomen l. To this tube, near the center of the swelling g k l, are fixed two small transparent bodies; that end of these which is connected with the tube is round, the other end is pointed; these small pieces cannot be discerned in every position of the eel.

I shall now shew how this duct is to be forced out of the eel. The body, when compressed, generally bursts either at the head or tail, and always at that part which is least pressed; hence when the mass of fluids contained in the body is forced towards the anterior part, they meet with a resistance in passing from the abdomen to the duct already described; the abdomen, being forced by the fluids which are made to act against it, bursts at the upper end, and the fluids, striking against the neck, force it, with all its contents, out of the body, through an opening at the anterior part; on lessening the pressure, the intestine thus discharged will float in the water between the two plates of glass.

Not to enter into a detail of those parts which have been supposed by some writers to constitute the heart, &c. of these minute animalcula, it will be sufficient here to describe those in which motion may be discovered, and to leave the rest to future observations on the subject. The parts which may be seen in motion within these minute creatures are, 1. the small tube or duct, from its origin at m, to the two appendages; 2. these appendages themselves, h; 3. the remainder of the tube, from the appendages to the insertion at the ventricle k; 4. in the swelling g k l. The rest of this duct, from the beginning by the oesophagus b c, to the second swelling, has no motion. There is a variety in the motions of the first part of this duct, sometimes it dilates and contracts, at other times it has an oscillatory motion. It is difficult to gain a good view of the appendages; but when the position of the little creature is favourable, they seem to have a two-fold motion, by which the pointed ends approach to, and then separate from, each other, and another by which they move up and down. The part g k l moves backwards and forwards alternately; the motion of each of these parts is independent of the rest. These are the principal parts, whose motion is connected with the life of the animal.

The other viscera that are contained in the body of the eel, and which may be observed by the aid of the microscope, are, the vessels which contain the food, those which are filled with a transparent substance, and the womb or ovary. The first form the abdomen and intestines; these are filled with a black substance, which prevents their being properly and clearly distinguished; these vessels, in their passage through the posterior part of the body, form an empty space, in which we may perceive that one side of the animalculum is occupied by the ovary q q q, which runs from j to u x; it is at these two extremities of the ovaries that the eggs begin to be formed, for the largest eggs are always to be found in the middle, and the smallest at the ends, as may be seen at j f and u x.

All the eels which bear eggs have two protuberances, y y, formed on the exterior part near the center of the ovary; it appears like a transparent semicircular membrane, but is really a kind of hernia or bag, in which one or two eggs may be sometimes seen; all the larger eels have this appendage, which also bears the marks of having been burst. Now, as the younger eels have not this appendage, nor any marks of a rupture, we may reasonably conclude that it is from hence that the little eels issue from the parent.

In the latter part of the year, and during the winter, these eels are oviparous, and the young eels may be seen to proceed from the egg; at other times they are viviparous; six live eels have been seen at one time in the belly of the parent, twenty-two eggs have been counted in the ovary. Müller suspected that there was a difference of sex in some of these animalcula, but it was left to M. Roffredi to afford the proof, and it was only from a variety of repeated observations that he could allow himself to be convinced of this truth. He continued his researches upon the same subject on other microscopic eels, and has since been able to distinguish the sexual parts of the vinegar eels.