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

by George Adams

They descend at pleasure to the bottom of the water, and ascend again, either by the sides, or upon some aquatic plants; they often hang from the surface of the water, resting as it were upon the tail; or, at other times they are suspended by one arm from it. They walk also with ease upon the surface of the water. If the extremity of the tail b, Fig. 7, be examined with a magnifying glass, a small part of it will be found to be dry, and above the surface of the water, and as it were in a little concave space, of which the tail forms the bottom, so that it seems to be suspended on the surface of the water, on the same principle that a small pin or needle is made to swim.

Hence, when a polype means to pass from the sides of the glass to the surface of the water, it has only to put that part out of the water by which it means to be supported, and give it time to dry, which it always does upon these occasions. They attach themselves so firmly by the tail to aquatic plants, stones, &c. as not to be easily driven from the place where they have fixed themselves; they often further strengthen these attachments by means of one or two of their arms, which they throw out and fix to adjacent substances, as so many anchors.

The mouth of the polype or hydra is situated at the fore-part of the body, in the middle between the shooting forth of the arms. The mouth assumes different appearances, according to the different purposes of the insect; sometimes it is lengthened out, and forms a little conical nipple, as in [Plate XXIII. A.] Fig. 13; sometimes it appears truncated, as in [Plate XXI.] Fig. 8; at other times the interval between the arms appears closed, as in [Plate XXIII. A.] Fig. 2 and 12; or hollow, as in Fig. 11 of the same plate. If it be observed with a deep magnifier in either of the two last cases, a small aperture may be discovered.

The mouth of the polype opens into the stomach, which is a kind of bag or gut that goes from head to tail; this may be perceived by the naked eye, when the animal is exposed to a strong light, or a candle placed on the opposite side to the eye; for the colour of the polype does not destroy the transparency thereof. The stomach will, however, be better seen, if the eye be assisted by a deep magnifier; one of them is represented as highly magnified in [Plate XXI.] Fig. 8. To be fully satisfied whether they were perforated throughout, Trembley cut one transversely into three parts; each piece immediately contracted itself, and became very short; being then placed in a shallow glass full of water, and viewed through the microscope, they were found to be visibly perforated. Their microscopic appearance is represented in [Plate XXIII. A.] Fig. 6, 7, 8; its mouth was at the anterior end a, Fig. 8, of one of these parts. The tail was at the end b of the third part, Fig. 6; as this piece was also perforated, it plainly appears that the tail of the hydra is open. The perforation, which is thus continued from one end to the other, is called the stomach, because it contains and digests the aliments. The skin which incloses the bag, and forms the stomach, is the skin of the polype itself; so that the animal may be said to consist of but one skin, disposed in the form of a tube or gut open at both ends. On opening the polype, no vessels are to be distinguished; and whatever be the nature of its organization, it must reside in the skin.

The skin must be so far organized, as to perform all the operations necessary for the nutrition and growth of the animal, without considering those that are necessary for its various motions. Whatever are the means the Author of Nature has employed for these purposes, we are ignorant of them. If their skin be examined by a microscope, it appears like shagreen, or as if it were covered with little grains; these are more or less separated from each other, according to the degree in which the body is extended or contracted.

If the lips of a polype be cut transversely, and placed so that the cut part of the skin may lie directly before the microscope, the skin throughout its whole thickness will be found to consist of an infinite number of these grains. To know whether the inside of the stomach was formed of similar grains, several of them have been laid open and examined by the microscope; the interior surface was then found to consist of an immense number of them, being as it were more shagreened than the exterior one, and less transparent. The grains are not strongly united to each other, but may be separated without much trouble. [Plate XXIII. A.] Fig. 10, represents a piece of skin thus laid open. To examine these particulars further, a piece of skin a, Fig. 9, was laid in a few drops of water, on a piece of glass before the microscope, and some of the grains were separated from it, as at b c d, by pressing them with the point of a pin; in endeavouring to open them, they spread themselves into all parts of the water, and at last remained in heaps, as at e and f.

If a polype be carefully placed before the microscope, without wounding it, you will seldom be disappointed in seeing some of these grains detach themselves from the superficies thereof, and that even in the most healthy.

But if the grains separate themselves in large quantities, it is the symptom of a very dangerous disorder; the surface of the polype thus attacked becomes more and more irregular, and is no longer well terminated and defined as before. The grains fall off on all sides, the body and arms contract and dilate, it becomes of a white shining colour, loses its form as at a, Fig. 4, and then dissolves into a heap of grains, as at b, Fig. 5. The progress of this disorder is most easily observed in the hydra viridis.

A very attentive and accurate examination shews that the skin is formed of a kind of glareous substance, a species of gum, which fills up the intervals between the grains, in which they are lodged, and by which they are attached, though weakly, together. It has been already observed, that it is to these grains that it owes its shagreen-like appearance; it is from them also that it derives its colour; for, when they are separated from the polype, they are the same colour with it, whereas the glareous matter is without any distinguishing colour. The construction of the polype seems then to be confined to these glandular grains, to the viscous matter, and the invisible fibres which act upon the glareous substance.

The structure of the arms of the polypes is very analogous to that of their body. When they are examined by the microscope, either in a contracted or dilated state, their surface is shagreened; if the arm be much contracted, it appears more so than the body; on the contrary, it appears less so in proportion as they are more extended; almost quite smooth when at their full extension; so that in the hydra viridis the appearance of the arms is continually varying, and these variations are more sensible towards the extremity of the arm than at its origin, as, in [Plate XXI.] Fig. 10; but more thinly scattered, or farther asunder, in the parts further on, as at Fig. 9. The hairs which are exhibited in this figure cannot be seen without a very deep magnifier, however they indicate a further degree of organization in this little animal. The extremity is often terminated by a knob.