[Plate X.] Fig. 9, is a magnified scale of the sea-perch, which is found on the English coast. Fig. 10, the same scale of the natural size.
[Plate XIX.] Fig. 7, a scale from the haddock, as seen in the microscope. Fig. 8, the same of the natural size.
[Plate XIX.] Fig. 9, a scale from a species of perch from the West-Indies, magnified. Fig. 10, the scale of its real size.
[Plate XIX.] Fig. 11, a scale from the sole-fish, delineated as it appears in the microscope; the pointed part is that which stands without the skin, as may be seen in Fig. 5, which represents a piece of the skin of a sole, as viewed by the opake microscope. Fig. 6 and 12, the same objects of their real size.
CHAP. VII.
THE NATURAL HISTORY OF THE HYDRA, OR FRESH-WATER POLYPE.
Having in the two preceding chapters given the reader such a general idea of the history and œconomy of a great variety of those minute animated bodies, called insects, as I am inclined to hope has not only afforded him entertainment and instruction, but tended to excite an emulation for further researches; I shall endeavour to gratify so laudable a disposition, by introducing him to a class of beings whose œconomy and singular properties equally engage the attention of the philosopher and the natural historian; a scene which opposes our general system of vitality, and which presents to the eye of the mind, as well as that of the body, a series of astonishing wonders. It is among the minutiæ of nature that we find her models most diversified, and displaying the marvellous fecundity of its powers.
The polypes described in this chapter are fresh-water insects, of the genus of hydra, of the order of zoophytes, and class of vermes. The body consists of a single tube, furnished at one end with long arms, by these it seizes small worms, and conveys them to its mouth. It has, according to our general notions, neither head, heart, stomach, nor intestines of any kind; and is without the distinction of sexes, yet extremely prolific. From the simplicity of its structure those of its œconomy and functions are probably derived. When they are cut or divided into a number of pieces, the separated parts in a very little time become so many perfect and distinct animals; each piece having a power of producing a head, a tail, and the other organs necessary for its existence.
They are generally known by the name of polype; but as this was thought by many to be improper, because that, strictly speaking, they have no feet, Linnæus called the genus hydra, probably from their property of re-producing the parts which are cut off, a circumstance that naturally brings to mind the fabulous story of the Lernean hydra. Dr. Hill called them biota, on account of the strong principle of life with which every part is endued.
Leeuwenhoeck, whose indefatigable industry in his researches after small insects permitted very few things to escape his notice, discovered these animals, and gave some account of them in the Philosophical Transactions for the year 1703. There is also in the same volume a letter from an anonymous hand on this subject. We had, however, no regular account of them, their various habits, their different species, or of their wonderful properties, till the year 1740, when they first engaged the attention of M. Trembley, to whose assiduity and observations we are indebted for the display of their nature and œconomy.