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along without rolling about, to which their round figure Would without thefe naturally have fubjefted them. But this can- not be agreed to on a general examination, when we find that there are a vaft number of other worms of as round bodies as thefe, which have no power of flattening themfelves, no vif- cous matter exfudated from their skin, nor either feet or tail, which yet are able to move along without rolling. Notwithftanding that Goedart did not perceive it, this fpecies of worms have legs alfo, and thofe in a fufficient number. They have feven pair of thefe placed at pretty regular diftan- ces, and the firft pair not far from the head of the creature, the laft but at a fmail diftance from the tail. Thefe are all jhort and membranaceous, but they are well contrived for lay- ing hold of things ; for they are round and large at the hot' torn, and are furrounded there with fine and fharp hooks. The creature has a power not only of retracting thefe hooks into the fubftance of its feet, but can even retract its feet into its body, and render them invifible, till it has occafion to thruft them out again to walk with.
To know the ufe of the tail of this worm, it is however ne- ceflary firft to know the nature of the worm itfelf. Thefe authors took it for a terreflrial worm ; but it is in reality an aquatic, as Swammerdam and Vallifneri rightly judged. 'Tis in the water that it acquires its growth, and it never leaves that element till the time that it is in a ftate to undergo its firft metamorpholis, in order to its change into the fly ftate. Thefe worms being ufually found on dry land, aud only picked up now and then fparingly, gave the world for a great while no room to believe that they were fo numerous as they really are, nor had people any idea that they were to look for them in the water. In examining veflels of corrupt- ed and {linking water however, they have been found in very great multitudes together lying among the mud at the bottom. Thefe taken out of this filth, and put into veflels of clean water, will foon {hew the curious obferver the ufe of their Jong tails. Notwithftanding that thefe worms are to live under water and in mud, there is no doubt to be made but that, like other of water fly worms, they have a neceflity of breathing frelh air ; and 'tis to this great purpofe that their long tail ferves them ; for while they lie buried in the mud, this is ex- tended up to the top of the water, and being open at the ex- tremity lets in the air into the body of the creature. It is a fleafing fight to obferve, that thefe animals, after having been put into a veffel of frefh water, all get quickly to the bot- tom, and each thrufts up the extremity of its tail to the fur face.
It is very eafy, from feeing that at whatever depths the worms lie their tails ftill reach the furface, to conclude, that they have a power of lengthening them at pleafure. The ready way to be allured of this is to add depth to the water by pouring more into the veflel ; for as the worms love the bottom, they muft, if they will remain there, lengthen their tails propor- tionably in order to breathe from the furface. The repeating feveral of thefe additions {hews, that the worm is able to ex- tend its tail to the length of five inches ; an extremely re- markable length for a creature, whofe body is not much more than half an inch, and whofe tail in its more common ftate is not much longer than that. Five inches is however the flint, beyond which they cannot go in this action ; and when the water in the veflel is made of a greater depth, the worms leave the bottom, and either crawl up the fides of the veffel to a proper height, or elfe fwim in the water at the depth of five inches, which is the depth at which they carry their tails up fo the furface.
The organization of fo remarkable a part as the tail of this animal, cannot but appear to the curious greatly worth the enquiring into ; and the tranfparence both of it, and of the body of the creature, is fuch as to favour very greatly this examination. And indeed in many of the younger of thefe worms, and fuch as have not yet come to the ftate at which they quit the water, one may diftinguifh the parts within the tody as plainly almoft as if placed for examination in a glafs tube. It is eafy in all ftates of the worm to diftinguifh that its tail is compofed of two hollow tubes, the one of which nicely fits, and is ealily retracted within the other, or thruft out of it again, as the creature's occafions require. The out- ter and larger tube feems an elongation of the flefhy matter that forms the rings of the body. It is white like the body, and is compofed of a vaft number of rings, as the body is. Within this is lodged a lefTer pipe, a great part of which is of a blackifh brown ; and by means of this colour, the inner tube may be diftinguifhed even when it is wholly within the other. This inner one may be called the tube of refpira- tion. It is eafy to fee now that the tail of the worm may be made more and more long, as his inner tube is thruft out of the other, which ferves it as a cafe or fheath. But this is not all i it is eafily known that thefe pipes are in themfelves capa- ble of extenlion, by obferving, that when the whole inner tube is lodged within the cafe, or outer one, yet this is of no determinate length, but is fometimes longer, fometimes fliorter. Both the inner and outer tube are indeed compofed of annu-
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lar fibres, and are capable of extenlion in length by the leffin- >ng then diameters. Thus when the tail is extended to its whole length the upper part of the tube of refpiration ap- pears no thicker than a large thread, or a horfe-hair The tube of refpiration is terminated by a fmail brown pro- minence, in wh.ch there may be diftinguifhed two apertures dehgned to give a paflage to the air; and around this there may be difcovered five fmail pencils of hairs, which, when the end o the tail is on the furface of the water, fpread on it like the rays of a ftar. The mechanifm by which the in- ner tube is extended out of its cafe, and is again retraQed in- to it, when fuch extenfion is no longer neceflary, is extreme- ly plain and fimple, and the eye is very well able to difcern the whole operation. There are in the body of the worm two large trachea laid in a parallel direaion to one another, and extended the whole length of the body from head to tail. 1 hey have their origin very near the head, the one on the right hand, and the other on the left, and their diameter in this place is very fmail ; they infallibly grow larger as they ap- proach the hinder part of the body, and when arrived at this hze they keep it to the extremity of the body. They are here two large veffels, whofe brightnefs and whitenefs referable white fatun. Near the origin of the tail thefe trachea; fud- denly become narrower ; and for the reft of their extent, each is no more than an extremely thin tube, but that fo long, that it has not room to be lodged in it without making feve- ral folds and convolutions in a very fmail fpace ; and this the more, as the fmaller tube, or tube of refpiration, is the more retracted within its cafe.
The long and {lender portions of thefe refervoirs of air are two hollow threads as it were, the ends of which are faftened to the tube of refpiration. When this tube is thruft in part out of its cafe, and the tail is by that means become longer, thefe threads follow it out; they unfold themfelves in propor- tion to the length it is thruft out to, and are drawn out of the body into the cafe, or larger outer tube of the tail. In this cafe the folds and convolutions they make in the body are few, and diflina ; and when the tail is yet much farther lengthened, and the tube of refpiration is carried out to its ut- moft length, and leaves the outer tube an empty cafe, thefe hollow threads are extended into two parallel ftrait lines, their folds and convolutions are all loft, and they are brought into the now empty cafe in form of two parallel tubes: and when this tube of refpiration is again retraAed into its cafe, thefe hollow threads recede into their old places, and make their former folds and convolutions at the end of the great trachea;, or the great refervoirs of air, of which they are properly no other than elongations.
It is mod probable, that to the unfolding and expanding of thefe two vefTels, the whole aflion of the thrufting°out of the tube of refpiration from its cafe, when the depth of water requires it, is owing; and that the retracling of it in again is, in like manner, the effia of the returning of thefe veflels into their original folds and convolutions : fince if we allow this veflel but a little ftrength, as they muft have if of a cartilaginous ftruflure, that will be enough on its expand- ing to throw out the eafily moveable tube of refpiration from its cafe ; and as they are faftened to the end of this tube, it is a mod neceflary confequence, that their contracting them- felves into their old convolutions muft pull it in again. Befide this, there muft alfo be a power in the animal of ex- tending and retrafling the parts of each of thefe pipes or tubes by means of their annular ftruAure. But this depends on the fame principle as mufcular motion in general, and is a fubjeft of too abftrufe enquiry to be entered into here. This tube of refpiration is however not only capable of being retrafied wholly into its cafe, but it is fometimes drawn into the body of the creature between the two principal tracheae, and car- ried up almoft to the head, and is even fometimes bent and folded.
The worm has the power of changing the figure of its head like the reft of this clafs. It fhews at times two flefhy horns, the infertions of which are in the under part of the body, and near thefe is the mouth. This is furrounded by a fort of arch of a brown cartilaginous fubftance ; and within this is a little fleftiy protuberance, which may betaken for the tongue. Its anus is a tranfverfe flit, placed between the infertion of the laft pair of legs, and the origin of the tail. This is furround- ed by a number of flefhy bodies forming a kind of fringe, which the creature either fhews or hides at pleafure. Thefe may not improperly be taken for a part of the inteftines, which the creature is obliged to puftl out of its body when it is to void its excrements ; and when that is done, it refracts them in again. The microfcope fhews them to be all hollow. Many of the water^ worms do not quit that element till in the^ ftate ; but this fpecies, as foon as they have acquired their full growth, leave the water, and crawl upon the earth. Their bodies in this new element become dirty and greyilh ■ and when they have found a propel fpot of ground where the earth is foft and moift, they bury themfelves, and after a nei ceffary time for their change come out in the form of flies. Their changes are all made under a {hell of their own skin,
which
