120 THEPHILOSOPHY 



of the water, and the tails uniformly rofe to the furface till It was 

 between five and fix inches high. When the water was raifed high- 

 er, the animals immediately quitted their ftation at the bottom, and 

 either mounted higher in the water, or fixed upon the fides of the 

 veflel, in fituations which rendered it convenient for them to reach 

 the furface with the points of their tails. Thefe tails confift of two 

 tubes, both of which are capable of extenfion and contradion. The 

 firft tube is always vifible ; but the fecond, which is the proper or- 

 gan of refpiration, is exferted only when the water is raifed to a cer- 

 tain height. Through this tube the air is conveyed into two large 

 tracheae or wind-pipes within the body of the animal, and main- 

 tains the principle of life. When the tails are below the furface, 

 they occafionally emit fmall bubbles of air, which are vifible to the 

 naked eye ; and immediately repair to the furface for frefli fupplies. 

 Thefe rat-tailed worms pafs the firft and longeft part of their lives 

 under water; when near the time of their transformation, they leave 

 the water, go under the ground, and are there transformed into 

 chryfalids ; and, laftly, from this ftate they are transformed into 

 flies, and fpend the remainder of their exiftence in the air. 



Another fpecies of aquatic worms merit attention. They frequent 

 jnarfhes, ditches, and ftagnating waters. Their general colour is a 

 greenifh brown. Their bodies confift of eleven rings ; and their 

 fkin is not cruftaceous, but rather refembles parchment. Though 

 thefe animals, before their transformation into flies, live in water, 

 air is neceflary to fupport their principle of life ; and the apparatus 

 with which Nature has furniQaed them for that important purpofe 

 deferves our notice. The laft ring, or termination of their bodies, 

 is open, and ferves as a condudor of air. From this laft ring pro- 

 ceed a number of hairs, which, when examined by the microfcope, 

 are found to be real feathers with regular vanes. In particular fi- 

 tuations, they bend the laft ring in fuch a manner as to reach the 

 o furface 



