ORGANS FOR BREATHING IN WATER-GRUBS. 159 



but consisting only of two tubes, the outer one serv- 

 ing as a sheath, within which the inner one can be 

 retracted at pleasure. Both of these are composed of 

 fibrous rings; and are so very extensile, thatR' aumur 

 has seen thein pushed out to twelve times the length 

 of the body of the maggot. The contrivance by which 

 the inner tube is pushed out is no less simple than 

 ingenious. It is furnished at the base with two 

 flexible pipes, coiled up, when it is sheathed, into 

 several folds, and communicating with the double 

 wind-pipe ( Trachea) in the body. When it wishes 

 to extend this breathing-tube, therefore, it inflates, by 

 means of air from the wind-pipe, the flexible pipes, 

 and in this way pushes them outward, and with them 

 the breathing -tube, to the extent required. The 

 breathing-tube itself is very slender, but terminates 

 similarly to those of the grub of the chameleon fly, in 

 five bristles. This breathing apparatus is admirably 

 adapted to the economy of these maggots ; as from 

 their seeking their food amongst ooze and mud, they 

 would often be exposed to suffocation, which their 

 extensile tube effectually prevents. 



Water worms (Nais). a a a, lialf-conce>iled in the sand. 

 bbb, their sand-tubes exposed. 



