236 STRUCTURE OF 



■which a numher of thread-like filaments protruded 

 from the lower part of the head are engaged from 

 time to time in feeling, and apparently examining. 

 When this hollow is sufficiently wide and deep, the 

 animal tilts its tube into it, by protruding until the 

 weight of its hody overbalances the supported part ; 

 it proceeds with its excavation, the tube becoming 

 more and more inclined, until at length it is brought 

 to the perpendicular, -when it descends straight down 

 till it is completely buried, the sand closing over its 

 disappearing extremity. 



This burrowing habit, the mouth of the tube being 

 downward, makes it needful that there should be a 

 posterior orifice in the tube. All the tribe to which 

 this species belongs are nourished by those minute 

 organic atoms which are held in suspension by the 

 water, and which are brought by strong cihary cur- 

 rents to the mouth. The currents thus produced are 

 subservient to the two functions of respiration and 

 digestion, the water thus hurled along giving off its 

 oxygen to the gills, and its organic atoms to the 

 stomach. The refuse water, kept in unflagging mo- 

 tion by vigorous cilia, is poured from the terminal 

 extremity of the body, and discharged through the 

 minute orifice that I have described. 



Dr. Williams, in his admirable ' Eeport on the Bri- 

 tish Annelida,' has, I think fallen into an error with 

 regard to this species ; or at least his statements in 

 this particular do not agree with my own observation. 

 After describing the mode in which the posterior 

 extremity in A. alveolata is contracted into a true 

 cylindrical tail, which, turning upwards, jeturns along 



