230 STRUCTURE OF 



■which a number 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 body 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 bm-ied, 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 to 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 ciliary 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 ofi" its 

 oxygen to the gills, and its organic atoms to the 

 stomach. The refuse water, kept in unflagging motion 

 by vigorous cilia, is poured fr'om the terminal extremity 

 of the body, and discharged through the minute orifice 

 that I have described. 



Dr. Williams, in his admirable " Report on the 

 British 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 observa- 

 tion. After describing the mode in which the pos- 

 terior extremity in A. alveolata is contracted into a 

 true cylindrical tail, which, turning upwards, returns 



