INGENIOUS MODE OF WELL-SINKIXG. 337 



of its extreme fragility. This tube is, in fact, left 

 behind as the animal progresses, and forms a com- 

 plete lining for the hole, which, like the brickwork 

 of a well, it supports, and keeps pervious throughout 

 its whole length. The creature is thus enabled to 

 ascend readily to the surface, and the water is ad- 

 mitted freely to the branchiae. 



A few of the anterior rings of the Arenicola pro- 

 gressively diminish in size, and each admits of being 

 completely folded within the ring immediately behind 

 it. Hence, when they are fully retracted, the anterior 

 extremity of the animal appears abruptly truncated ; 

 while it is prolonged in the shape of a regular cone 

 when they are projected to an extent just short of 

 developing the mouth. This constitutes its boring 

 apparatus. The rings being retracted, the flat head 

 of the Arenicola is directed against the sand, and 

 then the forcible projection of the cone opens a 

 passage for the rest of the body. The opening thus 

 made would, however, be insufficient, and the delicate 

 branchiae might be injured while forced through such 

 a narrow passage ; but, to avoid this, the animal imme- 

 diately on completing the penetration distends the 

 penetrating rings very considerably. By the advance 

 of the body the opening thus enlarged is then oc- 

 cupied, and the rings being again received into each 

 other, the cone is rea'dy to be again projected. During 

 the progress of the animal, the glutinous secretion 

 which exudes from it cements together the surround- 

 ing sand, and gives that support to the sides of the 

 hole, without which it would immediately fall in. 



It is not, however, simply for the purpose of exca- 



Q 



