Mr. Osler on burrowing and boring marine animals . 343 
the animal to which the sand adheres ; and hence, when the 
worm is dug up, this part is generally found to be covered 
with an imperfect arenaceous tube ; within which the mo- 
tions of the body are performed with perfect freedom, but 
which cannot be removed entire. This tube is in fact left 
behind in the progress of the animal, and forms a complete 
lining for the hole, which it supports, and keeps pervious 
through its whole length. The worm is thus enabled to as- 
cend readily to the surface, and the water is admitted freely 
to the branchiae. 
A few of the anterior rings of the Arenicola progressively 
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 is 
truncated abruptly ; while it is extended to a regular cone 
when they are projected to a point just short of developing 
the mouth. This is its boring apparatus. The rings being 
retracted, the flat head of the Arenicola is directed against the 
sand ; when the projection of the cone opens a passage 
for the body. The opening thus made would however 
be insufficient, and the branchiae might be injured while 
forced through a narrow passage ; but that the animal, im- 
mediately on completing the penetration, distends the pene- 
trating rings very considerably. By the advance of the body 
the opening made for it is occupied ; and the rings being thus 
received into each other, the cone is ready to be again pro- 
jected. In the progress of the animal, the gluten which 
exudes from it cements the surrounding sand, and gives that 
support to the sides of the hole, without which it would 
immediately fall in. 
