346 Mr. Osler on burrowing 
different fragments compactly, it presses them into their 
places with the erected scales, at the same time retracting the 
body a little. Hence, the fragments being raised by the 
scales, are generally fixed by their posterior edges ; and thus 
overlaying each other, often give the tube an imbricated 
appearance. 
When the tube is from half an inch to an inch in length, 
the animal proceeds to burrow ; directing its head against 
the sand, it penetrates by a slight extension of it, effected by 
the circular contraction of some of the posterior rings, which 
presses the viscera forward. The penetrating force is very 
inconsiderable, because the animal having to form its tube as 
it descends, can advance but slowly. It is now of course 
obliged to avail itself of the materials with which it may 
happen to come into contact. Its progress is marked by 
the gradual disappearance of the tail, which continues to 
advance into the tube till the whole has entered. Soon after 
the conchilega is found to have turned within the tube, and 
its head appears at the surface. This motion is clearly effected 
by the longitudinal muscles (fig. i, c. c. c.), which have been 
stated to throw the animal into folds, and which must also 
raise the tail to the surface whenever the faeces are to be 
discharged. 
In summer the whole task is completed in four or five 
hours ; but in cold weather, when the animals are more slug- 
gish, and the gluten appears to be secreted in smaller quan- 
tity, their progress is slower in proportion. 
The extremity of the tube is generally ornamented with 
a number of branches, composed of single rows of sandy or 
shelly particles. These are formed accidentally from time 
