238 MR. A. T. WASTON ON THE STRUCTURE 
satisfactory to find that his observations on the appearance of 
the finished tube are quite in accordance with my own, and con- 
sistent with what we might expect as the result of the process 
of building, which I have had the good fortune to witness 
on several occasions, and shall hereafter describe. From the 
fact that the sand in the Bay of Naples is singularly devoid 
of minute flat particles of stone or shell, his specimens would 
probably not show that most interesting characteristic, “ imbrica- 
tion,’ mentioned by Grube (2). Gilson consequently does not 
-refer to it; but I know from experience that when suitable 
material is supplied, these worms gladly employ it in the manner 
described, and the process by which the desired result is 
attained is wonderfully ingenious. De Saint-Joseph remarks 
(29. p. 898) that the fragments of shell are sometimes fixed at a 
right angle by the edge ; but the difference between his deserip- 
tion and that of Grube is due to a difference in the state of 
contraction of the tube at the time of observation. I have 
frequently seen a tube present both appearances, separated by 
an interval of, say, half an hour. The tube (PI. 24. fig. 12) 
consists of two distinct parts: (1) an external covering of foreign 
matter (s.c.) which may be very minute fragments of shells of 
molluscs, calcareous tubes of annelids, grains of quartz, or flat 
particles of other rocks ; and (2) an internal tube (¢.¢.) which 
is translucent, colourless, flexible, elastic, and exceedingly 
tough—this part being secreted by the worm. The tube as a 
whole differs, I believe, from all others in that the particles of 
foreign matter, instead of being more or less attached to one 
another, are designedly kept entirely apart, firmly affixed only 
to the inner membranous tube, with the attachment limited 
to one side or edge (Pl. 24. fig. 12). The structure thus pro- 
duced is thoroughly protective, and, at the same time, like a 
coat of mail, it accommodates itself easily to the violent contor- 
tions of the body, which are involved in the habits of the worm. 
Carrying its tube with it, the animal is capable of burrowing 
through the sandy sea-bottom, and consequently the position of 
the tube in relation to the surface is by no means constant, 
though it is usually more or less vertical. 
It is interesting to note that the tube itself has an anterior 
and a posterior end, and is always buried in a definite direction. 
The anterior, being the growing end (at all events as regards 
the stony covering), is invariably nearer the surface ; whilst the 
b) 
