240 MR. A. T. WATSON ON THE STRUCTURE 
produced by the thread-glands. Gilson’s figure, 23, is specially 
interesting as delineating a section through the complete tube 
not stripped of its earthy covering. The section was made by 
a method specially devised by him for the purpose, and shows 
both the inner sheath of concentric layers and the outer 
irregular zone. The former is the part first laid down, as 
shown in the sections of the tip; whilst the latter is formed of 
masses of foreign materials subsequently affixed to the former 
by secretion, in which the various fragments are seen to be 
more or less completely embedded. My own examination of 
the structure of the inner tube was made by the ruder method 
of scraping off the sandy covering, and, after subjecting the 
inner sheath to maceration in distilled water, tearing the layers 
asunder. In this way I found it easy to separate the external 
rugged layer (which represents the cement by which the stony 
covering is attached) from the tube proper, aud, by means of 
stains, to establish the fact that the fibres in the former (which 
are embedded in a homogeneous secretion) may assume either a 
radial or any other direction; whilst those of the sheath take 
a course which is usually more or less longitudinal or transverse 
in relation to the tube. 
This somewhat lengthy preliminary description will render 
intelligible the questions to which I have successfully directed 
my attention. They are as follows :— 
(1) The method of construction of the imbricated tube and 
the elastic conical tips. 
(2) The reason for the adoption of this particular arrangement. 
(3) The function of the “ Lippen-organ ” or “‘ Metastomium ” 
(Gilson). 
(4) The raison d’étre of the immense number of uncini and 
of the double form of septal valves. 
(5) The existence of a cephalic opening, and of anal pores. 
(6) The method of reproduction. 
(7) The larval form (Ahéraria). 
T do not now propose to deal with these questions seriatim, but 
rather with such points as have not already been dealt with, in 
my account of the habits of the worm. As already explained, 
the tube has a definite anterior and posterior extremity, and is 
usually embedded more or less vertically in the sand, the anterior 
end being always nearest the surface, and also that to which 
