AND HABITS OF THE AMMOCHARID®. 241 
external stony additions are made. During the day the tube, 
into which the inmate retreats, is frequently entirely buried ; for 
the Ammocharide, like many other annelids, appear to be largely 
nocturnal in their habits. It is not usually until towards 
evening that the animal becomes active, and, after exposing ten 
or more millimetres of its tube above the surface, ventures to 
protrude its branchie. 
In order to appreciate the builder’s difficulties we must realize 
the fact that the tube of an annelid is necessarily constructed 
From within; that while it is, so to say, secreted, or built by the 
worm round its own body, it 1s so arranged that the inmate shall 
all the time be entirely detached from it, and free to change its 
position within it at will. If we imagine the head of the worm 
protruding through the growing conical tip (c.z., Pl. 24. fig. 12), 
it will then be seen that, in order to produce the imbricated tube, 
the base of each new fragment added must be placed carefully 
underneath the fragments which are already attached, and must 
be there affixed to the membranous tube; in other words, it 
must be inserted and fixed between the inner sheath and the 
last row of fragments added to the outer covering. 
Were the process reversed and additions made at the other 
end of the tube, as by a tiler covering a roof, there would seem 
to be but little difficulty. The matter would be almost as simple 
as the placing of one stone above another; but the problem is 
as I have stated, and its solution (which absorbed considerable 
time) took me completely by surprise. The beautifully trans- 
parent, conical, elastic part of the tube (c.t., Pl. 24. fig. 12), 
which on retreat of the animal is drawn inwards (the worm 
thereby frequently pulling together the latest shelly additions 
and closing the tube with them), is figured as almost fully dis- 
tended by the water, which is being driven before it by the 
advancing worm. At first the animal is bidden by the stony 
sheath, but in another moment the branchial tentacles, con- 
tracted and twisted together in the form of a living cone, appear 
beyond it and fill the elastic transparent tip, which I might 
perhaps compare to an india-rubber teat. Then, cautiously 
thrusting the tip of a single tentacle through the terminal pore, 
the animal carefully feels round to make sure that all is safe. 
Satisfied on this point, a second tentacle follows the first, then 
several more; finally the whole branchial crown is forced through 
the minute opening, and the tentacles, spreading themseives out, 
LINN. JOURN.—ZOOLOGY, VOL. XXVIII. 17 
