AND HABITS OF THE AMMOCHARID®. 239 
posterior, which never receives additions except to its internal 
sheath, may be embedded deep in the sand. The membranous 
tube terminates at each end with a conical, very elastic tip 
(c.t., Pl. 24. figs. 12 & 13), which projects more or less beyond 
the stony covering, and has at its extremity a very minute 
perforation. At the posterior or buried end, this tip is often 
preceded by a length of from 10 to 15 mm. of naked tube 
(Pl. 24. fig. 13), which Gilson (20. p. 322) has mistaken for a 
secretion at the outset of the animal’s existence. It is simply 
an extension (possibly late in origin) underground, where the 
worm is unable to affix its stony covering. The animal is 
capabie of reversing in its tube, and thus exploring the sand at 
either end, and, like the Terebellide (17), the Ammocharide 
invariably adopt the sanitary method of ejecting the excreta, at 
the anterior end of the tube, into the open sea. 
When the anterior end of the tube of an English specimen is 
seen under a low power (PI. 24. fig. 12), the imbricated arrange- 
ment and transparent conical extremity of the internal sheath 
are very noticeable, the free edge of the bits of stone being 
directed upwards, whilst the tip, in which is the minute per- 
foration, is drawn in, as is common when the animal is just 
about to emerge. Gnulson (20. figs. 21, 22 & 23) gives excellent 
figures illustrating the structure of the tube. His longi- 
tudinal section of the posterior naked end shows that this 
membranous sheath consists of a number of layers, and that 
the lengthening of it is produced by internal additions, which 
advance by stages beyond the extremity of the external ones, 
each advance being marked externally by a stepped appear- 
ance. This section also shows that the tube is occasionaily 
strengthened by new external additions on the face of the layers 
previously formed. 
Both these points are clearly visible in the drawing of this 
portion of the tube of Owenia (Pl. 24. fiz. 13). A cross section 
shows the internal layers to be concentric. By removing the 
stony covering from the body of the tube, Gilson demonstrated 
that the inner sheath is imperforate, and constructed as de- 
scribed from end to end. By means of tangential sections he 
found (20. p. 3238), in the wall of the inner tube, a some- 
what irregular system of coarse striz which, under a high 
power, he was able to resolve into a large number of very fine, 
irregular, longitudinal strie, and to recognize as the filaments 
