248 MR, A. T. WATSON ON THE STRUCTURE 
the edge of the new tip (and sometimes the interior) was 
manipulated by the underside of the ventral end of the “ Lippen- 
organ,”’ and the animal once more retired into the stony tube. 
It was found that a beautifully perfect, though apparently 
delicate tip had been produced. After a short interval the other 
end of the tube was similarly renewed. 
The part taken by the sete in this operation is, I think, 
noteworthy. It has hitherto been overlooked, and as I have 
witnessed it in the tube-forming operations of Panthalis (24) 
and Nerine, it is probably true of other annelids. Although it 
was impossible, owing to the stony covering of the tube, to see 
what was taking place low down, there can be no doubt that the 
action, as regards the “abdominal” thread-glands, would be a 
repetition of what I have described. Gilson’s interesting ex- 
periment shows that a marked exhaustion of the contents of the 
thread-glands results from the repairing and extending of a 
fractured tube. As already stated, the lengthening of the tube 
is due to constant additions, internally, of new membranous 
tips, the old layer always being forced outwards ; consequently, 
the description of the formation of the membranous tip is really 
a description of the formation of the outer layer of the com- 
pleted tube. As I have shown, it is due to the secretion of 
the thread-glands of the “thoracic” segment, supplemented by 
those of the “ Lippen-organ.” Its outer coating, during the 
life of the animal, is quite unaffected by the sea-water, and it is 
obviously impossible for it to be renewed. It is an interesting 
fact, however, that the tube as a whole, though exceedingly 
tough during the life of the animal, speedily decomposes after 
its death or expulsion ; and it would seem probable that during 
life a constant renewal of the internal layers is taking place, and 
that these (which are doubtless due to the secretions of the larger 
glands) differ, in composition, from those of the outer sheath— 
a probability which is also indicated by the difference in the 
reaction to stains already alluded to. 
The tube is essential to the existence of the animal, which, 
when in great straits, bending its body into the form of the 
letter S, will make a tube only one third of its own length suffice 
temporarily for its protection; but if, by chance, the worm 
should be expelled from the tube, death will follow after a short 
time, as the animal is then, like the Serpualide and some other 
worms (19. p. 75), evidently quite incapable of replacing it. 
