246 MR. A. T. WATSON ON THE STRUCTURE 
The contents of the first two pairs of thread-glands, like those 
of the long cells (h.c., Pl. 24. fig. 9) of the ‘“‘ Lippen-organ,”’ 
strongly resist stains, and it seems very probable that their 
secretions are identical. The fact that the secretion of the 
“abdominal” thread-glands stains more readily, may possibly 
indicate some difference in composition. 
The membranous conical tip (c.¢., Pl. 24. fig. 12) is a character- 
istic and essential feature of the tube, and in the event of its 
accidental removal it is at once re-formed by the worm. By 
taking advantage of this fact, I have been enabled to make 
two very interesting observations on (1) the method by which 
the membranous tube is formed, and (2) the means by which the 
animal can easily, at will, cut off any desired portion. The 
latter is a habit which I had noticed to exist, from frequently 
finding fragments in the aquarium cut off evenly as though with 
a knife; showing that the worm possesses some special means of 
accomplishing the apparently difficult task of cutting through 
the tough membranous sheath. In order to make sure of these 
observations, a healthy vigorous worm was selected, carefully 
removed from the aquarium, and placed in sea-water in a glass 
dish. The tube, as usual, was considerably longer than the 
animal, and to wduce work I at once cut off enough from each 
end to reduce the tube to about the length of the tenant, taking 
care, of course, not to injure the worm. ‘This, in its shortened 
tube, was then transferred to a shallow glass dish containing 
fresh sea-water, and watched under the microscope. Very soon 
the animal became aware of the unusual “ draughtiness” of its 
dwelling, and proceeded to investigate the position by cautiously 
protruding ite tentacles and examining the edge of the tube. 
Having ascertamed the extent of the damage, and by re- 
peated search satisfied itself that no enemies were near, it 
retired just within the shelter of the tube, so that the tips only 
of the branchial tentacles were, now and then, visible at the 
fractured end. By transmitted light, it was just possible to see 
between the sand-grains, and through the membranous tube, 
that something was going on inside: the branchial tentacles 
were more or less completely twisted together ; the worm was 
revolving slowly within its tube, with the first bundle of sete in 
motion, and it occasionally advanced slightly and then retreated. 
This process continued for varying periods (apparently dependent 
upon the intensity of the light), which sometimes extended over 
