INTRODUCTION. 
XXXI 
When fully expelled ^ the thread or wire, which I distin- 
guish by the term ecthormim (Plate XI. fig. 7, n), is often 
twenty, thirty, or even forty times the length of the cnida ; 
though, in some species, as in most of the Sagarti(B, it 
frequently will not exceed one-and-a-half, or two times the 
length of the cnida. 
The ecthoraa, wliich are discharged by cliamhered cnidce, 
are invariably furnished with a peculiar armature. The 
basal portion, for a length equal to that of the cnida, or a 
little more, is distinctly swollen, but at the point indicated 
it becomes (often abruptly) attenuated, and runs on for the 
remainder of its length as an excessively slender wire of 
equal diameter throughout. In the short ecthorcea of 
Sagartia, the attenuated portion is obsolete. 
It is chiefly upon this ventricose basal portion that the 
elaborate armature is seen, which is so characteristic of 
these remarkable organs. For around its exterior wind 
one or more spiral thickened bands, varying in different 
species as to their number, the number of volutions made 
by each, and the angle which the spiral forms with 
the axis of the ecthorceum. The whole spiral, formed of 
these thickened bands, I designate the screw, or strehla 
(fig. 7, o). 
In the ecthorcea emitted by cliamhered cnidee from the 
craspeda of T. crassicornis, the screw is formed of a single 
band, ha\dng an inclination of 45° to the axis, and be- 
coming invisible when it has made seven volutions. In 
those from the same organ in S. parasitica we find a 
screw of two equidistant bands, each of which makes 
about six turns, — twelve in all, — having an inclination of 
70° from the common axis. Iii those similarly placed in 
Garyophyllia, the strehla is composed of three equidistant 
bands, each of which makes about ten volutions — thirty in 
all — with an inclination of about 40° from the axis. In 
every case the spiral runs from the east towards the north, 
supposing the axis to point perpendicularly upwards. 
{Sometimes, esi^ecially after having been expelled for 
some time, the wall of the ecthorceum becomes so attenu- 
ated as to be evanescent, while the strehla is still distinctly 
visible. An inexperienced observer would be liable, under 
such circumstances, to suppose that the screw, when formed 
of a single band, as in 'T. crassicornis, is itself the wire ; 
an error into which I myself had formerly fallen. An 
