178 = 
sub-genus Idiothecia (see Reports “ Discovery ”’, Expedition, 
Cephalodiscus, 1906, p. 7). The tubes are 1 to 1.2 mm. wide, 
rarely I.4mm., and they vary in length from 2.5 to 5 mm. 
They are of uniform diameter, but in some cases the opening 
or ostium is a little wider than the tube itself; the blind end 
is usually blunt (see plate 3, figs. 9-11). The inner or blind 
ends of the tubes curl in and out among one another, but the 
more superficial parts are straighter (figs. 2 and 3, plate 2). 
The shortest tubes are to be found at the extremities of the 
branches (fig. 10, plate 3). Here the paler, softer, and pre- 
sumably newer test that agglomerates the tubes and spines 
together is less abundant than elsewhere. It would seem that 
the young polypides of the colony crawl over the surface of 
the tubarium, and, on coming to rest at the end of a branch, 
first secrete short tubes and spines, and subsequently secrete 
the softer test. which fills in the intervals between the pro- 
jecting parts of the tubes (peristomes) and spines. The tubes 
are lengthened by additions at their mouths or ostia until they 
attain their maximum length of 5 mm., and soft test is de- 
posited around them pari passu so as to envelop them up to 
the mouths, and to cover in more and more of the basal parts 
of the spines. This may explain why the spines are shorter 
towards the rooted end of the tubarium than near the distal 
ends : they are more submerged in the common test. 
It not infrequently happens that a spine begun in relation 
with one tube becomes secondarily related to another and 
younger tube, as though a settling polypide, finding itself in 
the vicinity of a projecting spine, builds up its tube against 
it, and dispenses with the formation of a spine of its own. 
The relations of spines to tubes are best studied by reference 
to the ends of branches of the tubarium, and the following 
interpretation of three particular cases may serve to render 
the relations clearer than a generalised statement would. 
In fig. 9, plate 3, is shown a group of four tubes and five 
spines cut from the end of a branch. The cut surface is repre- 
sented by the irregular line at the bottom of the figure. The 
oldest spine here appears to be that marked a ; it is darker 
in colour than the others ; it extends all the way through the 
group of tubes under consideration, and in all probability it 
Was not originated by any of the polypides occupying these 
tubes, although the more terminal parts of the spine may have 
been put on by one or more of them. The tube which runs 
alongside the spine a is more remote from the observer than 
the spine itself, and has its aperture (6) facing away. The 
polypide which secreted and inhabited this tube commenced 
to construct it against the side of the spine a, and only com- 
