166 
THE VOYAGE OF H.M.S. CHALLENGEE. 
might, from the general structure of the tube, have been expected. Of these eight, four 
are primary, arranged as described in the stage of four, the remaining four are smaller 
secondary tubes, they lie in the outer angles of the four primaries, one between each pair 
of these. Stage of ten canals. — Eidley also figures a stage of ten, in which the four 
primary have the usual arrangement, but there are only two secondaries, one at each 
end of an axis of symmetry passing between the pairs of primaries ; the remaining four 
canals are of the third order (tertiaries), and occupy the outer angles between the 
primaries and secondaries. Stage of sixteen canals (PL XVIII, figs. 1, 18, 19). — In the 
largest cloacal tube in the Challenger collection is a stage of fifteen (sixteen, with one 
tube, required to complete the symmetry, suppressed), the four primary tubes (i., 
fig. 18) have the usual arrangement, then follow four secondaries (ii., fig. 18), 
situated in the outer angles formed by contiguous pairs of primaries, and finally seven 
(8 — 1) tertiaries (iii., fig. 18) are present, situated each in the outer angle between 
a primary and secondary canal. The eighth required for complete symmetry is altogether 
absent, not being represented even by a rudiment ; the basal part of the tube, however, 
is not present, or possibly a trace of the missing canal might be found near its proximal 
end. In Agilardiella, Marshall represents a stage of eight, in which aU the tubes are of 
the same value, ^.e., primaries. 
Proximally the cloacal tube is traceable into continuity with the cortex and choano- 
some ; the main excurrent canals of the latter are continued as the component tubes of 
the cloaca, and its radiating spicular sheaves are continued into the skeleton of these tubes. 
The spicular sheaves which approach the cloacal tube differ from the rest which enter 
the cortex, their cladomes are developed earlier, so that the young trisenes are found quite 
close to the spicular nucleus, and from the earliest developed cladomes of these a regular 
series of growing forms succeed till the cloacal tube is reached and entered. The arrange- 
ment of these spicules is similar to that already described in Trihrachium (p. 157); but 
as there are at least four tubes present in the cloaca of Tethyopsis, and never more than 
one in Trihrachium, so in the former case the spicules are more numerous ; in the latter 
the young spicules of the tube could be traced into connection with an inflexion of the 
cortex ; in the former nothing of the kind is to be observed, the spicules destined to 
enter the cloaca lying entirely within the choanosome. 
The youngest form of the cloacal cladoxea closely resembles that of the orthodisene of 
Trihrachium ; there is indeed no discernible difierence, except in the absence of a rudi- 
mentary third cladus, which I could not find, owing probably to the fact that I did not 
employ all the means of research available. Having found the rudiment in Trihrachium, 
there was less necessity to do so in this case. The young spicules observed are not only 
like those of Trihrachium, but, except for the absence of a third cladus, precisely similar 
to those of normal Stellettidse, the diflerences which distinguish the adult spicule being 
due to subsequent excess and defect of development. 
