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478 COLLECTIONS FROM M E LA N E S IA .
fascicles of spicule-shafts, a ray proceeding from the head of each of
the latter, extending along the membrane and supporting it (see
fig. I") ; in some parts stout acerate spicules (No. 2) take part in
the formation of the dermal skeleton. Sarcode transparent, of
very pale brown colour ; rendered suhopaque, when seen in the
mass, by immense numbers of small elongate stellate spicules.
Spicules :—(1) Triradiate of axis and dermal skeleton, consisting
of a straight shaft and two arms, one tapering to a sharp point and
boldly recúrvate, the other ending abortively in a rounded extremity
shortly after its origin ; the arms are set at right angles to the
shaft and at angles of about 160° to each other, but lie in different
planes. Length of shaft and long arm probably variable, and depending
on the position of the spicule ; the former attains a length of 5 5
millim., the latter of 2 millim. ; diameter about '05 millim. I t is
the shaft of this spicule which forms the longitudinal skeleton-bands.
(2) Large acerate of dermal skeleton, slightly curved, tapering from
centre to sharp points ; size about 1’8 by ■05--075 millim. (3)
Minute elongate stellate flesh-spicule, consisting of a straight or
occasionally curved or sinuous cylmdrical shaft, beset with numerous
irregular hluut processes, about 20 to the spicule, varying in length
from -001 to -002 millim., thickness about '001 millim. ; length of
spicule about ’OOOo millim., thickness of shaft alone ’001 millim.
Crowded over all ptfftè of the soft tissues.
JIab. Port Darwin, 7-12 fms., bottom sand and mud; Torres
Straits, 10 fms., bottom sand.
Of the two specimens from Port Darwin the larger is 74 millim.
(3 inches) long in its present state, viz. without its original base
and with tho apex somewhat abraded: it probably did not much
exceed this length when perfect ; its longest diameter (it is snb-
oblong in transverse section) is 8 millim., its shortest 5 millim., at
the present base. The smaller specimen has the base attached, hut
has lost the apex ; it is almost cylindrical, and has a diameter of
about 3'5 millim. throughout. In the dermis of the larger specimen
no acerates have been found, hut in the smaller one they
appear to replace the triradiates in this place ; it is in this specimen
that an axial canal traverses the sponge. The specimen from Torres
Straits is a fragment, forming the base of a specimen almost
certainly belonging to tho same species, but very imperfect. Its
acerate differs from that of the typical form by having a diameter of
•075 instead of -05 millim. ; the flesh-spicule shows no divergence.
The species differs very markedly from Stewart’s—(1) outwardly,
in having the surface level instead of bearing sharp points ; (2)
iuAvardly, in the presence of an axial cavity, in having tri- instead
of quadriradiate body-spicules, and in having a dermal acerate ;
tho stellates of T. columnifera, further, are normal globostellates
and not elongate, as here ; in the general arrangement of the
skeleton this species differs by possessing a number of longitudinal
lines, instead of the condensed central mass of th at species.
The species is obviously nearly related to a form named Tri-
hrachion Schinidtii, well described and illustrated as the type of
SrONGIlDA. 479
a new genus hy W. Weltner (‘Beitr. zur Kenntniss d. Spongien,’
Inaugural Dissertation, Lreiburg-in-Breisgau, 8vo, 1882, p. 30,
pi. iii. figs. 29-41, 43), from Prof. Agassiz’s dredgings in 1879
off the Morro Light, Gulf of Mexico, in 250-500 fathoms. In
point of fact Weltner’s species, ivhich differs from T. dissimilis principally
in the possession of a fully developed triradiate “ anchor,”
occupies a position almost exactly intermediate between T. dissimilis
and Stewart’s species. I gather from Weltner’s paper th at he has
not seen Stewart’s description; had he done so he would, I feel
sure, have at any rate mentioned the close affinity of his species
to th a t of Stewart, from which it differs chiefly by the elongate
form of the flesh-stellate and by the suppression of the third lateral
arm of the skeleton-spicule, a suppression already foreshadowed in
Stewart’s species by the great reduction of two out of the three
lateral arms in some of these spicules (see fig. 75, I. c.). I do not
think th a t Tribracliium can be upheld as distinct from Tethyopsis ;
the gradation of forms between T. columnifera and T. dissimilis,
by which ( I j the quadriradiate spicule of T. columnifera is reduced
to ^ a triradiate in Trihrachium, and to («) a hiradiate with aborted
third ray and (6) an acerate in T. dissimilis, together with the general
agreement between the minute spicules, the skeletal structure,
and _ the general form of tho sponge, appear to mark these three
species out as belonging to a natural though highly plastic circle
of forms comparable to the Tetractinellid genus Placina, Schulze,
of which the species (P. monolopha, dilopJia, and trilopha, Schulze)
each include hi-, tri-, and quadriradiate forms of the fundamental
quadriradiate type ; they are comparable also to many genera of
the Calcarea, where the fundamental (probably triradiate) type exhibits
great modifications, even within tbe limits of a single species.
Besides possessing three complete arms and the large skeleton-
spicule, Trihrachium schmidti is distinguished from Tethyopsis dis-
similis by :—(2) the exterior being unmarked by horizontal ridges ;
(3) the inferior length of the lateral arm of the triradiate spicule J
(4) the apparent absence of the long acerate spicule ; (5) the more
generally elongate form of the flesh-spicule and the superior number
of its lateral whorls of tubercles.
Weltner’s comparison of the form of the minute flesh-spicules
witb the similarly dendritic skeleton-spicules of the Ilhizomorine
Lithistids is invalidated by the fact th at the two classes of spicules
are not homologous with each other, the flesh-spicules oi Trihrachium
being represented in the Lithistid series only by the minute bihamates
and other flesh-spicules of Corallistes &c.
A striking analogy with the arrangement of the skeleton of
the Lyssakine Hexactinellida is afforded by the manner in which
tho arms and shafts of the large skeleton-spicules are employed in
Tethyopsis (inch Trihrachium) to form coherent rectangular meshes.
Weltner’s discovery is of great interest, apart from the peculiarities
of the type described, in the fact th at his species, though living in
the West Indies, is clearly intermediate between two types found
near the confines of the Indo-Australian region.
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