3 12 
THE TANGANYIKA PROBLEM. 
in Fig. I — d., is very exceptional. The axial thread, in contrast to spicules of class 
(a.), present a dilation corresponding to each swelling on the spicule. 
(7) The spicules of this class are of variable and irregular form, since the 
individual amphioxea or amphitornota which form them may fuse at any point and 
at any angle (Fig. 1 — b. ). As a rule these compound systems are formed from 
spicules from class ( a .), though occasionally a spicule of class (/ 3 ) is found to take 
part in their formation. 
With regard to their origin, two suppositions are possible ; first, that they are the 
result of irregular growth, and branching of a single spicule derived entirely from a 
single scleroblast ; secondly, that they arise by fusion of spicules primitively distinct, 
and formed each by its own scleroblast. Fig. 1 — b. might be taken as evidence of the 
former view, but such forms as that represented in Fig. 1 — b. render such a supposition 
highly improbable, to say the least. The view that these spicular systems are of 
compound origin receives strong support from the way in which their axial threads 
cross one another instead of branching. If these irregularities arose as outgrowths 
from one spicule formed in one mother cell, it might well be expected that their 
axial threads should be also formed as outgrowths from that of the main spicule ; 
but this is certainly not the case in many spicules of our Spongilla, as can be seen 
from the figures. In another sponge, which is probably a monaxonid of the family 
Axinellidce, viz., Tricentrium muricatum (Pallas 1756), Ehlers, 1870 (=Pleetronella 
papillosa, Sollas, 1879), there are branched spicules iu which the axial threads are 
continuous throughout, a fact which may indicate that the spicules themselves owe 
their form to branching. It seems clear, therefore, that the irregular spicules of 
Spongilla moorei have, in many cases, been produced by fusion. Judgment must 
be suspended for the present with regard to those systems in which no discontinuity 
can be detected in the axial threads of the component spicule rays ; such spicules 
may be simply branched. The question cannot be decided until the actual origin of 
the spicules has been studied ; and the same may be said for Tricentrium. Since 
now it has been shown that the triradiates and quadradiates of the Ascons are formed 
by fusion, there is no inherent improbability in a similar process occurring in other 
cases. 
Spicules of a similar character to the compound systems here described have been 
figured by many authors in various Spongillidre (Spongilla aspinosa, Potts) Lubomir- 
skia intermedia, Dybowski). All these authors regard them as abnormalities, but 
in moorei they are so frequent that they must be considered as a normal feature of 
the species. It is possible that in other Spongillidse these systems have not received 
the attention they deserve. 
In addition to the spicules described above there are small masses of silica in 
Spongilla moorei, comparable with those found in Spongilla aspinosa (Fig. 1 — e.). 
(B) The Arrangement ok the Sficules to form Fibres, etc. — The spicules 
which form the polyspiculous fibres belong mainly to the first and third classes 
above described. Spicules of the first class form the greater part of the fibies, while 
others lie about in the sponge tissue, presenting for the most part an ii regular 
method of arrangement, though many such spicules are placed so as to bridge over 
the spaces between the fibres in a perfectly definite way. Spicules of the second 
class, which are far less numerous than those of the first, seldom participate in the 
formation of the fibres, but, as a rule, lie scattered irregularly between the fibres. 
