784 
DB. J. S. BOWEEBANK ON THE ANATOMY 
of the parent sponge ; thus, the one that is represented by fig. 13, Plate XXVI. Phil Trans. 
1858, from the surface of Spongilla lacustris^ Johnston, is curved so as to accommodate 
it to the rotundity of the ovarium; and we do not find fits apices projecting as in those 
of S.Jiuviatilis, but instead of the projecting apices, the whole spiculum is covered with 
minute spines, assimilating it in character with the general structure of those spicula 
which combine the office of tension and defensive spicula, but differing considerably in 
their proportion from the tension spicula of the same sponge, S. lacustris, represented 
by fig. 21, Plate XXIV. Phil. Trans. 1858, the one being evidently destined to sustain 
and protect extended membranes, while the other is especially adapted for a small curved 
surface by its form and small size ; each of the figures being drawn with the same power, 
660 linear. 
On the surface of the ovarium of Spongilla cinerea, Cartee, we find this description 
of spiculum still more decidedly produced. It is of a cylindrical form and entirely 
spined, and has just the amount of curvature that is in unison with the curved surface 
on which it reposes. The spines on the middle of the shaft are cylindrical, and termi- 
nated bluntly, so as to strengthen its hold on its imbedment. Those of its apices, on the 
contrary, are acutely conical and recurved, and are strongly produced, so as to form very 
efficient weapons of defence. This spiculum is represented by fig. 17, Plate XXVL 
Phil. Trans. 1858. 
The birotulate and boletiform spicula of the second group appear to be more purely 
structural, as regards the skeleton of the ovarium. The rotulse are very closely packed 
at both the external and internal surfaces of that body, and the crenulation or dentation 
of each rotula is as well produced on the internal as on the external ones, and it appears 
to be very influential in maintaining each spiculum in its proper position. In the 
natural condition of the ovaria these spicula are entirely imbedded in its walls, and 
other spicula of a truly defensive nature are superimposed for its protection. The large 
spines in the shafts of the birotulate spiculum from S^ongilla plumosa. Carter, fig. 21, 
Plate XXVI. Phil. Trans. 1858, are also apparently subservient to strengthening and 
maintaining the spiculum in its proper situation, although they are acutely terminated, 
as defensive spines usually are ; but in the same relative position on the birotulate spi- 
cula of Spongilla Meyeni, Carter, we find the spines short, stout, and cylindrical, 
spreading or budding at their apices, and evidently more fitted for assisting to retain the 
spiculum in its proper place than for defensive purposes. This spiculum is represented 
by fig. 29, Plate XXVI. Phil. Trans. 1858. 
There is an apparent analogy between the expansions of the rotulse and those of the 
folio-peltate spicula, but they do not appear, like the latter, to be derived from the 
temate forms. The radiations of the canaliculi, as represented by fig. 32, Plate XXVI. 
Phil. Trans. 1858, are not derived from three primary rays, but each appears to emanate 
from a central cavity at the end of the shaft ; and their number, 22, at their proximal 
termination is not reconcileable with any regular number of bifurcations arising from 
three primary rays, however short we may imagine them to be. 
