AND PHYSIOLOGY OF THE SPONGIAD^. 
779 
straight elongated forms ; and they are also better adapted to membranes having unequal 
surfaces, such as those in Microciona armata, Bowerbank, MS., where we see them 
following the undulations of the membranes and sustaining them in their proper positions 
around the columnar parts of the skeletons. The varieties of form in these spicula are well 
represented by figs. 26, 27, & 28, Plate XXIV, Phil. Trans. 1858. They are all out of 
the same sponge. In Grantia compressa, and other closely allied species, where the 
structure is systematically membranous, the skeleton spicula are triradiate, supporting 
the membranes in uniform planes in the most effectual manner ; and they are, in fact, 
systematically tension spicula, as well as skeleton ones. In Grantia nivea, Johnston, 
which is not symmetrical in its structure like G. corrvpressa and its congeners, other 
forms of tension spicula are developed to suit their especial purposes, such as represented 
by figs. 30 & 31, Plate XXIV. Phil. Trans. 1858. 
In siliceous sponges we also occasionally find triradiate spicula developed, and per- 
forming the office of tension spicula, in the midst of comparatively large membranous 
areas ; but these forms, in every case under such circumstances in which I have seen them 
in situ, appear to belong to the exception, rather than the general rule obtaining in such 
sponges. 
The foliato-peltate spicula, for a full account of the progressive development of which 
I must refer to page 298, Phil. Trans. 1858, appear to be a development of the apices 
of connecting spicula into dermal tension ones, bearing a strong resemblance in form 
and purpose to the bony scutes in the skins of some of the higher animals, while the 
extreme crenulation of their margins probably served the purpose of facilitating the 
action of the porous system. 
In all the varieties in form which I have hitherto described, and with which I am 
acquainted, where they perform the office of tension spicula only, they are destitute of 
spines. In other cases the tension spicula not only fulfil their own especial office, but they 
subserve that of defensive spicula also. Thus in the dermal membrane of S^pongilla 
lacustris, Johnston, we find them dispersed rather numerously, covered with short 
acutely conical spines, as represented by fig. 21, Plate XXIV. Phil. Trans. 1858. 
In Sjpongilla alba, Carter, we find the tension spicula as abundantly spinous as those of 
8. lacustris, but in this case the spines are truncated (fig. 22, same Plate). They have a 
similarly blunted, imperfectly produced character in those of Pachymatisma Johnstonia, 
as represented by fig. 24. 
The production of tension spicula in the membranes of the Spongiadse is by no means 
a peculiarity of that class of animals. We find them in numerous beautiful forms in the 
skins of the Holothuriadse, varying in shape in the different parts of the animal to adapt 
themselves to the necessities of their situation ; but the closest approximation, both in size 
and form, to those of the Spongiadse are the bihamate ones that are found so abundantly 
dispersed on the membranous tubular suckers of Echinus sgglimra ; and I have also seen 
another variety of these spicula in the tubular tentacles of a large common species of 
Actinia, and in the latter case they were even more minute than those of the Spongiadae. 
WDCCCLXII. 5 0 
