eh oth 
Gemmules of Freshwater Sponges. * 169 
Batesti occurs upon the twigs and stems of trees which 
are under water during the months of the rainy season; and 
Carl Miiller* states, from the reports of the traveller Gustav 
Wallis, who also met with this sponge, called in its native 
country “ Canixi”’ (pron. Canisch1), that the sponge appears 
especially to be produced where air can act upon it—that is, 
upon stones which are alternately wet and dry during the 
rising and sinking of the stream. By means of the spicular 
capsules the gemmules are firmly affixed to the dried sponge, 
so that they cannot fall out and perish on the dry ground ; but 
when, in the rainy season, the rising water again reaches the 
sponges of the previous year, the capsules are ruptured (as 
may be artificially shown), and the true gemmules are set 
free and can become developed in the water. The armature 
of siliceous shields seems to prevent too great a collapse 
of the delicate gemmule-envelopes during desiccation; as 
their margins overlap the conditions are similar to those of a 
coat of scale-armour, which also is only compressible to a 
certain extent. When I removed the siliceous structures by 
hydrofluoric acid the envelopes became very flexible. The 
amphidisci of the gemmule-envelopes of the Meyenie seem 
to have a similar function, but a double disk would not be 
necessary for this purpose; this, as we shall see immediately, 
has been produced by another adaptation. 
The gemmules of the series of forms to which Spongilla 
lacustris belongs have homogeneous membranous walls pro- 
vided at the surface with projecting, often spinose, tangential 
or radiating spicules; hence they are certainly heavier than 
the gemmules of the nitens series, but at the same time always 
light enough to be able to swim in and upon the water. The 
superficial spicules, like the uncinate processes of many stato- 
blasts of Bryozoa, will act as adherent organs, by which the 
gemmules may on occasion be anchored. ‘This constitution 
of the gemmules is certainly advantageous to forms of Spon- 
gille which live in stagnant or slowly flowing water, but 
would be of doubtful advantage to those dwelling in brooks 
and rivers with strong currents; here, being continually 
rolled and driven along, they would have few chances of 
coming to rest, and a large percentage would be lost. To 
prevent this the gemmule would have to adopt, as it were, 
a check in the form of a heavier shell; and this is effected by 
the introduction of special siliceous elements, the amphidisci, 
which at the same time form a shield against the occasional 
‘shocks and contusions which are inevitable in running water, 
* See ‘Die Natur,’ Bd. xxiii. (1870), p. 181. 
Ann. & Mag. N. Hist. Ser. 5. Vol. xiii. 12 
