Sottas—On the Origin of Freshwater Faunas. 95 
It will be seen that, as the analysis proceeds, each group furnishes a large 
number of exclusively marine and a very small number of exclusively freshwater 
divisions; while the mixed freshwater and marine genera, omitting admittedly 
exceptional cases, are very few indeed. Thus, to take the Spongiz as an example, 
it is divided into six orders, of which only one, the Monaxonidez, is marked m. f.; 
all the rest are marine. Disregarding these, we find that the Monaxonide break 
up into five families, of which only one, the Renieridz, is marked fm. This, 
finally, is resolvable into several sub-families, all of which are marine, except one, 
the Spongillina, which is exclusively freshwater. When this Paper was first penned 
the Spongillina were regarded as a compact family, clearly distinguished from 
other Renieridz by the possession of “ statoblasts”; but recent discoveries have 
brought to light some ten species, belonging to four or five genera, which are said 
to be without this characteristic feature; and it is quite possible that some of 
these new forms may belong to genera otherwise known as marine. 
We now approach the inquiry as to how far the exemption we have predicted 
of freshwater animals, from a free larval existence, is capable of verification. 
Commencing with the sponges, we find, in this country, the closely allied genera 
Euspongilla and Ephydatia or Meyenia (Zuspongilla fluviatilis and Ephydatia lacus- 
tris). Elsewhere the rest of the world has furnished hitherto some additional seven 
or eight genera. Such of these (except Lubomirskia) as have been exhaustively 
studied have been found to propagate by means of so-called ‘‘ winter-eggs” or 
statoblasts. These are modified internal buds or gemmules, which are provided 
with a protective horny envelope, and generally one or more layers of surrounding 
siliceous spicules. Within this protecting case the bud rests during the winter, 
and on the advent of spring emerges as a young sponge. 
In tropical climates the statoblast is produced, not on the approach of 
winter, but just before the dry season ; and it, no doubt, primarily serves, in this 
case, as a means of protection against the fatal effects of dessication. Since, how- 
ever, in the dry state, the envelope of these statoblasts contains a considerable 
volume of air, which materially diminishes their specific gravity, it is possible, 
as William Marshall has suggested, that their formation may serve a secondary 
purpose ensuring for the sponge a wide dispersion by the agency of the 
winds. 
The pressing need for a rigid attachment of the reproductive gemmules, 
which exists in swift streams, or rivers liable to floods, is well illustrated by a case 
mentioned by Marshall; for he finds that in Parmula brownii, from the Rio Negro, 
the spicular layer of the statoblasts is continuous with that of the hard, dense ske- 
leton of the parent sponge, thus protecting them against all chance of being washed 
away till they can crawl out as young sponges and shift for themselves. On the 
whole, so far as definitely ascertained facts go, they point unmistakably to the 
general, if not universal, occurrence of a peculiar mode of propagation in the case 
TRANS. ROY. DUB. SOC., N.S. VOL. III. 
