Dr. W. Marshall on new Siliceous Sponges. 40 { 
the crocodiles, many fishes, &c.), we must have recourse to 
similar ancestors which have lived in the sea. 
For the explanation of the origin of the freshwater sponges 
the conditions, I believe, are much more favourable: here we 
have in truth still living sea-inhabiting forms which perfect] 
agree, except for a few, small, and easily intelligible differ- 
ences, with the former, as shall be immediately shown; in 
this case we have no occasion to reconstruct an ancestral form, 
which always has something doubtful about it. 
In seeking for forms of marine sponges which might be the 
ancestors of the freshwater sponges, we must at once leave 
out of consideration the Hexactinellide, Lithistide, and 
Tetractinellidee, less perhaps on account of the form of their 
skeletal elements than of their occurrence in deeper water; and 
of the Monactinellidzs which remain, the Rendere press, as it 
were, of themselves into the first line. There can be no 
doubt that these sponges are at present engaged very actively 
in adaptation. In them we have a group in which, notwith- 
standing strong differentiation, the individual forms are most 
multifariously bound together; the struggle for existence will 
not long be fought between them in so high a degree, through 
which only in course of time, under fundamentally altered 
conditions of existence, it will come to pass that the surviving 
members of the group, like the mountain-tops of a sunken 
land protruding as islands from a flood, as sharply-defined 
species will represent well-characterized genera. Renierse 
are distributed in all seas, from the tropics to Greenland and 
Kerguelen’s Land; they oceur (Pellina profunditatis, O. 
Schm.) from a depth of 324 fathoms*, upwards, as far as 
existence is still possible for truly aquatic animals. Of Re- 
niera caruncula of the English coast, Norman says: “ On 
rocks between tide-marks. ‘This is one of the regular tide-mark 
sponges.” On the rocky coast of Knoshima, Déderlein{ found 
feniere in places which at low tide were just sprinkled with 
water; the Mediterranean Reniera littoralis§ also descends 
only a few feet below the surface of the water, and R. luxu- 
rians 1s even frequently left dry by the ebb||. At the same 
time these sponges are always abundant, not only in indivi- 
duals, but also in species; in the northern Adriatic over a 
dozen occur, and near Naples they form more than 24 per 
cent. of the existing Monactinellide (fourteen out of fitty- 
* Schmidt, Spongien-Fauna des atlant. Gebietes, p. 42. 
+ Bowerbank’s ‘ British Sponges,’ vol. iv. suppl., ed. by A. M. Nor- 
man, p. 81. 
¢ Archiv f. Naturg. 1883, p. 111. 
§ U, Keller, Zeitsch. f. wiss. Zool. Bd. xxx. p. 580. 
|| Schmidt, Spongien des adriat. Meeres, p. 12. 
