392 Dr. W. Marshall on new Siliceous Sponges. 
special interest among naturalists. The extraordinary is always 
adapted to impress us particularly, and especially the extra- 
ordinary in the organic world: to the many mysteries which 
every creature hides within itself we have here added a new 
one, which asks us how does this particular creature come to 
differ so completely from the ordinary pattern? what were the 
constraining causes of this, and how is it modified by them in 
its whole organization? Parasitism and subterranean mode 
of life lead to such aberrant phenomena, as also the adaptation 
of terrestrial forms to an aquatic life, and vice versd; those 
organisms especially which, belonging to a group otherwise 
exclusively inhabiting fresh or salt water, have emancipated 
themselves from this general rule of life of their relatives, and 
taken up their abode in the sea or fresh water in opposition to 
it, have always attracted and still attract particular attention. 
Ten new marine Meduse could not have caused so much stir 
as Limnocodium palustre alone ! 
In this way also it comes to pass that much attention has 
for a long time been paid to the freshwater sponges, and that 
even small contributions to the knowledge of these remarkable 
organisms may expect to meet with more consideration and 
amore favourable reception than, for example, the descrip- 
tion of a number of new Ascones or Reniere. This convic- 
tion encouraged me in the following paper to make known 
some new freshwater sponges, which may perhaps also lay 
claim to a special interest on the ground of their origin. 
General Part. 
Before passing to the special descriptions I may be permitted 
to premise some remarks of a more general nature, in which 
I shall confine myself to a historical introduction, as it is my 
intention to revert to the subject in a memoir of wider extent 
upon the freshwater sponges. 
No zoologist of the present day can doubt that our freshwater 
sponges are derived from marine forms, and that they are the 
descendants of the latter modified by their mode of life. We 
have therefore only to ascertain what are the relationships of the 
freshwater sponges to one another and to the marine sponges, 
and how far they are altered by residence in fresh water. 
Most investigators of our objects seem tacitly to be of 
opinion that all non-marine siliceous sponges are nearly allied, 
at any rate more nearly than to marine sponges; and that 
they form a well-characterized group, the members of which 
have originated directly from each other, and these from a 
common marine form. ‘This is possible, but certainly not more! 
It is just as possible that im the freshwater sponges there 
. 
| 
