404 Dr. W. Marshall on new Siliceous Sponges. 
which, if we may so speak, are to be accounted for by a ten- 
dency to saving in Nature, scarcely occur more strikingly than 
with respect to offensive and defensive colours. These dis- 
appear immediately when the pressing necessity which called 
them forth is got rid of ; and it would seem that the enemies 
by which Lentere in sea-water might certainly be threatened 
and disturbed did not migrate after them into the fresh water, 
and there the varied coloration disappears, just as in the 
Renter which live concealed under stones &c. That green 
Spongille occasionally occur is due, as is well known, to a 
symbiotic process, and is no integral property of those 
sponges. 
Although the conviction of the connexion of the freshwater 
sponges with the Render has never, so far as | know, been deve- 
loped in detail in zoological literature, I have repeatedly sup- 
ported it, especially in conversation with scientific friends, and 
Claus gives expression to it in his text-book *, Other natural- 
ists indeed seem to be of a different opinion, such as Keller J, 
who regards the Spongille as well as the Hsperie as groups 
quite distinct from the Reniere ; and Carter}, who occupies the 
same standpoint in 1881 as in 1875; and although he places 
his ‘ Potamospongida”’ in the same order (Holorhaphidota) 
with the Renieride, separates the latter as his first from the 
former as his fifth family, by the Suberitide, Pachytragide, 
and Pachastrellide (¢. e. by the whole of the Tetractinellide, 
including the Lithistide), or, in other words, at least if his 
system is to be taken as expressing his ideas of the relation- 
ships, the relations of affinity between the freshwater sponges 
and the Renieridz are but slight, at any rate slighter than 
those of the two groups with the Geodia for example. 
Against my hypothesis of the polyphyletic origin of the 
freshwater sponges evidence from facts can hardly be brought 
forward; and although I cannot prove it, it seemed to me not 
without interest to bring this question under discussion. I 
may here, however, expressly state that it is not and cannot 
be at all my design to break up the group of the freshwater 
sponges ; even from considerations of convenience it is desir- 
able to adopt a Renierid group of “ Potamospongie.” 
* Grundziige der Zoologie, 8 Aufl, 1876, p. 194. 
+ ‘Upon the latter point the less weight can be laid, as a similar 
structure occurs in quite different groups (Spongilla, Esperia).”  Zeitschr. 
f. wiss. Zool. Bd. xxx. p. 564. 
t Ann. & Mag. Nat. Hist. ser. 5, vol. vii. p. 85. 
Dee 
