170 ‘On the Gemmules of Freshwater Sponges. 
especially with a stony bottom. The gemmules of the fluvia- 
tilis-series are heavier than those of the lacustris-series, as 
one may easily convince one’s self by scattering the two upon 
water; the fuviatilis-gemmules sink much sooner than those 
of lacustris. By moving water they will be rolled along 
slowly, and especially at the bottom. 
It would appear that the Meyenie are chiefly distributed 
in running waters, while the true Spongille are rather inhabi- 
tants of still water. This, of course, does not mean that the two 
forms are sharply separated in this respect; on the contrary, 
Meyenie will easily be found in stagnant water, although true 
Spongille are not so readily met with in strongly running 
water. 
A form of Meyenia, M. mirabilis, from the Ohlau, near 
Breslau, lately made known by Dr. Wilhelm Retzer*, is 
interesting in more than one respect ; but especially on account 
of its gemmules, which have a triple armature of amphi- 
disci one above the other. Ido not know the character of 
the Ohlau (not Ohle) and its subsidiary waters, but probably 
its system includes many rapid brooks, so that the gemmules 
of the sponges occurring in it have had to adapt themselves 
in this direction. 
I find it not inconceivable that, in the lapse of time, Spon- 
gillee (Euspongilla, Vejd.) may become converted into Meyenie 
(Ephydatia, Lamx., Gray) by long residence in running 
water, ¢. e. that their skeletal elements, and especially their 
gemmule-envelopes, may become modified in this direction ; 
and that, on the other hand, by a long undisturbed sojourn in 
standing water, Meyenie may revert to true Spongille, by the 
gradual loss of the amphidisci &c. From this point of view 
the ELuspongilla jordanensis, var. drulieformis, recently de- 
scribed by VejdovskyT, acquires a heightened interest. In it 
we have perhaps before us a J/eyenia in process of reversion. 
In the siliceous elements of the shell of the gemmule drawn by 
Vejdovsky, in his pl. ii. fig. 19, we have before us a series of 
transitions from the amphidisci (c) to the simple disk (e). 
Moreover, these structures are much more sparingly scattered 
in the membrane of the gemmule (see pl. 11. fig. 14) than in 
the true Meyenie. I should not be at all surprised if, in 
course of time, in the quiet water of the pool of the Jordan, 
this siliceous armature were entirely to disappear—that is, 
supposing that no “ new blood” was introduced from with- 
* W. Retzer, ‘Die deutschen Siisswasserschwamme,” Inaug. Diss, 
Tiibingen, 1483, p. 25, pl. il. fig. 18. 
+ Abhandl. d. kén, bohm. Gesellsch. d. Wiss. 6 Folge, Bd. xii. p. 22, 
Taf, ii. figs, 14-19. 
