400 Dr. W. Marshall on new Siliccous Sponges. 
of their gemmules the Spongille show peculiar and very 
significant differences. Some of them (Sp. lacustris &e.) are 
adapted for swimming; others (Sp. Cartert, nitens, &c.) for 
swimming and passive movement; others, again, areessentially 
heavier (Meyenia) ; but all can easily get from one locality to 
another by the well-known means of transport of the lower 
aquatic organisms, namely birds*. 
The occurrence of Sp. Cartert (which has hydro- and aero- 
statically adapted gemmules, and was previously known only 
from India, although possessing allies in Africa, namely Sp. 
nitens from the White Nile in the Leipzig Museum) in the 
Mauritius } is very remarkable, although that island lies in 
the south-east trade-wind belt and in the southerly diverging 
branch of the trade-drift flowing from east to west; but we 
see that the Mauritius, besides some autochthonous forms, 
ossesses a very remarkable mixed fauna, in which Ethiopic 
and Oriental, and even some Australian, elements meet to- 
gether. 
These considerations may, in a certain sense, be compared 
with those which Huxley} has put forward as to the origin 
and derivations of the freshwater Crayfish. The great biolo- 
gist shows that there are two families, well characterized by 
certain peculiarities, of such Crustaceans, one of which, that 
of the Potamobiide, inhabits the northern, and the other, the 
Parastacide, the southern hemisphere. He supposes that 
both families descend from a widely distributed primitive 
form, living in the sea, which he names Protastacus, and which 
has wandered into the fresh waters, and here become differen- 
tiated into the ancestor of the Potamobiide in the northern 
parts of the earth, and of the Parastacide in the southern ; 
and hence that the river Crayfish, notwithstanding their differ- 
ences, are of monophyletic origin §. 
It is, however, in my opinion, very possible that this hypo- 
thetical Protastacus, while still an inhabitant of the sea, may 
have existed under two, three, or even more different forms, 
local races, or whatever they may be called, and that these, 
after passing into a different medium of existence, still fur- 
ther adapted themselves to the latter. Hor a whole series of 
other organisms of the freshwater, the geographical distribu- 
tion of which would otherwise be quite unintelligible (such as 
* I reserve these various modifications of the gemmules and their pro- 
bable causes for treatment in a subsequent memoir. 
+ According to a kind epistolary communication from Mr, Carter. 
{ Proc. Zool. Soc. Lond. 1878, p. 752. 
§ See also Jhering, “ Die Thierwelt der Alpenseen und ihre Bedeutung 
fiir die Frage nach der Entstehung der Arten, in Nord und Sud,” Band x. 
p. 242 (1879). 
