Dr. W. Marshall on new Siliceous Sponges. 395 
great interest tome. These were of two different sizes; some 
were on the average about 3 centim. in diameter, the others 
only 1 centim.; in other respects they had exactly similar 
skeletal elements and exactly the same structure. In this 
indeed there was nothing surprising ; one could simply regard 
the smaller specimens as younger; but there was one pheno- 
menon that could not easily be brought into agreemen; with 
this view—the larger specimens, so far as they were investi- 
gated, showed no sexual products (ova) in the mesoderm, but 
were nearly all engaged in budding in different degrees ; the 
smaller ones, on the contrary, without exception had ova, but 
never buds. In presence of these facts one could imagine two 
possibilities—the sponge in question might propagate sexually 
when young, perhaps during the summer, but asexually when 
older and in the winter ; or we might have to do here with an 
alternation of generations, in that from the ova of a sexual 
smaller form an asexual larger one was developed, from which 
again the first sexual generation would bud. 
With these buds of the marine sponges the gemmules of the 
freshwater sponges may very well be compared ; both occur in 
addition to the sexual products and separated from them in 
point of time ; both are primarily parts of the mesoderm, 
which, however, in the sea-sponges when separated pass 
naked to the surface and immediately commence an indepen- 
dent existence, apparently without any permanent injury to 
the parent animal; while in the freshwater sponges they bo- 
come encapsuled, and for a time pass a latent existence in the 
interior of the dead parent individual. ‘These are certain 
noteworthy distinctions, but still not so very difficult to under- 
stand; we need only bear in mind that we have to do here 
with freshwater animals, and that the conditions of existence 
are essentially different for these and for marine animals. 
The latter have to suffer very little or not at all under periodi- 
cally recurrent persistent want of food, their existence is not 
threatened for a time by the cold of winter or the drought oi 
summer, their conditions of life remain trom month to month, 
in one season of the year as in another, nearly the same, or 
vary too little to superinduce profound changes in the economy 
of these marine organisms. Matters are quite otherwise with 
the creatures dwelling in fresh water, which in this respect 
rather resemble inhabitants of the land than of the sea. 
Some of them belonging to the faunas of warm countries 
are exposed during the hot season to the drying-up of the 
element in which they reside; and although this only takes 
place partially on the banks and here and there elsewhere, 
it suffices, especially in the case of adherent ET to pro- 
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