FRESH-WATER ANIMALS 87 



water animal, enabling it to hatch of greater size 

 and strength and consequently better able to resist 

 the currents of the river. Hence we find that in 

 fresh-water animals the eggs are usually larger 

 than in their marine allies. A crayfish, for ex- 

 ample, though only a third the length of a lobster 

 lays actually bigger eggs. Having thus seen 

 what are the special conditions under which fresh- 

 water animals exist, we may now proceed with our 

 inquiry and consider in what way the fresh-water 

 representatives of the several groups of animals 

 meet these conditions. 



Concerning the Protozoa I am not aware of any 

 points, either in structure or life-history, that 

 would distinguish the fresh-water from the marine 

 forms. The mode of life is apparently the same in 

 the two cases ; though it is very possible that 

 closer examination would show that differences do 

 exist, at any rate in certain cases. From this point 

 of view a careful study of the fresh-water Protozoa 

 might very possibly yield results of interest and 

 importance. Among Sponges Spongilla is the most 

 familiar one occurring in fresh water. Spongilla 

 reproduces sexually, like the marine sponges ; but, 

 unlike the latter, it can also reproduce by means of 

 special buds or " gemmules," Each gemmule 

 consists of a little spherical group of cells, formed 

 in the deeper part of the sponge, of which the 

 superficially placed cells become specially modified 

 so as to form a thick projecting capsule strengthened 

 by peculiarly formed silicious spicules. Such gem- 

 mules are formed usually in the autumn. Owing 



