But if there are so many thousands of mouths to feed, on the tree-like Sertulariæ as well as in all these Infusoria, where does the food come from?
Partly from the numerous atoms of decaying life all around, and the minute eggs of animals and spores of plants; but besides these, the pool is full of minute living plants—small jelly masses with solid coats of flint which are moulded into most lovely shapes. Plants formed of jelly and flint! You will think I am joking, but I am not. These plants, called Diatoms, which live both in salt and fresh water, are single cells feeding and growing just like those we took from the water-butt (Fig. 29, p. 78), only that instead of a soft covering they build up a flinty skeleton. They are so small, that many of them must be magnified to fifty times their real size before you can even see them distinctly. Yet the skeletons of these almost invisible plants are carved and chiselled in the most delicate patterns. I showed you a group of these in our lecture on magic glasses (p. 39), and now I have brought a few living ones that we may learn to know them. The diagram (Fig. 69) shows the chief forms you will see on the different slides.
Fig. 69.
Living diatoms.
a, Cocconema lanceolatum. b, Bacillaria paradoxa. c, Gomphonema marinum. d, Diatoma hyalina.
The first one, Bacillaria paradoxa (b, Fig. 69), looks like a number of rods clinging one to another in a string, but each one of these is a single-celled plant with a jelly cell surrounding the flinty skeleton. You will see that they move to and fro over each other in the water.
Fig. 70.
