thin them.

[Illustration: FIG. 7. LIVING DIATOMS.

_a, Cocconema lanceolatum. b, Bacillaria paradoxa. c, Gomphonema
marinum. d, Diatoma hyalina_.]

But if there are so many thousands of mouths to feed, on the tree-like
Sertulariae 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, 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, and now I have brought
a few living ones that we may learn to know them. The diagram (Fig. 7)
shows the chief forms you will see on the different slides.

The first one, _Sacillaria paradoxa_ (_b_, Fig. 7), 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.

The next two forms, _a_ and _c_, look much more like plants, for the
cells arrange themselves on a jelly stem, which by and by disappears,
leaving only the separate flint skeletons. The last form, _d_, is
something midway between the other forms, the separate cells hang on
to each other and also on to a straight jelly stem.

[Illustration: FIG. 8. A DIATOM (_Diatoma vulgare_) GROWING.

_a, b,_ Flint skeleton inside the jelly-cell. _a, c_ and _d, b_, Two
flint skeletons formed by new valves, _c_ and _d_, forming within the
first skeleton.]

Another species of Diatoma (Fig. 8) called _Diatoma vulgare_, is a
very simple and common form, and will help to explain how these plants
grow. The two flinty valves _a, b_ inside the cell are not quite the
same size; the older one _a_ is larger than the younger one _b_ and
fits over it like the cover of a pil