22 LIFE AND HER CHILDREN. 
elsewhere. All these wondrous little beings are some 
of life's simplest children, and one and all are made 
of nothing but slime, while yet they live, and move, 
and seek their daily food. 
But all these are naked and homeless, and to a 
great extent unprotected. Gulped down in thou- 
sands and millions by each other, and by other 
animals, they are defenceless and weak against 
attacks. It would certainly be better for them 
if they could have solid shells to cover their soft 
bodies, and to protect them in many dangers. And 
so we find that even in this lowest stage of life 
necessity is the " mother of invention ;" and drops of 
slime, no higher than the thread-slime (Fig. i), have 
learned to build shells around their delicate bodies. 
These shell-builders live chiefly in the sea, and 
there you may find them if you search carefully by 
the help of a strong magnifying glass in the ooze of 
oyster-beds, or under the leaves of the delicate green 
seaweed, or in the muddy sand of the sea-shore. 
The most common forms will be those shown at a, e, 
f, and g in Fig. 4 ; and, though they are so very 
small, you may if you are fortunate see them clinging 
by their fine slime-threads to the weeds or the mud. 
These animals are, as I have said, simple slime- 
drops like the thread-slime, but they add to the list of 
wonderful things that such slime can do, for they take 
out of the sea-water, particle by particle, the lime 
which is dissolved in it, and build around their soft 
bodies the solid shell or skeleton in which they live. 
Nor is this all ; even if they all built the same simple 
shell, it would be very puzzling to imagine how they 
do it, but they do much more. They build shells in 
