In the larger Sertularia (2, Fig. 66) the whole branched tree is connected by jelly threads running through the stem, and all the thousands of mouths are spread out in the water. One large form called the sea-fir _Sertularia cupressina_ grows sometimes three feet high, and bears as many as a hundred thousand cups, with living mouths, on its branches.
The next of my minute friends I can only show to the class in a diagram, but you will see it under the fourth microscope by and by. I had great trouble in finding it yesterday, though I knew its haunts upon the green weed, for it is so minute and transparent that even when the weed is in a trough a magnifying-glass will scarcely detect it. And I must warn you that if you want to know any of the minute creatures we are studying, you must visit one place constantly. You may in a casual way find many of them on seaweed, or in the damp ooze and mud, but it will be by chance only; to look for them with any certainty you must take trouble in making their acquaintance.
Turning then to the diagram (Fig. 68) I will describe it as I hope you will see it under the microscope--a curious tiny, perfectly transparent open-mouthed vase standing upright on the weed, and having an equally transparent being rising up in it and waving its tiny lashes in the water. This is really all one animal, the vase _hc_ being the horny covering or carapace of the body, which last stands up like a tube in the centre. If you watch carefully, you may even see the minute atoms of food twisting round inside the tube until they are digested, after they have been swept in at the wide open mouth by the whirling lashes. You will see this more clearly if you put a little rice-flour, very minutely powdered and coloured by carmine, into the water; for you can trace these red atoms into some round spaces called _vacuoles_ which are dotted over the body of the animal, and are really globules of watery fluid in which the food is probably partly digested.
_Thuricolla folliculata_ and _Chilomonas amygdalum_. (Saville Kent.)
1, Thuricolla erect; 2, retracted; 3, dividing. 4, _Chilomonas amygdalum_. _hc_, Horny carapace. _cv_, Contractile vesicle. _v_, Closing valves.]
You will notice, however, one round clear space (_cv_) into which they do not go, and after a time you will be able to observe that this round spot closes up or contracts very quickly, and then expands again very slowly. As it expands it fills with a clear fluid, and naturalists have not yet decided exactly what work it does. It may serve the animal either for breathing, or as a very simple heart, making the fluids circulate in the tube. The next interesting point about this little being is the way it retreats into its sheltering vase. Even while you are watching, it is quite likely it may all at once draw itself down to the bottom as in No. 2, and folding down the valves _v_, _v_ of horny teeth which grow on each side, shut itself in from some fancied danger. Another very curious point is that, besides sending forth young ones, these creatures multiply by dividing in two (see No. 3, Fig. 68), each one closing its own part of the vase into a new home.