From the microscopist’s point of view the most interesting feature of the snail is its rasping organ, often wrongly termed the tongue. To find this organ it is necessary to open up the mouth of a dead snail, and if we seek the assistance of our lens while doing so, we shall have no difficulty in finding the rasp—it may be recognised by the minute teeth with which it is furnished. The whole structure should be carefully removed and mounted upon a slide. In some kinds of snails there are but a hundred teeth, other kinds, however, possess as many as twenty-six thousand eight hundred. The snail makes use of this remarkable organ to procure its food. Vegetation is pressed against a plate at the top of the creature’s mouth and literally filed into small pieces by the rasping organ. Captive water snails may be watched while using their rasp upon the water plants or upon the green slime which soon accumulates in aquaria. The eggs of snails are easily found and should be examined, the queer little inmates may be studied through the transparent shells, in all stages of development.
The naturalist whose inclinations lead him towards the study of animal life will find plenty to occupy his time and his microscope. All kinds of eggs of small creatures may be watched as they develop. Frogs’ eggs are of interest in this respect, so too are tadpoles which hatch from them. The whole blood circulation in a young tadpole may easily be studied under the microscope, the structure of the external gills, the gradual change to internal gills, the development of legs, the absorption of the tail. The tadpole and its marvellous changes will afford sufficient microscopic material to last for many weeks.
CHAPTER IX
THE STUDY OF THE ROCKS
The study of rocks and minerals by means of the microscope is apt to be disappointing. In the first place, to study them seriously we require a special microscope, the ordinary instrument, with which we may poke into the deepest secrets of the animal and plant world cannot translate for us half the story of the rocks. Again, to understand rocks and minerals we must study them somewhat deeply. Geology, as the science of rocks is called, is no more difficult than botany or zoology, the sciences of plants and animals respectively. Botany and zoology, however, appeal in some degree to nearly all of us; we may learn a good deal concerning the structure of the cockroach with the help of our microscope and be interested in the revelations of our instrument, but to embark on a detailed course of the minute internal anatomy of insects would appeal to few of us. Animal or plant life may be studied piecemeal and enjoyed on account of its absorbing interest. Geology must be studied from its very beginnings if we are really to understand what we see beneath the microscope.
In the hand, a lump of rock, say of granite, may be of exceeding beauty. The body of the rock is, perhaps, a delicate pink, scattered here and there are the flat glittering plates of mica and brilliant crystals of quartz. Other rocks, less common, vie with the rare gems for beauty of colouring and lustre. As thin microscope sections these once gorgeous specimens are colourless, dull and, unless we understand them, uninteresting.
There are, however, many mineral substances which we may study with advantage for, if our investigations do not take us very far towards elucidating the story of the rocks, we shall at any rate discover something that is new to us. We may well commence our studies with the examination of ordinary sand. This is not a rock, you will probably exclaim. You are right but one day it may be a rock, it all depends upon circumstances.
Before we take out our microscope let us have a short talk about rocks in general, then we may understand better where we are. Rocks of one kind and another make up the crust of the earth, that is pretty obvious anyway. Thousands and thousands of years ago, how many we are not prepared to guess, this old earth of ours was a sphere of molten rock. Needless to say it was far too hot for any plants or animals to dwell upon it. Very, very gradually the outer crust cooled down and in time it became sufficiently cool to support animal and vegetable life. Then there were rivers and seas and then came, from time to time, rain and wind and frost and even earthquakes. The earthquakes cracked the crust of the earth, moisture entered the cracks and, when the frosts came, pieces of rock were broken away, owing to the expansion of the moisture in the cracks as it became converted into ice. The rain and wind helped to carry the broken pieces of rock, ever downwards towards the sea, but before the sea was reached the big boulders became broken up, by their buffeting, into shingle and sand and mud. In the course of long ages, longer than it is easy to imagine, these broken pieces of rock, gathered together as we have seen from various districts, may have been left high and dry, for the face of the earth has not always been as we know it now.
In time all the little particles became welded together to form a new kind of rock. Sometimes animals and plants were buried in the mud destined to become a rock and their parts were so well preserved that they may not only be recognised by present-day scientists, but in many cases their structure may be made out so well with the aid of a microscope that no one would guess they had been buried thousands of years ago.