« IndietroContinua »
The nervous system consists of a supra-oesophageal ganglion with four posterior nerve-cords-one dorsal, two lateral, and one (or perhaps two) ventral. There were probably also remains of the old plexus, but this is fast disappearing. The excretory system consists of a pair of tubes discharging through the sides of the · body-wall, and having each a ciliated, funnel-shaped opening in the perivisceral cavity. These have received the name of nephridia. Through these also the eggs and spermatozoa are discharged. The reproductive organs are modified patches of the peritoneum, or lining of the perivisceral cavity.
The number of muscles or muscular layers has been reduced in this animal. But such a reduction in the number of like parts in any animal is a sign of progAnd the longitudinal muscles have increased in size and strength, and the animal moves by writhing. Such a worm has the general plan of the body of the higher forms fairly well, though rudely, sketched. Many improvements will come, and details be added. But the rudiments of the trunk of even our own bodies are already visible. Head, in any proper sense of the term, and skeleton are still lacking; they remain to be developed.
And yet, taking the most hopeful view possible concerning the animal kingdom, its prospects of attaining anything very lofty seem at this point poor. Its highest representative is a headless trunk, without skeleton or legs. It has no brain in any proper sense of the word, its sense-organs are feeble; it moves by writhing. Its life is devoted to digestion and reproduction. Whatever higher organs it has are subsidiary to these lower functions. And yet it has taken
If this is the
ages on ages to develop this much. highest visible result of ages on ages of development, what hope is there for the future? Can such a thing be the ancestor of a thinking, moral, religious person, like man? "That is not first which is spiritual, but that which is natural (animal, sensuous); and afterward that which is spiritual." First, in order of time, must come the body, and then the mind and spirit shall be enthroned in it. The little knot of nervous material which forms the supra-oesophageal ganglion is so small that it might easily escape our notice; but it is the promise of an infinite future. The atom of nervous power shall increase until it subdues and dominates the whole mass.
WORMS TO VERTEBRATES: SKELETON AND HEAD
IN tracing the genealogy of any American family it is often difficult or impossible to say whether a certain branch is descended from John Oldworthy or his cousin or second cousin. In the latter cases to find the common ancestor we must go back to the grandfather or great-grandfather. The same difficulty, but greatly enhanced, meets us when we try to make a genealogical tree of the animal kingdom. Thus it seems altogether probable that all higher forms are descended from an ancestor of the same general structure and grade of organization as the turbellaria, although probably free swimming, and hence with somewhat different form and development, especially of the muscular system. It seems to me altogether probable that all, except possibly Mollusca, are descended from a common ancestor closely resembling the schematic worm last described. Some would, however, maintain that they diverged rather earlier than even the turbellaria; others after the schematic worm, if such ever existed. As far as our argument is concerned it makes little difference which of these views we adopt.
From our turbellaria, or possibly from some even more primitive ancestor, many lines diverged. And this was to be expected. The cœlenterata, as we saw in hydra, had developed rude digestive and reproduc
56 THE WHENCE AND THE WHITHER OF MAN
tive systems. The higher groups of this kingdom had
different in different areas.
We must remember, too, the great age of this group.
seems almost to
them are largely or entirely parasitic, and in connection with this mode of life have undergone modifications and degeneration which make it exceedingly difficult to decipher their descent or relationships.
Four of these branches have reached great prominence in numbers and importance. One or two others were formerly equally numerous and have since become almost extinct; so the brachiopoda, which have been almost entirely replaced by mollusks. The same may very possibly be true of others. For of the amount of extinction of larger groups we have generally but an exceedingly faint conception. Indeed in this respect the worms have been well compared to the relics which fill the shelves of one of our grandmother's china-closets.
The four great branches are the echinoderms, mollusks, articulates, and vertebrates. The echinoderms, including starfishes, sea-urchins, and others straggled early from the great army. We know as yet almost nothing of their history; when deciphered it will be as strange as any romance. The vertebrates are of course the most important line, as including the ancestors of man. But we must take a little glance at mollusks, including our clams, snails, and cuttle-fishes; and at the articulates, including annelids and culminating in insects. The molluscan and articulate lines, though divergent, are of great importance to us as throwing a certain amount of light on vertebrate development; and still more as showing how a certain line of development may seem, and at first really be, advantageous, and still lead to degeneration, or at best to but partial
When we compare the forms which represent fairly well the direction of development of these three lines,