tive systems. The higher groups of this kingdom had developed all, or nearly all, the tissues used in building the bodies of higher animals-muscular, reproductive, connectile, glandular, nervous, etc. But these are mostly very diffuse. The muscular fibrils of a jellyfish are mostly isolated or parallel in bands, rarely in compact well-defined bundles. The tissues have generally not yet been moulded into compact masses of definite form. There are as yet very few structures to which we can give the name of organs. To form organs and group them in a body of compact definite form was the work pre-eminently of worms. The material

for the building was ready, but the architecture of the bilateral animal was not even sketched. And different worms were their own architects, untrammelled by convention or heredity, hence they built very different, sometimes almost fantastic, structures.

We must remember, too, the great age of this group. They are present in highly modified forms in the very oldest paleozoic strata, and probably therefore came into existence as the first traces of continental areas were beginning to rise above the primeval ocean. They are literally "older than the hills." They were exposed to a host of rapidly changing conditions, very different in different areas. This prepares us for the fact that the worms represent a stage in animal life corresponding fairly well to the Tower of Babel in biblical history. The animal kingdom seems almost to explode into a host of fragments. Our genealogical tree fairly bristles with branches, but the branches do not seem to form any regular whorls or spirals. Few of them have developed into more than feeble growths. They now contain generally but few species. Many of

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

success.

When we compare the forms which represent fairly well the direction of development of these three lines,

a snail or a clam with an insect and a fish, we find clearly, I think, that the fundamental anatomical difference lies in the skeleton; and that this resulted from, and almost irrevocably fixed, certain habits of life.

We may picture to ourselves the primitive ancestor of mollusks as a worm having the short and broad form of the turbellaria, but much thicker or deeper vertically. A fuller description can be found in the "Encyclopædia Britannica," Art., Mollusca. It was hemiovoid in form. It had apparently the perivisceral cavity and nephridia of the schematic worm, and a circulatory system. In this latter respect it stood higher than any form which we have yet studied. Its nervous system also was rather more advanced. It had apparently already taken to a creeping mode of life and the muscles of its ventral surface were strongly developed, while its exposed and far less muscular dorsal surface was protected by a cap-like shell covering the most important internal organs. But the integument of the whole dorsal surface was, as is not uncommon in invertebrates, hardening by the deposition of carbonate of lime in the integument. And this in time increased to such an extent as to replace the primitive, probably horny, shell.

Into the anatomy of this animal or of its descendants we have no time to enter, for here we must be very brief. We have already noticed that the most important viscera were lodged safely under the shell. And as these increased in size or were crowded upward by the muscles of the creeping disk, their portion of the body grew upward in the form of a "visceral hump." Apparently the animal could not increase

much in length and retain the advantage of the protection of the shell; and the shell was the dominating structure. It had entered upon a defensive campaign. Motion, slow at the outset, became more difficult, and the protection of the shell therefore all the more necessary. The shell increased in size and weight and motion became almost impossible. The snail represents the average result of the experiment. It can crawl, but that is about all; it is neither swift nor energetic. Even the earthworm can outcrawl it. It has feelers and eyes, and is thus better provided with senseorgans than almost any worm. It has a supra-œsophageal ganglion of fair size.

The clams and oysters show even more clearly what we might call the logical results of molluscan structure. They increased the shell until it formed two heavy "valves" hanging down on each side of the body and completely enclosing it. They became almost sessile, living generally buried in the mud and gaining their food, consisting mostly of minute particles of organic matter, by means of currents created by cilia covering the large curtain-like gills. Their muscular system disappeared except in the ploughshare-shaped "foot" used mostly for burrowing, and in the muscles for closing the shell. That portion of the body which corresponds to the head of the snail practically aborted with nearly all the sense-organs. The nervous system degenerated and became reduced to a rudiment. They had given up locomotion, had withdrawn, so to speak, from the world; all the sense they needed was just enough to distinguish the particles of food as they swept past the mouth in the current of water. They have an abundance of food, and "wax fat."

The clam is so completely protected by his shell and the mud that he has little to fear from enemies. They have increased and multiplied and filled the mud. "Requiescat in pace."

But zoology has its tragedies as well as human history. Let us turn to the development of a third molluscan line terminating in the cuttle-fishes. The ancestors of these cephalopods, although still possessed of a shell and a high visceral hump, regained the swimming life. First, apparently, by means of fins, and then by a simple but very effective use of a current of water, they acquired an often rapid locomotion. The highest forms gave up the purely defensive campaign, developed a powerful beak, led a life like that of the old Norse pirates, and were for a time the rulers and terrors of the sea. With their more rapid locomotion the supra-oesophageal ganglion reached a higher degree of development, and it was served by senseorgans of great efficiency. They reduced the external shell, and succeeded, in the highest forms, of almost ridding themselves of this burden and encumbrance. Traces of it remain in the squids, but transformed into an internal quill-like, supporting, not defensive, skeleton. They have retraced the downward steps of their ancestors as far as they could. And the high development of their supra-oesophageal ganglion and senseorgans, and their powerful jaws and arms, or tentacles, show to what good purpose they have struggled. But the struggle was in vain, as far as the supremacy of the animal kingdom was concerned. Their ancestors had taken a course which rendered it impossible for their descendants to reach the goal. Their progress became ever slower. They were entirely and hope