| Photo, American Museum of Natural History |
| CORAL FORMATIONS OF THE GREAT BARRIER REEF, AUSTRALIA |
If now you were to cut off its stalk, lay the crinoid on the sand, mouth down and arms outspread, beside a brittle star or a basket fish, which also have many-branched arms, it might be difficult to tell them apart, yet they represent different orders; and from this, by way of the naked serpent star, it is but a short transition to the starfish, where the arms are no longer tentaclelike, but are simply pointed extensions of a central body; this, in fact, is the case, for now they are no longer prehensile organs, but are supports, mainly serviceable in locomotion, and the stomach and ovaries are partly lodged in them. The main point just now, however, is the fact that here, and in the successive changes of form to be shown, the pattern of plates that form a strengthening mosaic in the skin of the central part of the body remains identical.
All starfishes are not as prettily symmetric as our familiar five-finger. Some are shorter in the arms, and much broader and thicker in the body; and if you will examine a collection of preserved specimens of the echinoderms you will see that you can trace gradation of form right around to the bun-shaped cake urchin, on whose top the five-pointed star is printed, and thence to the globular sea egg, which the French called "sea urchin," using one of their names for the hedgehog. Furthermore, the five sections of the shell of the urchin, which represent the five arms of the starfish folded forward and grown together into a spherical case, are to be traced again, outlined by appendages, in the elongated and leathery hide of the trepangs and sea cucumbers of the order Holothuria.
It is as an illustration of homology, that is, the resemblance between parts that have the same relation to the typical plan of structure, and as an example of how almost endless variations of form may arise within a single type, that the echinoderms are of most interest. Otherwise it may be said that they serve as food for fishes and some other creatures, including coastwise savages, and as curiosities in geological museums and in aquaria; and that starfishes are sadly destructive of cultivated oyster beds. We may therefore dismiss them, and devote a page or two to the worms.
EARTHWORMS AND BEACHWORMS
Although various parasitic creatures have been described as flat "worms," round "worms," and so forth, naturalists regard as true worms only those of higher organization classified in the phylum Annulata, or annelids, the distinctive characteristic of which is that its members have elongated bodies divided into ringlike sections. These represent a division of the internal parts into a series of structural segments or "matemeres," each supplied with its own set of organs, yet connected by blood vessels and nerves, and the whole traversed by tubular organs serviceable to the entire animal. The nervous system consists of a "brain" in the head, and a double, ventral nerve-cord with a ganglion in every segment, foreshadowing the nervous system in insects and other arthropods. The phylum embraces three classes: 1. Chætopoda—earthworms and marine annelids; 2. Gephyrea—marine worms, otherwise called sipunculoids; and 3. Hirudinidæ—leeches.
The earthworm or "angleworm" (that is, angler's worm, bait worm) of the "common garden variety," to use the phrase of old-fashioned encyclopedias, is a typical example of the first class, whose Latin name refers to the bristles (setæ) on the flattened lower surface of the body that serve the worm as "feet." A magnifying glass shows them in four double rows allowing eight to each of the rings into which the body is so plainly divided; their extremities are directed rearward, and by their means the worm pushes itself along, and is able to cling to and climb not only the walls of its burrow but vertical surfaces when not too smooth. Thus they are found frequently on roofs and in other elevated and surprising places, to which they have crawled in the night, when, as well as in warm, rainy weather, they are likely to wander a great deal. The long and greatly extensible and elastic body tapers almost equally at each end, but the head end is that which goes forward in crawling, and a lens will show a mouth on its lower surface, beneath a sort of thick lip. A long gullet leads into an expansion called the crop, and that into a large, tough-walled stomach, beyond which an intestine leads to the last segment. The thirty-third to thirty-seventh segments are swollen, forming the "belt" (clitellum), which denotes maturity, but seems to have no special functions. The senses are few and dull. No eyes exist, nor sense of hearing, but the skin is extremely sensitive to vibrations, and to bright light, as might be expected in a nocturnal animal. The sense of taste is discriminating. The eggs are extruded in such a way as to form a glutinous ring about the body, which, when complete, is slipped over the head of the worm, and left to hatch in warm soil under a stone.
Earthworms live underground in burrows that are sunk well below the frost line. In digging they work head downward, gnawing—although they have no hard jaws—and swallowing the earth that is not easily crowded aside and then throwing it out and perhaps heaping it up as "castings." The tunnel must be wide enough to let its occupant turn around in it, and it ends in a deep chamber in which one or more worms may pass the winter without freezing. These worms naturally seek a loose, damp soil, not only for ease of working, but because moisture is a necessity, as they breathe through their skin; hence they abound in meadows and cultivated soil, and are not found on high, dry plains. During the day they lie near the surface, often with the head just protruding. Here they are discovered by sharp-eyed birds and garter snakes, and sacrificed by thousands, notwithstanding the strength with which they hang on to their retreats by the tail. When it retires to the depths of its burrow this worm plugs the mouth of the tunnel with leaves which it draws always by the base, exhibiting considerable intelligence in manipulating the various shapes of leaves to that end.
The world-wide distribution of the earthworm is to some extent owing to man's agency. On our northwestern plains, for example, these worms originally were absent, but are now widely distributed and flourishing there, having been carried from the east, as eggs or small worms, in the soil packed about the roots of trees and shrubs transplanted to western orchards and gardens. This fact may have something to do with the recent westward spread of the robin, which, more than any other of our birds, is a hunter of them. Except where excessively numerous these worms do far more good than harm in a garden.
The naids (Naidæ) are small transparent worms that creep about on vegetation in fresh water, and, besides laying large eggs, they occasionally divide into two at a place in the body that appears arranged for this purpose, for it consists of a zone of very elementary tissue. "Gradually," as Minot records, "the tissue of this interpolated zone transforms itself into muscles, nerves, etc., and, growing meanwhile, it forms in front a new tailpiece to patch out the anterior half of the worm, and behind it forms a new head for the posterior half of the original body. The zone then breaks and there are now two worms." A relative, the lumbriculus, does the trick in a much more prosaic way, breaking in two first, and letting the separate halves acquire head or tail as best they may. This ability to reproduce lost parts is of much service in the life of the species and often of the individual, which may still live after some water tiger has bitten it in two—and these worms are at the base of the food supply of rivers and ponds, and would soon be exterminated were they not capable of rapid and profuse multiplication.