Seaweeds and rocks at and below the limit of the ebbing tide are often covered with small bushy growths, or with lacelike incrustations that are alive. These are moss animals, representing the class Polyzoa of the phylum Molluscoida. They are minute, soft creatures that live in colonies formed by the repeated budding of the members, all connected by a fleshy base so that each contributes to the nourishment of all. "Each little animal occupies a separate stony or horny capsule, into which it may withdraw and even close the opening with a lid.... The mouth is surrounded by tentacles that in many species arise from a horseshoe-shaped or disklike base. These tentacles are always beset with hairlike bristles which by their movements serve to set up currents, and thus to drive minute organisms into the mouth."

A typical example of these polyzoans (or bryozoans) is Bugula turrita, so abundant wherever our northeastern coast is rocky that the rocks below tide level appear covered with its mossy tufts, which are often ten inches long and profusely branched. The main stems are orange-yellow, while the terminal branches are yellowish white. The delicate tracery so frequently seen on the fronds of kelp, and on shells and stones along both shores of the Atlantic indicate colonies, or their remains, of the lace coralline (Membranipora); and the dull red or pinkish crust so common on shells and stones in shaded tidepools represents successive colonies of the "red-crust" polyzoan (Escharella variabilis), layer crusting over layer. A similar history accounts for the curious nodules called "false coral" so common in moderately deep water in Long Island Sound. Similar polyzoans, which exist in great variety, both modern and fossil, contributed extensively to the formation of the older strata of sedimentary limestones.

ANCIENT LAMP SHELLS

Associated in structure with these minute colonists is the ancient race of brachiopods (Brachiopoda, "arm-footed") or lamp shells, although they much more nearly resemble bivalved mollusks, whence, by the way, comes the name of the phylum to which both belong—Molluscoida, which means "mollusklike."

The race of the brachiopods goes back to the beginning of the geologic record. A few living examples are still found in the ocean, some of which, as lingula, have changed so little that they can hardly be told from the most ancient fossils of their family. Certain species are dredged abundantly on both coasts of the Atlantic from water a few fathoms deep where the bottom is rocky. They look like small mussels at first sight, but on examination show a vast difference in structure. The bivalve shells, instead of growing on the right and left sides of the animal, as in bivalve mollusks, cover its back and front, and the head parts are at the gape of the valves. At the hinge end of the shell the lower valve overlaps (it is the shape of this lower shell, like that of an old Roman lamp, which suggests their common name, "lamp shells") and the hinder end of the body projects as a stalk, by which the animal fastens itself to the rock. "The mouth in the brachiopods is flanked by two curiously coiled and feathered arms which lie within the cavity between the shells, and are supported by skeletal rods attached to the upper shell. These serve as gills, and also to capture the minute creatures upon which the brachiopod feeds."

Owing to their great abundance, world-wide distribution, and remote antiquity, as well as their excellent state of preservation, brachiopods occupy a very conspicuous rank among extinct invertebrates, and furnish us besides with a large number of important index fossils. They are to be found in immense variety from the Cambrian to the present, most numerously in formations from Silurian to Permian times.

STARFISHES, SEA URCHINS, AND TREPANGS

We have now arrived at the point (phylum Echinodermata, "spiny-skinned") where a distinctly new type of interior structure appears in the possession by animals of a hollow space (coelom) between the outer skin and the wall of the digestive tube which now becomes occupied by definite organs instead of by an almost uniform mesenchyme, as in the sponges and coelenterates. These organs arise from an interior lining membrane called "mesoderm."

Henceforth, therefore, we shall deal with coelomate animals, among which the echinoderms are lowest in rank. The simplest of them is the "sea lily" which lives rooted on the bottom in deep water, and sways about on a slender, jointed stalk, looking much like the flower after which it is named. It is of interest chiefly as a survivor of the tribe of crinoids that were so varied and numerous in early Paleozic times that massive Devonian limestones are composed largely of their remains; and the type has changed little through the ages. It consists typically of a cup, mounted on its stem like the calyx of a flower, and composed of circles of calcareous plates, definite in form and in relative position, that contain and protect a well-organized body. Surrounding the open mouth of the cup is a circle of long, jointed, much-branched tentacles that sweep the water, capture passing prey, and bring it into the mouth of the crinoid within the circling base of the arms.

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.

Worms of this class dwell in great numbers and variety in the sea and in salt-water meadows and beaches, and are often beautiful as well as interesting objects of study for the visitor at the shore. The sea mouse (Aphrodite), for instance, which is about three inches long and of oval shape, is covered with hairlike bristles that glisten with brilliant green, red, and yellow iridescence; it is to be looked for on the mud just below the low-tide line, and inhabits both coasts of the North Atlantic. The body of the common "clay worm," dug for bait at low tide, which is olive in general tone, gleams with pearly iridescence, while its innumerable feet bear gills that are green and salmon-red. Another (Lumbriconereis) is known as "opal worm" for good reason; and our sands abound in slender scarlet worms of the same genus named "red thread." All these worms bury themselves in the sand, or wander through it in search of prey, for they are carnivorous, and do not hesitate to kill and eat each other. Some are fairly sedentary, and protect themselves against fishes, crabs, mollusks, and bigger annelids that seek them, by forming tubes by means in some cases of a shelly secretion, but more usually by cementing bits of shell, stones, and grains of sand into an irregular tube lining the burrow; the slender, limy serpentine tubes often seen on stones or dead shells in tide pools, are, or were, the homes of such protected worms, most commonly of the "shell worm" (Serpula). "Often a number of these calcareous worm tubes are seen clustered together. When undisturbed the worm protrudes its beautiful feathered gills, which resemble a little passion flower projecting from the mouth of the tube. These gills are variously colored in different individuals, some being purplish brown, banded with white and yellow, while others are yellowish green, orange, or lemon-yellow. At the least disturbance, such as a shock or a shadow, the gills are instantly withdrawn into the stony tube, and the opening stopped by a horny disk." In the Gulf of Mexico extensive colonies of these worms often form, and as the early generations die others erect their tubes above them; as this goes on sand and shell fragments fill around and between the tubes, and after a long time the whole mass becomes a solid reddish, loose-lying rock, composed chiefly of serpula tubes, which in Florida is dragged up from the beach and used as building stone.

The third class (HirudinidÆ) of Annulata is that of the leeches, those ugly, but useful, worms of land and sea. In spite of their sluglike appearance the leeches are segmented worms, although the wrinkles on their gray, mottled skins do not indicate the position of the segments beneath. The mouth on the under side of the head is armed with jaws and sharp teeth that make three or more cuts through the skin, whence the blood is sucked; there is also a holding sucker near the tail. Their attacks cause little pain, and that fact has led physicians to put them into use when bleeding is required. The eggs of leeches are laid in moist earth in little packets, and hatch in five or six weeks. The growth to maturity is slow, and continues during a long life. Many species abound in ponds and stagnant waters. Asia has terrestrial leeches, swarming in moist vegetation; and in Ceylon the minute leeches are a terrible plague in certain regions. Many also are wholly marine. Some of the larger forms attack fishes directly, and quickly kill them by sucking their blood away; others are true parasites. On the other hand the leeches of our lakes are fed on by the whitefish and similar fishes. They are a great pest to our fresh-water turtles.