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.