The segmented worms differ from most other invertebrates in that they lack a true exoskeleton. In place of the tough outer covering that protects the other creatures to which they are related, segmented worms have developed fluid-filled bodies for rigidity of form. Although given only scant consideration by most people, including aquarists, this phylum contains creatures of immense importance to hobbyists and nonhobbyists alike. Earthworms, for example, are vital to the functioning of most temperate ecosystems and are important food sources for a variety of wild and captive animals. Other relatives such as marine worms and leeches are also vital links in ecosystems in which they live, and many are fascinating subjects for home aquariums.

Leeches, segmented worms of the class Hirudinea, are familiar to most of us as blood-sucking parasites. The majority are, however, predatory. The next time you purchase black worms as food for your fish, look closely for a small type of leech that usually lives with them — their ability to quickly suck down black worms is quite interesting. One family of leeches lives in the sea and consumes the blood of marine fish.

Among the class of segmented worms known as bristle worms are a number of creatures of interest to marine aquarists, both as exhibit animals and as somewhat noxious pests. Many of the marine species are gorgeously colored, but, unfortunately, we know little of their husbandry. The sea mouse, which actually resembles its namesake far more that it does a worm, is occasionally offered for sale but should only be purchased by serious hobbyists with experience in keeping similar creatures. The bristles, or chaetae, of the sea mouse form a furlike covering over the plump body and lend this odd creature its common name. These bristles are actually quite sharp to the touch and render the sea mouse unpalatable to most predators.

The tubeworms, or fan worms, are the Polychaetes of greatest current interest to marine aquarists. These worms live within winding calcareous tubes that are attached to rocks, docks, or the shells of snails, horseshoe crabs, and other invertebrates. The worms extend a stiff ring of tentacles from the tube's end, appearing, at first glance, to be brightly colored underwater flowers. These tentacles serve both as gills and as food-gathering organs, trapping organic material from the surrounding water.

Tubeworms occur in a great variety of colors, including blue, red, orange, green, and purple. In contrast to the general rule that animals dwelling in temperate regions are more somberly colored than are their tropical relatives, tubeworms from the northern parts of their range are often quite brilliant.

Where it is legal, tubeworms are easily collected from rocks, dock pilings, and even from the shells of other invertebrates. Tubeworms retreat with an amazing rapidity into their shelters at the slightest disturbance, but they are quick to reappear and resume their feeding. Most tubeworms are quite easy to maintain if provided with good water quality and an ample supply of food. Food is most easily provided in the form of commercial pastes formulated for filter-feeding invertebrates.