Introduction to the Hemichordata

Of sawblades and acorns. . .

Not familiar creatures to most people, hemichordates form a small phylum (only a few hundred species). Their importance for the study of vertebrate evolution, however, cannot be underestimated. The fossil record of one group of hemichordates, the graptolites, is very well known and is often used to correlate rocks.

Hemichordates are distinguished by a tripartite (threefold) division of the body. At the forward end of the body is a preoral lobe, behind this is a collar, and last comes a trunk. The name "hemichordate" means "half chordate," and hemichordates share some (but not all) of the typical chordate characteristics. There are branchial openings, or "gill slits," that open into the pharynx; there is a rudimentary structure in the collar region, the stomochord, that is similar to a notochord; and there is a dorsal nerve cord, in addition to a smaller ventral nerve cord. However, hemichordates are not classified as true chordates, although they are quite closely related. Some DNA-based studies of evolution suggest that hemichordates are actually closer to echinoderms than to true chordates. This is supported by the fact that the larvae of at least some hemichordates look very much like those of some echinoderms.

Of the three classes of hemichordates, the most familiar living ones are the Enteropneusta, the acorn worms. One is pictured at the top right of the page. The triple division of the body is obvious. Acorn worms also have multiple branchial openings, as many as 200 in some species. They are slow burrowers, using the proboscis to burrow through sediment, and may either deposit feed (consume sediment and digest the organic matter, rather like earthworms in soil) or suspension feed (collect suspended particles from the water). Some of these worms may be very large; one species may reach a length of 2.5 meters (almost eight feet), although most are much smaller.

The second living class is the Pterobranchia, an obscure group with only about 20 living species. Pterobranchs as very different from acorn worms; they form colonies in which the individuals are interconnected by stems, or stolons. Individuals, or zooids, are often less than 1 millimeter long. The proboscis is not elongated, as it is in acorn worms, but shield-shaped. The second division of the body bears a pair of branched tentacles that collect small food particles from the water. There is only one branchial opening. Most strikingly, almost all pterobranch species create and live within a network of tubes, the coenecium. These tubes are made up of the protein collagen, secreted by special glands in the proboscis. Yet similar larvae and a similar tripartite body plan unite the enteropneusts and pterobranchs. This diagram shows a colony of pterobranchs in the genus Rhabdopleura.

What is now considered a third class of hemichordates, the Graptolithina or graptolites, has a checkered history. Graptolites are common fossils in Ordovician and Silurian rocks, but for a long time no one was sure what kind of animals they were. Most fossil graptolites look like nothing so much as tiny sawblades. However, well-preserved graptolites can be seen to be tubular in cross-section, with the "teeth of the saw" formed by short open branches from the main tube. Careful study of the microscopic structure of the tubes of graptolites showed that they are very similar to the tubes of pterobranchs.

Unlike their pterobranch kin, which typically live encrusting rocks and shells, most graptolites are thought to have been planktonic, floating or slowly sinking through the water. The spiral shape of some was probably an adaptation to slow sinking. Other graptolites may have been connected to gas-filled sacs, keeping them buoyant.

Source: Ruppert, E.E. and Barnes, R.D. 1994. Invertebrate Zoology. Sixth edition. Saunders College Publishing, Fort Worth.