Code IF999999

Nemertea (Nemertina: Rhynchocoela), or ribbon worms, are somewhat similar to flatworms (Phylum: Platyhelminthes) but are distinguished by a long, eversible proboscis held in a hollow proboscis sheath (rhynchocoel) above the digestive tract. They also differ from flatworms in having a distinct circulatory system, and an anus.

Almost all nemerteans are marine, but a few species live in freshwater or are terrestrial. Two families are recorded from fresh water in New Zealand and one in Australia.

The one Australian freshwater genus is Prostoma . The one species reliably identified is the cosmopolitan P. graecense , which may or may not be introduced (Gibson and Moore, 1976; Williams, 1980). Other species may occur. P. graecense inhabits small, permanent, reedy ponds and backwaters where it glides by cilia, flatworm-like, on submerged vegetation.

Prostoma spp. generally are small, rarely more than 25mm long. The anterior is blunt with a rounded frontal lobe lying wholly before the brain, the posterior slightly tapered. The rhynchocoel (proboscis sheath) extends about 2/3 to 3/4 the length of the adult body, almost the full length in juveniles. Colour varies with age, being typically white or cream in juveniles and orange or red or green in mature specimens. Eye number is typically 2 in juveniles, 4-6 in adults. The muscular proboscis generally is longer than the body and lies coiled in the rhynchocoel. It is armed with a stylet (terminal when the proboscis is extended, thus about half-way along when the proboscis is retracted). This stylet comprises a single central thorn on a conical base, plus two or more lateral pockets containing reserve thorns in various stages of development. The mouth opens within a combined rhynchodaeum/ buccal cavity, from underneath the rhynchocoel. The gut is straight. The anus is at the posterior tip of the animal.

A second family, Prosorhochmidae, is represented in New Zealand but apparently not in Australian inland waters. The family contains two freshwater genera, both monotypic: Potamonemertes from the Selwyn River, Canterbury, and Cambellonemertes from Campbell Island. These two prosorhochmids can be distinguished from any known Australasian tetrastemmatid by: (i) the absence of eyes, (ii) the presence of a comparatively large cephalic gland, (iii) a rhynchocoel which extends the full body length in adults as well as in juveniles, (iv) rhyncocoel musculature in which the circular and longitudinal layers not separated, and (v) the anterior or all blood vessels large and thin-walled, not narrow and muscular (Gibson and Moore, 1976).

References:

Gibson, R. and Moore, J. (1976) Freshwater nemerteans. Zoological Journal of the Linnean Society 58(3) : 177-218.

Parker, S.P. (1982) Synopsis and Classification of Living Organisms. Volume 2. McGraw-Hill Book Company, New York.