Ribbon worms

Code IH999999

Key terminates here

Nemertea (ribbon worms) are similar to Platyhelminthes (flatworms) but are distinguished by a long, eversible proboscis held in a hollow proboscis sheath (rhynchocoel) above the digestive tract. They differ also from flatworms in having a distinct circulatory system and a tubular gut with anus.

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

Tetrastemmatidae

Only one species, the cosmopolitan Prostome graecense , is reliably identified from Australia. It inhabits small, permanent reedy ponds and backwaters and may or may not be introduced (Gibson and Moore, 1976; Williams, 1980).

Other species may occur. Prostoma spp. generally are less 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, 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 so lies coiled in the rhynchocoel. The proboscis is armed with a stylet (terminal when the proboscis is extended, thus about half-way along when the proboscis is retracted) comprising a single central thorn on a conical base. Two or more lateral pockets contain reserve thorns in various stages of development. The mouth opens within a combined rhynchodaeum/buccal cavity from underneath the rhynchocoel.

Prosorhochmidae occurs in New Zealand. This 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 known Australasian tetrastemmatid by the following characters (Gibson and Moore, 1976): (i) eyes absent, (ii) large cephalic gland present, (iii) rhynchocoel extends full body length in adults as well as in juveniles, (iv) rhynchocoel musculature has the circular and longitudinal layers not separated, and (v) anterior or all blood vessels large and thin-walled, not narrow and muscular.

References:

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

Williams, W.D. (1980) Australian Freshwater Life: The Invertebrates of Australian Inland Waters. The Macmillan Company of Australia, Melbourne.