Phylum ECHIURA

Boniella sp.

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Phylum ECHIURA

Compiler and date details

26 March 2005 - Rick Johnson

Introduction

The phylum Echiura comprises a group of soft-bodied, non-segmented, coelomate, bilaterally symmetrical, worm-like marine invertebrates. Echiurans have a sausage-shaped muscular trunk and an anteriorly placed extensible proboscis. They are commonly known as spoonworms, a name derived from the function of the proboscis which, in most species, is used to collect sediment from around the burrow. Urechis is known as the "innkeeper worm" because a number of marine organisms live as commensals inside the burrow (Urechis is not recorded from Australian waters).

The saccular trunk is usually light to dark green in colour, or sometimes reddish brown, and usually bears numerous flat or swollen glandular and sensory papillae. A pair of golden-brown chaetae is usually present on the ventral surface of the trunk, just posterior to the mouth. In a few species, a number of chaetae may form a complex; in others, chaetae are absent. The proboscis is usually flattened and ribbon-like, but may be fleshy and spatulate. It is highly extensible and contractile, but cannot be withdrawn into the body cavity like the introvert of sipunculans. The distal end of the proboscis is usually truncate or bifid. In some deep-sea species the proboscis is modified considerably and assists in the collection of food.

A mouth is situated antero-ventrally at the base of the proboscis and the anus is at the posterior extremity of the trunk. Echiurans are detritus-feeders, except for Urechis which trap very fine particles by secreting a mucous net. A long, convoluted alimentary system lies in a spacious coelom. The very long intestine is attached to the coelomic wall by threads and mesenteries. A special feature of the alimentary system is the collateral intestine or siphon, a narrow tube, that runs parallel to all or part of the intestine.

The sexes are separate. Mature eggs and sperm pass out through the nephridia and fertilisation is external, except in the Bonelliidae. Cleavage is spiral and the larva is a trochophore. Males and females are indistinguishable externally except in the Bonelliidae. Sexual dimorphism is very marked in bonelliids; the male is minute and usually lives in the female's nephridia. The eggs of bonelliids are fertilised in the nephridia.

Echiurans are marine animals that occasionally occur in brackish waters. They live predominantly in the benthos, making burrows in mud and sand, living under rocks and in the shells of molluscs and tests of sand dollars, and in galleries made by other animals in rock and coral. Due to their cryptic lifestyle, echiurans are not often seen and are difficult to collect. The body remains concealed in sediment or amongst rocks, whilst the long protrusible proboscis extends across the substrate and is the only part of the worm seen by an underwater observer. When disturbed the proboscis retracts quickly, but cannot be fully withdrawn into the body cavity as in sipunculans. If damaged or lost, the proboscis can be regenerated. Echiurans burrow head first via peristalsis-like movements of the body wall (the proboscis lies dorsally along the body pointing backwards and seems to play no part in digging). There is no evidence that they are able to bore into rocks as do sipunculans. They are found in tropical, temperate and polar seas and their bathymetric range is wide. Bonelliids have also been collected from the abyssal and ultra-abyssal depths of the oceans (Zenkevitch 1958, 1966).

There are over 150 species of echiurans known worldwide, however, only 12 described species and 1 undescribed species from 3 families (Bonelliidae, Ikedidae and Thalassematidae) have been recorded from Australian waters (Edmonds, 2000). Seven species are endemic to Australia.

The classification used in this database follows Edmonds (2000). Two Echiura classes are recognised — Echiuridea Fisher, 1946 and Sactosomatidea Fisher, 1946. All echiurans, except the little known species Sactosoma vitreum, belong to Class Echiuridea. Edmonds (2000) gives a key to families.