Echiura

Echiurans are non-segmented, coelomate, bilaterally symmetrical worm-like marine animals (Fig. 22.41). They have an elongate muscular trunk and an anterior extensible proboscis. They are commonly referred to as 'spoon worms', as their proboscis when extended is used to collect sediment from around its burrow.

The muscular trunk is usually light to dark green in colour, but sometimes reddish brown, and typically 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 behind the mouth. The proboscis is usually flattened and ribbon-like but may be fleshy and spatulate. It is highly extensible and contractile but cannot be withdrawn completely into the body cavity like that of sipuncu-lans. The distal end of the proboscis is usually truncate or bifid. It is this proboscis that is typically seen on the reef, especially on night dives, and when touched it retracts and is smooth and slippery (Fig. 22.4/). The mouth of an echuiran is present antero-ventrally at the base of the proboscis and the anus is at the posterior extremity of the trunk.

Echuirans are detritus feeders, except for species of Urechis that trap very fine particles by secreting a mucous net.

The sexes are separate, mature eggs and sperm pass out through the paired nephridia that range from 1-10 in number. Fertilisation is external, and a pelagic tro-chophore is produced. The only exceptions are species of Bonellia (Fig. 22.4!) that are common on the reef, where the male is minute and lives permanently within the female body.

Echiurans make burrows in sand and mud, live in the shells of molluscs and tests of sand dollars and in galleries made by other animals in the coral. There is no evidence that they themselves are capable of burrowing into the coral. They also occur on tropical mud flats intertidally.

To date only 13 species or subspecies of echiurans have been described from Australia and seven of these are endemic. While they are present on the GBR, it is not known how many species are present and unde-scribed species may occur.

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