Segmented worms: Earthworms

Code LO999999

Key terminates here

Class: Oligochaeta

Annelids are segmented worms with a subterminal mouth, straight gut with or without diverticulae, posterior anus, dorsal pre-oral brain, paired ganglionated ventral nerve cords, a closed blood circulatory system (occasionally reduced or absent), a well-developed coelom divided by septa (corresponding to the external segmentation, except in leeches) and typically with moveable setae in bundles on most body segments.

Oligochaeta (earthworms, etc.) are predominantly an aquatic and terrestrial class, though a few species are marine. The aquatic oligochaetes are placed in two superorders, Megadrili and Microdrili. Megadrili are relatively robust worms related to earthworms. An aberrant family, Aeolosomatidae, may be treated as a separate class, Aphanononeura (Timm, 1981; Parker, 1982). Microdrili are mostly smaller, thin-bodied worms without any close terrestrial relatives.

Oligochaetes are distinguished from polychaetes by the lack of fleshy parapodia, lack of sensory head appendages, and relative paucity of setae. The aberrant polychaete family Capitellidae, known in Australian inland waters from one saline lake in South Australia, superficially resembles Oligochaeta but does not develop a clitellum.

Oligochaetes are readily distinguished from Hirudinea (leeches) by the presence of setae in bundles on most body segments, lack of suckers, lack of secondary annulation of the body segments, variability in segment number amongst species and earthworm-like locomotion.

The characters on which oligochaete families are defined involve the position and arrangement of the reproductive organs. Specimens in reproductive phase show a ring of thickened, glandular epithelium (a clitellum) around the genital segments. Identification of non-reproductive specimens to family level can be difficult or impossible although certain taxa can be recognised by overall body shape and size or by distinctive gills or setae.

Two megadrile, seven microdrile and the one aphanononeuran family have been recorded from Australian waters.

MEGADRILI:

Large and robust worms which look much like earthworms. The aquatic families are:

1 Lumbricidae (Code LO099999)

Dorsal setal bundles located dorsolaterally, laterally, or ventrally. Body section cylindrical, quadrangular, octagonal or trapezoid. Ovaries small and discoidal. Oocytes forming a single egg string. Clitellum often long, covering 4-32 segments, starting from segment XVII-LII.

2 Megascolecidae (Code LO109999)

Either with dorsal setal bundles located dorsolaterally or ventrally, or else with numerous setae arranged around the equator of each segment. Ovaries large and fan- or rosette-shaped. Oocytes forming several egg strings.

MICRODRILI:

Smaller and more lightly built worms. Some species are short with few segments and with the body clearly divided into specialised regions, others are unregionalised with a few to several hundred segments. A few genera or species carry conspicuous external gills and/or construct characteristic tubes. The aquatic families are:

3 Lumbriculidae (Code LO069999)

Somewhat robust worms having the testes and male ducts in the same segment. Ventral and dorsal setae begin at segment II and each setal bundle comprises two setae. All setae at least rudimentarily bifid. Hair setae absent. No eyespots or proboscis. There are two species, both introduced: Lumbriculus variegatus up to 100mm long, green anteriorly with the remainder red to black; Stylodrilus heringianus shorter, to 40mm, and pale to white in colour.

4 Enchytraeidae (Pot worms) (Code LO089999)

Ventral and dorsal setae beginning from segment II. Setae as Lumbriculidae except usually four per bundle. All setae simple pointed and either straight or sigmoid. No eyespots or proboscis. Testes in segment XI, ovaries and male pore in XII, paired spermathecae in V.

5 Naididae (Code LO059999)

Length 1-10 mm, but usually reproduces by fragmentation and chains of zooids may be longer. Ventral setae begin at segment II and are bifid, those of segments II to V often longer and thinner than the rest. Dorsal setae begin usually at V and are thinner than the ventral setae. Eye spots sometimes present on the peristomium. The prostomium may bear a proboscis. Gills often present as dorso-lateral filaments on most segments or else located within a branchial fossa on the terminal segment.

6 Tubificidae (Sludge worms: some species tolerate grossly polluted water)

(Code LO049999)

Some species cosmopolitan, others narrowly restricted in distribution. Body usually about 20mm by 1mm. Ventral setae usually bifid, rarely simple-pointed or pectinate or hair-like. Dorsal setae usually begin at segment II, of indefinite number, generally resembling ventral setae in size and shape. No eye spots. No proboscis. Testes and spermathecae in segment X, ovaries and male pore in XI. Mid-dorsal and mid-ventral gill filaments present on posterior segments in one species.

7 Phreodrilidae (one genus, Phreodrilus ) (Code LO039999)

Generally slender worms, length less than 30mm. Ventral setae in bundles of two and either bifid, simple pointed, or one of each. Dorsal setae from III, in bundles of one to many hairs with fine support hairs not emergent from the body wall. Testes in segment XI, ovaries, male pores and spermathecae in XII, female pores in XIII. Some species have posterior lateral gills or a proboscis on the prostomium, others have neither. Some species are commensal on crayfish. This family is of gondwanan distribution.

8 Haplotaxidae (one genus, Haplotaxis ) (Code LO019999)

Moderate to very long (2-30 mm), uniformly thin worms with many equal segments. All setae simple, pointed, 1-2 per bundle. Hair setae absent. Prostomium elongate, forming an extended preoral region.

9 Capilloventridae (Code LO029999)

Small (under 5mm) with hair setae in both dorsal and ventral bundles. Ventral setae of male segment (XII) modified as a bundle of stout hairs in mature specimens. Prostomium glandular. Pharynx with a ventral pouch.

Aphanononeura:

10 Aeolosomatidae (Code LO079999)

Small worms (under 1mm) with reduced segment number, a unique configuration of the reproductive organs, and paired ciliated organs on the head region, used in locomotion. Most are free-living but a few are commensal on crayfish. At first sight some might be mistaken for a flatworm (Platyhelminthes) or else for a histriobdellid polychaete. The internal structures are, however, more or less of oligochaete type. All individuals are hermaphrodite with paired sexual organs in several segments. Most species also can reproduce asexually by transverse fission. Chains of zooids may range in length from 1 to about 1mm.

References:

Timm, T. (1981) On the origin and evolution of aquatic Oligochaeta . Easti NSV Teaduste Akadeemia Toimetised Biologia 30 :174-181.

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