Annelida (segmented worms, including earthworms, leeches and polychaetes)

Life > Eukaryotes > Opisthokonta > Metazoa (animals) > Bilateria > Lophotrochozoa 

Earthworm (Oligochaeta). [photo H. Robertson, Iziko ©]	Polychaete worm (Pseudonereis variegata) collected by fisherman for bait, from mussel bed. [photo H. Robertson, Iziko ©]

Introduction

Annelids are segmented worms. The well developed coelom (fluid filled body cavity) acts as a hydrostatic skeleton with muscles of the body walls acting on it. The body segments of an annelid are partitioned by internal divisions called septa, giving independence of movement to each of them. Segmentation and a tube within a tube body plan have resulted in relative specialisation of the digestive tract into a pharynx, a stomach, and accessory glands. Cephalisation or head formation is apparent in some annelids, most notably the polychaetes, and in these tentacles, palps and eyespots are common.   

There are over 15,000 species of annelids known globally and they are generally divided into three classes. The Polychaeta have separate sexes and posses soft projections called parapodia with many bristle-like chetae per segment, no clitellum (fused mid-body segment that secretes mucous), a distinct head (with eyes and tentacles), approximately 200 segments, with the majority being marine. The Oligochaeta or eathworms are hermaphroditic and posses no or few chaetae, no parapodia, a clitellum, no distinct head and 100-120 segments with very few marine forms.   The Hirudinea or leeches are blood eating parasites, which are hermaphroditic and posses no chaetae or parapodia, a temporary clitellum, no head, a fixed 33-34 segments and a single sucker at the posterior and anterior end, with very few marine forms.  At present ~766 marine Annelid species are known from South Africa.  

Ecology

Polychaetes represent the largest class of annelids and are predominantly marine. This class is divided into two groups; the sedentary polychaetes and the errant or free living polychaetes. Sedentary polychaetes spend most of their time in tubes or permanent burrows. Errant polychaetes include free-moving pelagic forms, active burrowers, crawlers and the tube worms that leave their tubes for breeding and feeding. Some polychaetes are ferocious predators, using their strong chitinous jaws that extend with a part of the pharynx when the animal is feeding. Others are sessile, housed in a burrow or tube, with only their tentacles that form a funnel-shaped fan extended into the surrounding water to filter out prey items. Members of the Oligochaeta and Hirudinea are primarily found in terrestrial or freshwater habitats respectively.

The Cape reef-worm Gunnarea capensis, a common sedentary polychaete found all round the southern African coast,extracted from its burrow. [photo H. Robertson, Iziko ©]	Gunnarea capensis contracted within burrow. [photo H. Robertson, Iziko ©]

Annelids and humans

The presence of earthworms in soil increases crop production. The burrows they produce improve aeration of the soil, facilitating water movement, while their castings readily combine with organic debris to form humus and improve fertility of the soil. Generally people cringe when even the slightest mention of a leech is made. Their parasitic blood sucking mode of feeding stirs emotions of disgust, but medicinally harnessed, this ability has been used for centuries in blood-letting and a number of other procedures, including reconstructive surgery of severed digits and plastic surgery. Polychaetes play a role in biomonitoring of the marine environment (e.g. pollution and mining) and are commonly used as bait organisms, but may be destructive as demonstrated by their boring and fouling activity.

Classification

Polychaeta (polychaetes)
Oligochaeta (earthworms)
Hirudinea (leaches)