Station 5: Phylum Annelida

 The Nereidids, also known as the bristle worms, are a family of annelid worms, generally marine.  Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. As such, Nereidids are sometimes referred to as bristle worms. More than 10,000 species are described in this family. Common representatives include the lugworm (Arenicola marina) and the sandworm or clam worm, Nereis.

Nereidids as a family are robust and widespread, with species that live in the coldest ocean temperatures of the abyssal plain, to forms which tolerate the extremely high temperatures near hydrothermal vents.  Nereidids occur throughout the Earth's oceans at all depths, from forms that live as plankton near the surface, to a 2- to 3-cm specimen (still unclassified) observed by the robot ocean probe Nereus at the bottom of the Challenger Deep, the deepest known spot in the Earth's oceans.  Only 168 species (less than 2% of all Nereidids) are known from fresh waters.

 Lumbricidae is a family of animals in the phylum Annelida, which is made up of many types of aquatic and terrestrial worms, and including all of the various earthworms.  Specifically, the lumbricids contain the terrestrial megadrile earthworms (some of which are semiaquatic or fully aquatic), and freshwater or semiterrestrial microdrile forms, including the tubificids, pot worms and ice worms (Enchytraeidae), blackworms and several interstitial marine worms.

With around 10,000 known species, the Lumbricids make up about half of the phylum Annelida. These worms usually have few setae (chaetae) or "bristles" on their outer body surfaces, and lack parapodia, unlike polychaet worms (Class Errantia; Family Nereididae).

Lumbricids are well-segmented worms and most have a spacious body cavity (coelom) used as a hydroskeleton.  They range in length from less than 0.5 mm (0.02 in) up to 2 to 3 metres (6.6 to 9.8 ft) in the 'giant' species such as the giant Gippsland earthworm (Megascolides australis).

Terrestrial oligochaetes are commonly known as earthworms and burrow into the soil. Earthworms are found in all parts of the world except for deserts. They have a requirement for moist surroundings and the larger species create burrows that may go down several meters (yards) while young individuals and smaller species are restricted to the top few centimeters of soil. The largest numbers are found in humus-rich soils and acid soils.  A few species are found in trees, among damp moss and in the debris that accumulates in leaf axils and crevices; some others make their homes in the rosettes of bromeliads.

The majority of aquatic oligochaetes are small, slender worms, whose organs can be seen through the transparent body wall.  They burrow into the sediment or live among the vegetation mostly in shallow, freshwater environments.

Leeches are segmented worms that belong to the phylum Annelida and comprise the family Hirudinidae.  Like the lumbricids, such as earthworms, leeches share a clitellum and are hermaphrodites. Nevertheless, they differ from the lumbricids in significant ways.  For example, leeches do not have bristles and the external segmentation of their bodies does not correspond with the internal segmentation of their organs.  Their bodies are much more solid as the spaces in their coelom are dense with connective tissues. They also have two suckers, one at each end.

The majority of leeches live in freshwater environments, while some species can be found in terrestrial and marine environments. The best-known leeches, such as the medicinal leech, Hirudo medicinalis, are hematophagous, feeding on vertebrate blood and invertebrate hemolymph.  Most leech species, however, are predatory, feeding primarily by swallowing other invertebrates. 

Almost 700 species of leeches are currently recognized, of which some 100 are marine, 90 terrestrial and the remainder freshwater taxa.

Leeches, such as the Hirudo medicinalis, have been historically used in medicine to remove blood from patients.  The practice of leeching can be traced to ancient India and Greece, and continued well into the 18th and 19th centuries in both Europe and North America.  In modern times, leeches are used medically in procedures such as the reattachment of body parts and reconstructive and plastic surgeries and, in Germany, treating osteoarthritis.