Well worms

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Well worms
Systematics
Class : Belt worms (Clitellata)
Subclass : Little bristle (Oligochaeta)
Subclass : Diplotesticulata
Superordinate : Metagynophora
Order : Haplotaxida
Family : Well worms
Scientific name
Haplotaxidae
Michaelsen , 1900

The well worms ( Haplotaxidae ) are a family of the little bristle (Oligochaeta) in the annelid worm class of belt worms (Clitellata), which are distributed worldwide in groundwater and spring waters. About 30 species are known.

features

The well worms have a regular segmentation similar to the earthworms, but with a length of 2 cm to 10 cm, sometimes even 40 cm and a width of only about 0.5 mm to 2 mm, they are much longer than wide and thus appear thread-like. Living well worms are often pink or brown in color. The animals have a strong, muscular pharynx , which they use to suck in other small annelids.

Like all girdle worms, well worms are hermaphrodites , which usually have two pairs of testes and ovaries each, i.e. eight gonads, which is why they are also known as octogonadal. The testes are in the 10th and 11th segment, while the ovaries are in the 12th and 13th segment. This characteristic is considered to be a very original characteristic of belt worms. In some species of well worms, the back pair of testicles and ovaries are missing. Haplotaxis forbesi has only one pair of testes in the 10th segment, while the two pairs of ovaries are located in the 15th and 16th segments, in deviation from the basic plan. The receptacula seminis are located in the segments in front of the testicles and open outwards on the abdomen. In Haplotaxis gordioides there are three pairs in the 7th, 8th and 9th segments. Penis bristles or penises are missing. In contrast to earthworms, the clitellum of the Haplotaxidae consists of only one layer of cells and is therefore inconspicuous. In 1821, when GL Hartmann first described the “roundworm-like earthworm” Lumbricus Gordioides, no belt was found. The size and position of the mostly invisible belt vary according to the species: For example, in Haplotaxis ascarioides the clitellum extends from the 11th to the 16th segment, in Haplotaxis gordioides from the 10th to the 13th segment.

The Haplotaxidae show the typical original features of the paraphyletic few bristles. They have a fully developed coelom, septate according to segments and acting as a hydroskeleton, as well as a primary closed blood vessel system . This is of simple construction, but several main blood vessels protruding into the coelom can run parallel and take up a considerable proportion of the worm volume, while a fine capillary system, as is known from earthworms and also from many bristles, is completely or largely absent. The longitudinal blood vessels are connected to one another by contractile ring vessels.

The well worms have four or two individual bristles ( chaetae ) on each segment , with which the animals anchor themselves in the ground and provide the necessary support for the locomotion made possible by the longitudinal and circular muscles of the skin muscle tube . The belly-side Chaetae are large and sickle-shaped, while the back-side chaetae are small and straight, in many species not present in all or some segments.

During copulation, two animals wrap their front ends around one another on the abdomen and exchange sperm, which enter the receptaculum seminis of the sex partner. A secretion from the barely visible clitellum forms an egg cocoon into which the wellworm first releases its own eggs and then the sperm of its sex partner from its receptacula seminis, thus enabling fertilization in the cocoon. Here the embryos develop into finished small worms.

Distribution, habitat and way of life

The Haplotaxidae are distributed worldwide in groundwater, spring waters, streams and cave waters, whereby they are mostly only found scattered, but locally also in large numbers.

Well worms prey on other small annelid worms such as Tubifex or smaller well worms , which they suck in as a whole with the help of their muscular pharynx.

The first described by GL Hartmann in 1821 , about 1 mm wide and up to 30 cm long well burial worm Haplotaxis gordioides inhabits spring waters and wells as a cosmopolitan , but also rivers and lakes. The sites include Europe and North America. The Haplotaxis brinkhursti , known from West Virginia, is a cave dweller.

Genera

The approximately 30 species of the Haplotaxidae are divided into seven genera :

literature

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