Milnesium tardigradum

Water bear Milnesium tardigradum.  Natural size of this individuum 0.5 mm

Milnesium tardigradum, one of the most abundant and ubiquitous terrestric tardigrade species in Europe and possibly worldwide, represents a well-defined and unique position with respect to the rest of tardigrade species, a fact which becomes evident when comparing the anatomy and motion characteristics of this animal with other water bears. Whereas some other species can be identified only by a professional biologist Milnesium is easily recognized also by the microscope amateur due to its unique properties.
Whereas most of the water bears prefer vegetarian food, Milnesium is considered as one of the more carnivorous species, eating rotifers and nematodes.
The animals are really tough and long-living. This seems to be one of the main reasons why Milnesium is one of the best-studied species so far.
It can be kept in a petri dish over a longer period without problems whereas e.g. many Echiniscus species can't survive without regular desiccation.

Anatomy

The typical body length is about 0.5 mm. In the scientific literature we can find maximum values up to 1.2 mm. The body shape is lengthy and resembles a fish, in particular the strongly curved end of the body has a close similarity with a shrimp. The last pair of legs is somewhat apart and is orientated to the back, not to the side. Its main purpose seems to be for grasping objects or gaining distance from objects, not so much for crawling.
The rapid motion of Milnesium and its head sometimes have been compared to a lizard.
In particular when seen in incident light the body fluid sometimes appears in reddish (rose) colour. The buccal tube is very short and wide and the pharynx has pear form.

Front part of the body of Milnesium tardigradum with pear shaped pharynx (P)
and broad buccal tube (B)

Around the mouth opening are six characteristic sensory and moveable lobes which are not found at other species' mouths.
There are two more lobes (papillae), roughly cylindrical, on both cheeks, which can be perceived only faintly below.

Front part of the body of Milnesium tardigradum with buccal region

Also the claws form a class by itself called "Milnesium type" claws.

Detail: Claws of a leg of Milnesium tardigradum  

© Photomicrographs by  Martin Mach

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Literature:

A good characterization of Milnesium can be found in Kinchin's recent monograph.
Ian Kinchin: The Biology of Tardigrades. p. 126 - 128. London 1994.

First publication of Milnesium: Doyère, L., Mémoire sur les Tardigrades (1. Teil), in: Ann. sc. nat., sér. 2, Bd. 14 (Zool.), p. 282 - 284, Tab. 13, 17, 19. Paris 1840.

The classic reference, with nice drawings:
Ernst Marcus: Bärtierchen (Tardigrada). Gustav Fischer Verlag, Jena 1928.
Milnesium on pp. 217 - 219.

Out of print, it is a pity:
Hartmut Greven: Die Bärtierchen. p. 7, 15. Neue Brehm-Bücherei, Wittenberg, 1980.