Nematode. Said to be the most boring animals in the world, nematodes (eel worms) are also among the most widespread of the animals, found in almost all habitats. Possibly more renowned for their pathogenic influences of plants, animals (including humans - elephantiasis) or because some (Caenorhabditis) have been adopted as model organisms in the study of development. Nematodes are also common in natural habitats, where they accumulate detritus, aggregates of bacteria etc., sucking it into their bodies using the muscular pharynx - the grey central structure extending inside the anterior part (left) of the organism. They have a very characteristic writhing. Phase contrast.