Nematodes: the most numerous animals on Earth

Nematodes, aka threadworms, aka roundworms are said to be the most numerous animals on Earth. Some are parasites but many are soil-dwellers living on decaying organic matter. A single hectare of  heathy soil will be home to many billions of them. One study found an estimated 90 thousand in a single rotting apple, another found three million in a square metre of fertile ground.

Most are tiny: these, found in a pile of well-rotted leaves in deciduous woodland, were less than a centimetre long when full extended. They play a crucial role, along with soil fungi, in recycling nutrients from decaying plants. 

Because they are translucent, you can see the food particles that they've ingested passing along their gut during digestion - visible in these pictures as a series of dark particles along the length of the animal.

Click here for more pictures revealing their microscopic structure.