Nematodes
The "roundworms" or "nematodes" species are very difficult to distinguish; over 80,000 have been described, of which over 15,000 are parasitic, including pathogens in most plants and animals. From an agricultural perspective, there are two categories of nematode: predatory ones, which will kill garden pests and pest nematodes which attack plants and those that act as vectors spreading plant viruses between crop plants.