Nematodes do not have flame cells. Most nematodes are dioecious. Fertilization takes place when males use special copulatory spines to open the females' reproductive tracts and inject sperm into them. The sperm are unique in that they lack flagellae and move by pseudopodia, like amoebas. Development of fertilized eggs is usually direct.
Nematodes are almost unbelievably abundant.
One study reported around 90,000 individual nematodes in a single rotting apple. Another reported 236 species living in a few cubic centimeters of mud. The number of described species is around 12,000, but too little attention has been paid to these animals and the true number may be closer to 500,000. A good example of the latter is a species of nematode that is known only from felt coasters placed under beer mugs in a few towns in Germany. Many nematodes are free living and play critical ecological roles as decomposers and predators on microorganisms.