Minor Invertebrate Phyla 8 Phylum Nematoda Phylum Nematoda Nematodes , or roundworms, are very important and found in nearly every ecological niche. There are at least 10,000 species, both free-living in aquatic and terrestrial habitats and as parasitic forms are very successful, found in virtually every type of animal.
Nematodes can be carnivorous, herbivorous or parasitic.
The body is always long, slender and tapered at the end and circular in transverse section. A muscular pharynx is required to pump food through the gut since there is no peristalsis. The posterior end of the male is curled like a hook to hold onto the female during copulation. Ascaris is a parasite of the intestine of man, pigs and horses and although infections in man are rare in the UK, this is not the case in a number of developing countries.