Evolution and Phylogenetic Relationships


         Nematode fossils are hard to find because the organisms are microscopic and lack hard structures.  However fossils have been found dating from the Cambrian period so it is very likely that nematodes have been around since then (Waggoner 2004).  As rather small and primitive organisms, nematodes display mostly simple evolutionary developments.

The important steps in evolution follow a pattern similar to this.

  1. No symmetry (ex: unicellular organisms) to radial symmetry (ex: jellyfish) to bilateral symmetry (ex: vertebrates, worms, crustaceans etc.)
  2. Segmentation (ex: earth worms)
  3. No coelom or body cavity (ex: unicellular organisms) to coelom (ex: vertebrates, Annelids)
  4. Vertebrae (ex: mammals, fish, birds)

The following animal phylogeny illustrates many of the important relationships between Nematodes and other phyla:


This phylogeny was obtained from http://www.whozoo.org/inverts/animalphylo.htm


     The phylum Nematoda or roundworms obviously do not contain vertebrae.  They are bilaterally symmetrical but lack segmentation.  This characteristic distinguishes nematodes from other common segmented worms such as those in phylum Annelida.  The difference between other bilaterally symmetrical organisms and worms lies in the presence of an internal body cavity or coelom in those organisms.  The pseudocoelomates represent the first organisms to have an internal body cavity.  This is significant in that it promotes more sophisticated and efficient mobility (Raven et al. 743).   

         Again, the insufficiency of nematode study makes comprehensive classification very difficult.  Because only a small percentage of the different species of nematodes have been classified, constructing true phylogenetic relationships is hard.  Similarly, because nematodes are so uniform in structure, classifying them is tough.  It is widely believed that the shared ancestor of present day nematodes had the same basic characteristics that we see in all species of roundworms.  Thus, the differences between the most primitive and the most evolved nematode species is fairly small. Even where evolution is seen from primitive to advanced specimens, it is almost uniformly present in every branch  (Malakhov 175-76). This idea of parallelism presents further difficulty in classifying nematodes.  Nonetheless, nematodes are all classified as pseudocoelomates because they have a primitive body cavity.

        The phylum Nematoda is usually divided into two classes based largely on morphological characteristics.  The two classes of nematodes are Secernentea and Adenophorea.  The class Secernentea is also known as Phasmidia because phasmids (structures sensitive to chemicals found on the tail of some nematodes) are present, and these nematodes are not usually found in water.  Adenophorea, on the other hand, can also be called Aphasmidia because it lacks phasmids and this class is usually found in water.  This division of nematodes into two classes in effect distinguishes between the more advanced in Secernentea and the more primitive in Adenophorea (Poinar Jr. 18-20).  As technology and taxonomy have become more advanced, the classification of nematodes has changed significantly.  However, when considering the phylogenetic tree for nematodes it is imperative to keep in mind that nematologists have not reached a consensus.  There is no single comprehensive tree that all scientists agree on for nematodes. 




Evolution & Phylogenetic Relationships

Features Shared by Nematodes with Related Groups

Features Unique to Nematodes

World Distribution of Roundworms

Nematode Habitat Use

Energy/Modes of Nutrition

Ecological Roles of Nematodes

Impacts on Human Society

References Page