Secernentea are the main class

Class (biology)

In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

 of nematodes, characterised by numerous caudal papillae and an excretory system

Excretory system

The excretory system is a passive biological system that removes excess, unnecessary or dangerous materials from an organism, so as to help maintain homeostasis within the organism and prevent damage to the body. It is responsible for the elimination of the waste products of metabolism as well as...

 possessing lateral canals. Like all nematodes, they have no circulatory or respiratory system.

The Secernentea contain several significant parasitic species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 among the Rhabditia

Rhabditia

Subclass Rhabditia is mostly composed of parasitic nematodes , though there are some free-living species as well...

 and Spiruria

Spiruria

Subclass Spiruria comprises mostly parasitic secernentean nematodes. In an alternate classification, they are treated as suborder Spirurina, with the orders listed here being ranked as infraorders....

. Caenorhabditis elegans

Caenorhabditis elegans

Caenorhabditis elegans is a free-living, transparent nematode , about 1 mm in length, which lives in temperate soil environments. Research into the molecular and developmental biology of C. elegans was begun in 1974 by Sydney Brenner and it has since been used extensively as a model...

, the famous laboratory animal, is one of the many harmless members of the Rhabditia.

Systematics

Subclass

Class (biology)

In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

es and order

Order (biology)

In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

s of Secernentea are:

• Subclass Rhabditia
  Rhabditia
  Subclass Rhabditia is mostly composed of parasitic nematodes , though there are some free-living species as well...
  
   (paraphyletic?)
  • Rhabditida
    Rhabditida
    Rhabditida is an order of free-living, zooparasitic and phytoparasitic microbivorous nematodes .Order include 8 superfamilies.Diagnosis:...
    
    
  • Strongylida
    Strongylida
    The Suborder Strongylida includes many of the important nematodes found in the gastrointestinal tracts of ruminants, horses, and swine, as well as the lungworms of ruminants and, the hookworms of dogs and cats.This Suborder includes The Suborder Strongylida includes many of the important nematodes...
    
    
• Subclass Spiruria
  Spiruria
  Subclass Spiruria comprises mostly parasitic secernentean nematodes. In an alternate classification, they are treated as suborder Spirurina, with the orders listed here being ranked as infraorders....
  
  
  • Ascaridida
    Ascaridida
    The order Ascaridida includes several families of parasitic roundworms with three "lips" on the anterior end. They were formerly placed in the subclass Rhabditia by some, but morphological and DNA sequence data rather unequivocally assigns them to the Spiruria...
    
    
  • Camallanida
    Camallanida
    The Camallanida are an order of nematodes.* Parasites of terrestrial and aquatic vertebrates* Copepods as obligatory secondary hostsThey are sometimes included in the Spirurida as a suborderCamallanina....
    
     (sometimes included in Spirurida)
  • Drilonematida (sometimes included in Spirurida)
  • Oxyurida
    Oxyurida
    Oxyurida is an order of nematode worms of the class Secernentea. It consists of four families, one of which contains the human pinworm ....
    
     (= Rhabdiasida)
  • Rhigonematida (formerly in Tylenchia)
  • Spirurida
    Spirurida
    Spirurida is an order of spirurian nematodes. Like all nematodes, they have neither a circulatory nor a respiratory system.Some Spirurida, like the genus Gongylonema, can cause disease in humans...
    
    
• Subclass Diplogasteria (may belong in Rhabditia)
  • Diplogasterida
    Diplogasterida
    Diplogasterida is an order of nematodes. It is sometimes placed in a monotypic subclass Diplogasteria, but as it seems it is better included in the Rhabditia. If the Tylenchida are included in the Rhabditia, the Diplogasterida certainly would have to be merged there too-References: : . Version of...
    
    
• Subclass Tylenchia (may belong in Rhabditia)
  • Aphelenchida
    Aphelenchida
    A moderately large order of nematodes. Aphelenchida have a stylet for feeding and a very prominent median bulb in the oesophagus. They are cosmopolitan. Some are associated with insects, and may be ectoparasites or endoparasites, or merely use the insect as transport. Others are associated with...
    
    
  • Tylenchida
    Tylenchida
    Tylenchida is an order of nematodes.- List of families :* Superfamily Criconematoidea** Criconematidae** Tylenchulidae* Superfamily Tylenchoidea** Anguinidae ** Belonolaimidae** Dolichodoridae** Ecphyadophoridae** Hoplolaimidae...
    
    

Some families traditionally considered to be Rhabditida seem to be closer to the Tylenchida. If the Tylenchia are to be maintained as separate, they probably will be included therein.
These are also called as Phasmidians as they have the phasmids which are the posterior glandulo sensory and excretory structure.