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~20,000 species but likely to be 4 to 50 times more yet undescribed.
Why is this phyla so underdescribed?

Extremely abundant!   Million per square meter of aquatic sediment and nearly as abundant in soil and leaf litter.  In terms of individuals, nematodes account for an estimated four of every five animals in the world.

How does the success of nematodes differ from that of arthropods?  How could the success of arthropods be explained? How could the success of nematodes be explained?  What’s the LESSON here?
 

Characteristics

• Obviously from the common name they are vermiform and round. But why round?

• non-cellular molting cuticle (but of collagen rather than chitin).

• lack circular muscle.  Is annelid-like motion possible? How does this affect the design of the digestive system?

• no organized respiratory or circulatory system.  Sensory system simple with chemoreceptors at both ends and general body surface appears to be light sensitive.

Life History
 

Life cycles relatively simple in free-living groups being one of the few invertebrates with no true larval stage

Parasitism as evolve several times within the nematodes (Blaxter et al, 1998) (Why? ) and host vary widely.

Numerous ways health is affected:

• indirect effects due to competition for nutrients in the intestines of their host, as with many pinworms and large roundworms of the intestine, direct effects as with hookworms.

 
 
 
• block lymphatic system as with elephantiasis

 
 
• migrate through host as with Onchocerca in eyes results in river blindness (though the story may be more complex).

 
• Trichinella infects muscle tissue and occurs from eating undercooked pork and other meats.  It may act as a ‘viral animal’!  (vascular endothelial growth factor that promotes blood vessel growth, normally to heal wounds, provides greater nutrition for the parasite).

 
• affects hosts’  behavior in ways that improve chances of completing parasitic lifecycle:
• A nematode free-living in streams, infects aquatic larvae of mayflies and this nematode changes mayfly males into females (both anatomically and behaviorally) thus ensuring the adult parasite a chance for finding a mate and host for its offspring.
• Dracunculus (guinea worm) causing dracunculiasis drives its human host to water where the Guinea worm’s intermediate vector (copepod zooplankton) reside (click here for gross picture).

 
 
 

Effects of nematode parasites often overlooked:

• Affects the poorest people of poorest nations (public health, not medicine, has largely decreased parasites in USA).
• Often the infection is not outright fatal

Sometimes nematodes are the victim

 
 

Beneficial nematodes:

• pest control

• research model in developmental biology (view the breeders newsletter) in part because of a constant number of cells (959) in the mature adult (eutely)
  

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