BIO 385 Invertebrate Zoology
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Invertebrate Diversity
Cycloneuralia: Nematodes & Relatives

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Phylum Nematoda Roundworms


  • Body elongate, round in cross section, tapered at ends
  • Unsegmented
  • Complete gut
  • Pseudocoelomate or acoelomate
  • Widespread and abundant in all habitats; some parasitic
Vinegar Eel
Vinegar Eel, Turbatrix aceti, a free-living nematode.
Marine Nematodes
Free-living marine nematodes
Dog Hookworm
Dog Hookworm, Ancylostoma caninum, female (stained whole mount)
Ascaris C.S.
Ascaris lumbricoides; stained cross-sections of female, male, and through pharynx.
See also labeled photo.
Trichina worm
Adult female trichina worm, Trichinella spiralis (stained whole mount)
Encysted larva of trichina worm, Trichinella spiralis, in muscle tissue (Trichinosis); (stained whole mount).
Whipworm, Trichuris sp., an intestinal parasite; (stained whole mount).

Phylum Nematomorpha Horsehair Worms


  • Body greatly elongate, thread-like (up to 1 m long), often curled up and folded on itself
  • Unsegmented
  • Lack functional gut
  • Pseudocoelomate
  • Adults mostly in fresh water; larvae parasitic on insects
Horsehair Worm
Horsehair Worm, Gordius sp., KY

Phylum Cephalorhyncha penis worms & mud dragons


  • Fully retractable introvert that is covered in spines
  • Chitinous cuticle that is molted
  • Most are tiny (< 1 mm, but a few to 20 cm)
  • Benthic marine
  • Includes 3 groups formerly considered separate phyla: Priapulida (penis worms), Kinorhyncha (mud dragons), & Loricifera
  • Not represented by specimens in lab
Ottoia fossil
Fossil of penis worm, Ottoia sp.; Cambrian Period
Ottoia Model
Model of penis worm, Ottoia sp.; Cambrian Period
This page last updated 28 June 2015 by Udo M. Savalli ()
Images and text Udo M. Savalli. All rights reserved.