The Lower Metazoan Phyla

Rev. 10/30/2019


Kingdom Animalia, sub-kingdom Metazoa - Acoelomata

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Phylum Coelenterata

e.g., Hydra.

  1. Diploblastic - lower metazoa.

  2. Primarily radial structure.

  3. Body cavity, the enteron, with a single opening for ingestion and egestion.

  4. Nervous system a network of cells.

  5. Reproduce asexually by budding and sexually.

  6. Sedentary or free-swimming.


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Phylum Platyhelminthes 


  • Turbellaria - flatworms - free-living predators of invertebrates, e.g. Planaria, Bipalium.
  • Trematoda - flukes - vertebrate parasites, e.g. Fasciola hepatica, Schistosoma haematobium.
  • Cestoda - tapeworms - vertebrate parasites, e.g. Taenia solium.
  1. Parasitic or free-living unsegmented worms (Cestoda are strobilated)
  2. Triploblastic, acoelomate, bilaterally symmetrical; usually flattened dorsoventrally
  3. Complex intestine usually present
  4. Cephalized; simple central nervous system with anterior ganglion
  5. Protonephridia as excretory structures
  6. Hermaphroditic with complex reproductive systems

Anterior portion of flatworm - probably Bipalium sp.

Bipalium sp.
Flatworm - Bipalium sp.?
Photograph by Joan Maurer, Milwaukee County Zoo, Milwaukee, WI

Bipalium sp. - Yolo County, California

Turbellaria are "free-living" flatworms. They are generally predators of soil invertebrates, including earthworms.  Some species can become pests in commercial earthworm beds.  The worms are not able to conserve water so they are usually in humid habitats like earthworm burrows. They only emerge when the soil is saturated and their refuges waterlogged; then they are often seen in water puddles.

A Turbellarian that is frequently seen in northern California is Bipalium sp. (maybe B. kewense but there are other species).  Bipalium kewense can be up to a foot long and the head is slightly expanded; the basic color is dark orange, but it tends to look darker due to black longitudinal stripes. The ventral surface is less dark. It has numerous eyes scattered over the anterior end.  Thought to reproduce by fragmentation (asexually), as well as sexually.

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Phylum Nemertea


  • Anopla (mouth posterior to main nerve ganglion, nerve chord just below epidermis, proboscis unarmed).

  • Enopla (mouth anterior to main nerve ganglion, nerve chord below body wall muscles, proboscis armed).

  1. Flattened unsegmented worms.

  2. Ciliated ectoderm.

  3. Evertible proboscis.

  4. Digestive system with mouth and anus.

  5. Excretory system with flame cells.

  6. Circulatory system present.

  7. Simple, repeated gonads.

  8. Sexes separate.


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Phylum Nematoda


  • Secernentea

  • Adenophorea

  1. Triploblastic, bilaterally symmetrical, vermiform, unsegmented pseudocoelomate, roundworms.
  2. Body round in cross section , covered by a cuticle; growth in juveniles accompanied by molting
  3. Unique cephalic sense organs, amphids; some with caudal sense organs, phasmids
  4. Intestine complete and regionally specialized
  5. Unique excretory system of collecting tubules or renette cells (excretory glands)
  6. Without circulatory or respiratory structures
  7. Longitudinal nerve in epidermal cord
  8. Body wall with longitudinal muscles only
  9. Sexes usually separate
  10. Free-living or parasitic; marine, freshwater, terrestrial environments

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Phylum Nematomorpha

Nematomorpha (from the Greek nema, "thread," and morphe, "shape"); horsehair or gordian worms, e.g. Gordius

  1. Similar to nematodes but no excretory pore or lateral chords.

  2. Nervous system of dorsal ganglion and ventral chord.

  3. Gonads of both sexes open into posterior cloaca with intestine.

  4. Intestine somewhat degenerate.

Female anterior (top), female posterior (lower left) and male posterior (lower right) of Paragordius.

The first known fossil record of Nematomorpha dates from the Eocene (40 - 70 million years ago), but some suggest the group may have evolved in the Paleozoic Era (over 500 million years ago).

Nematomorpha larvae are parasites aquatic and terrestrial invertebrates, including notably grasshoppers, crickets, locusts, katydids, beetles, caddisflies, dragonflies, spiders, millipedes, centipedes, crustaceans, and leeches.  Horsehair worms are usually seen in the adult, free-living stage near edges of ponds and streams.

Nematomorphs can be 1 m long and 1-3 mm diam. Color ranges between light tan to dark brown.  Males are smaller than females. Locomotion of adult horsehair worms is a slow whipping action; males are more active than females.

Horsehair worms reproduce sexually, in spring, early summer, or autumn. Eggs, often numbering in the millions, are laid in long gelatinous strings. For aquatic forms, the incubation period may range between 15 and 80 days, depending upon water temperature. After hatching, larvae are thought to encyst on vegetation along the water's edge soon after hatching where they are ingested by their hosts. The larva then burrows its way through the intestinal wall into the host's body cavity, continuing its development.

When ingested by inappropriate hosts, the cyst may degenerate and then reencyst in the tissues of the host. If this inappropriate host is then ingested by one of its predators, the cyst may again disintegrate and continue its life cycle in the new host.

Horsehair worm

Horsehair worm, El Dorado County, California

After entering the body cavity of an appropriate host, the larva grows and develops into a tightly coiled mass in the host. One to several horsehair worms may occur in a single host. It has been suggested that hosts seek water when the horsehair worm is ready to emerge. Once the host enters the water, the horsehair worm breaks through the body wall of the host. Newly released horsehair worms soon die if they do not have access to water.

The common name, gordian worm, is traced back to the gordian knot tied by Gordius, king of Phrygia around 330 B.C. According to mythology, Gordius tied his chariot to a pole and declared that whoever could undo the knot would be ruler of all Asia. Horsehair worm is probably derived from the appearance of the worms in water.

Over 230 species of nematomorphs have been described; between 4 and 16 genera occur in North America. Identification of species is based on microscopic surface patterns and sculpturing of the cuticle.

Source:  Wetzel and Watermolen



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Phylum Acanthocephala

  1. Similar to nematodes but with an evertible, hooked proboscis.

  2. Circular and longitudinal muscles.

  3. Nervous system of anterior ganglion and two lateral chords.

  4. Excretory system of nephridia and modified flame cells.


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Phylum Rotifera  

  1. Have features of platyhelminthes and nematodes. 

  2. Among the smallest of the acoelomate metazoa.

  3. Triploblastic, bilaterally symmetrical, unsegmented pseudocoelomates
  4. Intestine complete and regionally specialized
  5. Pharynx modified with jaw-like elements
  6. Anterior with ciliated disk fields for wafting food particles
  7. Posterior end often with toes and an adhesive gland for anchorage
  8. Cuticle well developed, secreted by epidermis
  9. Protonephridia as excretory structures, but no circulatory or respiratory structures
  10. Usually parthenogenic; males absent or reduced
  11. Marine, freshwater, terrestrial; sessile or free-swimming
Class Bdelloidea. female of Philodena roseola

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  1. Minute, worm-like, unsegmented.
  2. Bristles and cilia on surface.
  3. Digestive system with esophagus like nematodes and tubular intestine.
  4. Nervous system of anterior ganglion and two lateral chords.
  5. Excretory system of paired nephrida.
  6. Hermaphroditic and parthenogenic.
  7. Total egg cleavage and direct development.
  8. Features in common with rotifers and nematodes.  

A. Order Macrodasyida  B. Chaetonotoid type.

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Phylum Kinorhynchia

  1. Minute, worm-like, unsegmented

  2. Bristles on cuticle, no cilia.

  3. Hypodermis a syncytium.

  4. Esophagus and intestine similar to nematodes.

  5. Longitudinal muscles run length of body.

  6. Excretory system of protonephridia, each with a single flame cell.

  7. Nervous system of anterior ganglion and four chords.

  8. Usually have males and females.

  9. Similarities with rotifers and gastrotrichs.

Class Homalorhagea; Pycnopyes frequens.

Adult on left, anterior retracted;  anterior extended on right.

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Phylum Priapulida

  1. Unsegmented worm-shaped.

  2. Digestive system with anterior mouth and posterior anus.

  3. Nervous system and anterior ganglion around esophagus and ventral chord.

  4. Marine, mud dwellers.

  5. Posterior appendages.

  6. No circulatory system.

  7. Excretory ducts.

  8. Affinities with nematodes, rotifers and gastrotrichs.


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Phylum Endoprocta

  1. Simple, archaic animals

  2. A coelomate

  3. Tentacles.

  4. Excretory system of protonephridia and flame cells.

  5. Gonads with separate duct.


All the above phyla contain unsegemented organisms. In earlier classifications (e.g. ref Hyman, 1930s) all were considered classes of a single phylum, the Aschelminthes.

Note: Annelida, Arthropoda etc. are coelomate, and segmented.

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Maggenti, A.R. 1981. General Nematology.

Borradaile, L.A., L.E.S. Eastham, F.A. Potts and J.T. Saunders. 1961. The Invertebrata. Cambridge University Press.

Dindal, D.L.(ed). 1990.  Soil Biology Guide.  John Wiley, NY.

Mark J. Wetzel, INHS Center for Biodiversity, and Dreux J. Watermolen, Wisconsin Department of Natural Resources, Madison.

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