• Acanthocephala

• Kinorhynchia

• Platyhelminthes

• Coelenterata

• Nematoda

• Priapulida

• Endoprocta

• Nematomorpha

• Rotifera

• Gastrotricha

• Nemertea

• References

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.

Phylum Platyhelminthes 

Include: 

• 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

Phylum Nemertea

Classes:

• 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.

Phylum Nematoda

Classes:

• 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

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.

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.

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

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.

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

Phylum Gastrotricha

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.  

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.

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.

Phylum Endoprocta

1. Simple, archaic animals

2. A coelomate

3. Tentacles.

4. Excretory system of protonephridia and flame cells.

5. Gonads with separate duct.