Eight phyla of animals have body plans based on the pseudocoelom, a fluid-filled cavity that develops between the endoderm and the mesoderm. The pseudocoelom helps the animals to move more efficiently than cnidarians or acoelomate worms. Pseudocoelomates, like the acoelomates, have a one-way digestive tract but lack a circulatory system.
         The most important group of Pseudocoelomates is the phylum Nematoda, roundworms. A few nematodes are parasitic, causing a variety of serious diseases in plants and animals, including humans. Unique to the roundworms is their style of movement. They use longitudinal muscles and move in a whipping manner.
         Other Pseudocoelomate phyla include the phylum Nematomorpha, horsehair worms. The adults are free-living and reproduce in water. The younger members live as parasites on arthropods.
            Spiny-head worms, phlyum Acanthocephala, live their lives entirely as parasites. The youth develop in the tissues of arthropods and the adults live and reproduce in the intestines of vertebrates.
           Four pseudocoelomate phyla-Kinorhyncha, Gastrotricha, Loricifera, and Rotifera-reside in the sand and silt of shorelines. The animals of the Loricifera phylum are unique from the other three phyla because their larvae are free-swimming and the adults are free-swimming. The adults of the other three phyla are motile. Phylum Kinorhyncha includes tiny, burrowing marine worms that feed on diatoms. These worms are short, covered with spines and have a tiny, retractile probosis. The members of the phylum Gastrotricha are hermaphrodites. They live in fresh and salt water and feed on protists. Species in the phylum Rotifera live in fresh water, while others are marine or terrestrial. Rotifers are sometimes called "wheel animalcules" because the beating of a crown of cilia around the mouth causes them to spin through the water like tiny wheels. In some groups of rotifers, the female can produce eggs that can develop without fertilization, a phenomenon known as parthenogenesis.
         Another phylum, Entoprocta, conatins animals that resemble both hydrozoans and a group of coelomate animals. However, their internal system identifies them as pseudocoelomates. They have a U-shaped digestive tract, with the mouth and anus located within a circle of tentacles. Reproductive patterns of entoprocts varies. Some species have separate sexes, some are simultaneous hermaphrodites, and in others a single reproductive organ produces sperm at one stage in the life cycle and eggs at a later stage. This last method is known as sequential hermaphroditism.
 

Nematoda:

Ancanthocephala:    Nematomorpha:

Rotifera:     Entoprocta:

Thanks to the websites: http://204.154.117.95/AnimalBiology/Pseudo/Pseudocoelomates.html and and http://rainbow.ldeo.columbia.edu/ees/life/slides/oldec/entoprocta.html for their pictures.