Platyhelminthes

Flat worms are unsegmented, bilaterally symmetrical worms that have no coelom, but that do have three germ layers. Many are parasitic organisms that are harmful to humans, but some are free living. Flatworms contain a cephalized nervous system that is composed of a head ganglion, usually attached to nerve cords that are connected across the body by transverse branches. Excretion and osmoregulation by flatwroms is regulated by "flame cells" lovated in protonephridia. Flatworms lack a respiratory or a circulatory system; these functions take place by absorption through the body wall. Free living forms contain a non-developed gut.

Movement in some flatworms is controlled by longitudinal, circular, and oblique layers of muscle. Others move along slime tails by the beating of epidermal cilia. The development of directional movement in the head of light-sensitive organs is called "ocelli". Other sense organs found in at least some members of this group include chemoreceptors, balance receptors, and receptors that sense water movement.

Flatworms can produce either asexually or sexually. Most are monoecious. Most have ways of avoiding self-fertilization. Development can be direct or indirect.

The Phylum Platyhelminthes...


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