FLATWORMS
Members of the phyla Platyhelminthes, have a wormlike shape and are the first in this manual to have a third layer of cells. The mesoderm produces muscular layers, an excretory system, and an elaborate reproductive system. The nematodes and rotifers, along with some other minor phyla, are sometimes referred to as the "aschelminths." The platyhelminths have bilateral symmetry and meet the environment in a more aggressive, head-on fashion than the radiate animals. Cephalization, a specialization of the anterior end of the animal, can be seen in the clustering of sensory structures and clumps of nerves (ganglia or "brains") in the head region. The body cavities of these and succeeding phyla have become more complex through the addition of specialized internal organs. The fluid in the body cavity may act as a hydrostatic skeleton, a site for the collection of wastes or gametes, and/or as a primitive circulatory system. Platyhelminths are commonly known as "flatworms," owing to their almost paper-thin structure. Flatworms have their body cavities loosely filled with cells and are said to exhibit an acoelomate body plan. Their digestive system is incomplete, and they possess only a mouth. Flame cells are often present as part of an osmoregulatory system, and all members have a spongy body structure. Members of one class (the turbellaria) are free-living, while the other two classes (trematoda and cestoda) are obligate parasites.
| Development: Protostomous |
| Body cavity (between endo- and ectoderm): None (true acoelomates, since they do have mesoderm) |
| Digestive System: Incomplete (Absent in tapeworms) |
| Circulatory System: Absent |
| Excretory System: Often there are protonephridia |
| Nervous System: Often two longitudinal nerves with ganglia at the anterior end |
| Reproductive system: Almost always hermaphroditic and complex |
| Claim to Fame: Known for their parasitic members, although many are beautiful, free-living reef inhabitants |
Diagram of a Platehelminthes
Penis-fencing flatworms
EXCRETORY SYSTEM OF PLATYHELMINTHES
