Placoverms (Plated Worms)
Placoverms are soft bodied ribbon-shaped, flattened dorso-ventrally, bilaterally symmetric worms with silica dorsal plating and a primitive hollow neural tube. Placoverm bodies are cylindrical, with four main parts: the acorn-shaped proboscis, a short fleshy collar that lies behind it, a long trunk, and a postanal tail which sometimes shows weak signs of segmentation. They have a circulatory system with a heart that also functions as a kidney, and complete digestive tracts, but food is mostly transported through it by using the cilia that cover its inside surface. Placoderms move via undulating locomotion. Depending on species and age, individuals range from almost microscopic to over 20 m long.
The head has various adhesive and feeding structures. In some species, there may be a number of glands secreting sticky adhesive substances and shallow muscular suckers, used for attachment. In other species there is an oral sucker, with various degrees of muscularisation that surrounds the mouth. In either case, there is a highly branched guts with one or more anuses. They breathe by drawing in oxygenated water through their mouth. The water then flows out the animal's gills which are on its trunk. The gills are also used for feeding
Some species have a collection of ganglia acting as a rudimentary brain to integrate signals from sensory organs such as eyespots.
Reproduction is hermaphroditic, with individual producing both eggs and sperm. When two Plate Worms mate, they exchange sperm so both become fertilized.