A. Movement/locomotion in the flatworms 1. Use of the mesoderm as a hydrostatic skeleton a. Mesoderm is a parenchyma (mesenchyme). Since cells are a large percent water, the parenchyma functions as a "water" skeleton. b. Flatworms have both dorsal ventral muscle bands and anterior/posterior muscle bands. This act against each other and against the parenchymal cells. This can change the shape of the worm, but isn't typically the means of locomotion 2. Organisms use the ciliated ventral surface as the means of loco- motion a. requires very small size b. is facilitated by the production of a mucus film by special cells in the epidermis B. Other specialized devices used by platyhelminths 1. Use of an extensible pharynx (proboscis) a. The is a highly muscular structure which is forced out of sack by contraction of body muscles. b. Pharynx has longitudinal and circular muscles which are used both to change the shape of the pharynx and to pump fluids into the gut cavity 2. Parasitic forms (Trematoda and Cestoda) have a variety of structures to stay in the host a. Trematodes have suckers (anterior and ventral) b. Cestodes may not only have suckers, but often hooks around or in the vicinity of the sucker(s) C. The Rhynchocoela (Nemertina)--Ribbon Worms 1. Have solid mesoderm 2. Have evertible proboscis, often armed with stylet a. Proboscis is in a sack-like structure. When animal contracts walls of this structure, proboscis is extended. b. This takes place very rapidly and with great force.