Flukes Trematodes are internal or external parasites of living vertebrate hosts. The body consists of an anterior scolex solely for attachment to the host's gut and a string of proglottids, each of which possesses both male and female organs. Ribbon Worms Members of the phylum Nemertea are likely related to turbellarians because of the external cilia and tissue organization. The most unique feature is the proboscis, which normally lies in a special cavity until pushed out by hydrostatic pressure to stab some prey. They are bilateral and possess a slender tapered body.
Reproductive organs lie in a false coelom filled with fluid.
A tough cuticle covers and protects the body. Structurally they are remarkably complex, possessing a crown of cilia, jaws, digestive tract, protonephridia, nerve cells, eyespots, and muscles. Chitons have eight plates covering the dorsal body. As the gastropod larva develops, the internal organs undergo a unique twisting, or torsion, that places the anus anteriorly near the mouth. Nudibranchs seem to have undergone detorsion and have exposed ctenidia.