The flatworms, flat worms, Platyhelminthes, or platyhelminths are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates. Being acoelomates, and having no specialised circulatory and respiratory organs, they are restricted to having flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion. The digestive cavity has only one opening for both ingestion and egestion ; as a result, the food can not be processed continuously.
Flatworm
Various parasitic flatworms from Haeckel's Kunstformen der Natur (1904)
The turbellarian Pseudoceros dimidiatus
Life cycle of the eucestode Taenia: Inset 5 shows the scolex, which has four Taenia solium, a disk with hooks on the end. Inset 6 shows the tapeworm's whole body, in which the scolex is the tiny, round tip in the top left corner, and a mature proglottid has just detached.
Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limbs, and usually no eyes.
Lumbricus terrestris, an earthworm
White tentacles of Loimia medusa, a spaghetti worm
Paragordius tricuspidatus, a nematomorphan
Pseudoceros dimidiatus, a flatworm
