Platyhelminthes

Platyhelminthes, or flatworms, are a phylum of relatively simple bilateral animals that are dorsoventrally flattened. They are typically acoelomates, lacking a true body cavity, and have no specialized circulatory or respiratory systems. Most rely on diffusion for gas exchange. Their digestive arrangements range from a branched gastrovascular cavity with a single opening to a markedly reduced or absent digestive tract in parasitic forms, which absorb nutrients across the body surface or through a tegument.

Anatomically, flatworms exhibit a simple organization with a central nervous system that includes a pair of cerebralganglia (brain) and longitudinal nerve cords. They possess mesodermally derived tissues called parenchyma and excretory structures called protonephridia with flame cells for osmoregulation and excretion. Many free-living species have a ciliated epidermis, while parasitic forms often have a syncytial ( multinucleate) tegument that protects against host defenses and facilitates nutrient absorption.

Taxonomically, Platyhelminthes comprises four major groups: Turbellaria (mostly free-living, including planarians), Monogenea (ectoparasites of fish), Trematoda (flukes, endoparasites), and Cestoda (tapeworms). They occupy diverse habitats, from marine and freshwater to damp terrestrial environments, and many are obligate parasites of other animals, including humans and domestic animals.

Reproduction is typically hermaphroditic, with cross-fertilization common, and many species can reproduce asexually by fission. Life cycles of parasitic flatworms are often complex, involving one or more intermediate hosts and larval stages such as miracidia, sporocysts, rediae, and cercariae.

Platyhelminthes play significant ecological roles as both predators and parasites. They have been important in medical and research contexts, particularly the planarian models used in regeneration studies.