Phylum Platyhelminthes is a taxonomic group of acoelomate animals whose classes are referred to monolithically as platyhelminths or flatworms. They are known for being the largest taxonomic group in the clade Bilateria. They possesses the most primitive organisms that evolved bilateral symmetry, cephalization, and three distinct germ layers while lacking segmentation and vertebrae which are commonly attributed to other phyla in Bilateria.
That being said, this phylumβs characteristic trait is its dorsoventral flatness. Platyhelminths are so flat that their cells are positioned either directly next to their environmentβcommonly one which is marine or nutrient-richβor on the lining of their incomplete gut. They have a greater surface area to volume ratio, and their shape and ratio allows them to use their entire body for diffusion and nutrient exchange.
Platyhelminths are triploblastic. Their embryos undergo spiral, determinate cleavage; their blastopores develop early into a mouth, as their mesoderm develops next to their blastopores. Their mesoderm develops into their muscles, gonads, and the parenchyma, here referring to a βfillerβ tissue unique to platyhelminths. Although they are primitive and do not possess fully evolved organ systems seen in many Vertebrata phyla, they do possess organs and several organ systems. They have a developed excretory system known as the protonephridia. This organismal development does not fully extend to their digestive system. Most platyhelminths have an incomplete digestive system. They possess a digestive tract with a pharynx and esophagus which leads to an incomplete gut and a blind-ended gastrovascular cavity, causing slow digestion and excretion. Some members, such as those of class Cestoda, have less developed digestive systems due to their parasitic dependence. In addition, all flatworms do not have a circulatory system or a respiratory system.
All platyhelminths are monoecious. Most perform both sexual and asexual reproduction. In the case of the former, they rarely use their own gonads to self-fertilize. Parasitic platyhelminths have incredible reproductive capacity without resorting to self-fertilization. In several of their early life stages, a platyhelminth will use polyembryony to produce many offspring from a single egg. In later stages, they perform reciprocal mating with local platyhelminths to increase the number of offspring produced. Some, but not all flatworms can perform reproduction through fission, such as Planaria.
All platyhelminths possess a degree of forward-traveling motility. Turbellaria are free-living and locomote for their entire life. The other flatworms, which are all parasitic, possess motility as larvae, but they lose it when they become adults. In adulthood, parasitic flatworms attach to a host and become sessile. This loss promotes platyhelminthic monoecy because reciprocal mating allows local platyhelminth to survive and reproduce after attaching to a common host.
Phyla Platyhelminthes is divided into three major classes and a fourth minor class. These classes are sectioned by the degrees of their parasitism, an evolution behavior found in many flatworms.
- Class Turbellaria comprises of all free-living flatworms. These flatworms are not parasitic and have evolved to survive on their own. They have the most developed cephalization of the phylum.
- Class Monogenea comprises of ectoparasitic flatworms who use simple life cycles without intermediate hosts to survive and reproduce. They are primarily found attached to marine definitive hosts.
- Class Trematoda comprises of endoparasitic flatworms (commonly referred to as flukes) who use complex life cycles with multiple intermediate hosts to survive and reproduce. They have less developed cephalization than Turbellaria.
- Class Cestoda comprises of endoparasitic flatworms (commonly referred to as tapeworms) who use complex life cycles with a single intermediate host to survive and reproduce. They have the least developed cephalization: their anterior and posterior sides are indistinguishable.