The species is known to serve as host for at least 51 species of parasites, most of which are roundworms ( nematodes ), flatworms ( digeneans ) and tapeworms ( cestodes ) carried internally ; a single species of louse is its only known external parasite.

A genus of trematodes, Schistosoma, commonly known as blood-flukes and bilharzia, includes flatworms which are responsible for a highly significant parasitic infection of humans by causing the disease schistosomiasis, and is considered by the World Health Organization as the second most socioeconomically devastating parasitic disease, next only to malaria, with hundreds of millions infected worldwide.

Another phylum is Platyhelminthes, the flatworms.

Circulatory systems are absent in some animals, including flatworms ( phylum Platyhelminthes ).

Trematoda is a class within the phylum Platyhelminthes that contains two groups of parasitic flatworms, commonly referred to as " flukes ".

The Turbellaria are one of the traditional sub-divisions of the phylum Platyhelminthes ( flatworms ), and include all the sub-groups that are not exclusively parasitic.

Digenea ( Gr. Dis-double, Genos-race ) is a subclass within the Platyhelminthes consisting of parasitic flatworms with a syncytial tegument and, usually, two suckers, one ventral and one oral.

Cavalier-Smith included in Platyzoa the Phylum Platyhelminthes or flatworms, and a new phylum, Acanthognatha, into which he gathered several previously described phyla of microscopic animals.

Unlike some non-mammalian animals ( such as lizards that shed their tails, salamanders that can regrow many missing body parts, and hydras, flatworms, and starfish that can regrow entire bodies from small fragments ), once removed, human extremities do not grow back, unlike portions of some organs, such as the liver.

They are now classified into over 30 phyla, from simple organisms such as sea sponges and flatworms to complex animals such as arthropods and molluscs.

In parts of Europe, notably the Atlantic fringe of northwestern Europe, it is now locally endangered due to predation by the New Zealand flatworm ( Arthurdendyus triangulatus ) and the Australian flatworm ( Australoplana sanguinea ), two predatory flatworms accidentally introduced from New Zealand and Australia.

All higher animals ( from flatworms to humans ) are triploblastic, possessing a mesoderm in addition to the germ layers found in Diploblasts.

However, flatworms differ from more advanced animals in that their mouths are in the center of their bodies, not at the anterior end.

By observing spiral cleavage in molluscs, flatworms and annelids he concluded that the same organs came from the same group of cells and concluded that all these organisms must have a common ancestor.

Life-history stages of the trematode flatworm Fasciola hepatica from 1911 Encyclopaedia BritannicaTrematodes are small parasitic flatworms that use vertebrates as their definitive host, and molluscs ( usually freshwater snails or land snails ) as their intermediate host.

Young flatworms vary in colour from white to pale orange and develop their adult colouration as they grow.

Stichocotylidae is a monospecific family of flatworms comprising the single species Stichocotyle nephropis, which is an internal parasite of elasmobranch fish, originally described from the Norway lobster, Nephrops norvegicus, by J. T. Cunningham in 1884.

There are a few types of existing bilaterians that lack a recognizable brain, including echinoderms, tunicates, and a group of primitive flatworms called Acoelomorpha.

Free-living flatworms are mostly predators, and live in water or in shaded, humid terrestrial environments such as leaf litter.

Like flatworms they have a ciliated epidermis, but are unique in having but one cilium per cell.

it was still unclear whether they are degenerate platyhelminthes ( flatworms ) or truly-primitive, basal metazoans.

Examples in animals are more common in the ' lower ' forms such as flatworms, leeches, and brine shrimp.

Another related mechanism is that of the biologically immortal planarian flatworms, which have “ apparently limitless regenerative capacity fueled by a population of highly proliferative adult stem cells .” These organisms are biologically immortal but not immortal in the traditional sense as they are nonetheless susceptible to trauma and infectious and non-infectious disease.

Cestodes ( tapeworms ) and other parasitic flatworms have complex life-cycles, in which specific developmental stages are completed in a sequence of several different hosts.

When it comes to the fossil record, soft-bodied and minuscule invertebrates — such as hydras, jellies, flatworms, hairworms, nematodes, ribbon worms, rotifers and roundworms -- are infrequently fossilized.

Green algae are also found symbiotically in the ciliate Paramecium, and in Hydra viridis and flatworms.

They are unique among trematodes or any other flatworms in that they are dioecious with distinct sexual dimorphism between male and female.

Exclusively soft-bodied animals -- such as jellyfish, flatworms, nematodes, and insects -- are consequently rarely fossilized, as these groups do not produce hard organic parts.

Monogenea are very small parasitic flatworms mainly found on skin or gills of fish.

Sea squirts are the natural prey of many animals, including nudibranchs, flatworms, molluscs, rock crabs, sea stars, fish, birds, and sea otters.

A flame cell is a specialized excretory cell found in the simplest freshwater invertebrates, including flatworms ( except the turbellarian order Acoela ), rotifers and nemerteans ; these are the simplest animals to have a dedicated excretory system.

One experiment that was purported to show a chemical basis for memory involved training planaria ( flatworms ) to solve an extremely simple " maze ", then grinding them up and feeding them to untrained planaria to see if they would be able to learn more quickly.

The tips of these tufts enclose a flame-cell like those found in flatworms and other animals, and these probably function as excretory organs.

* Acoelomate animals, like flatworms, have no body cavity at all.

The organ level of organisation in animals can be first detected in flatworms and the more advanced phyla.

These ecosystems also include bacteria and fungi, as well as animals like flatworms and crustaceans.

Fragmentation is as a method of reproduction is seen in many organisms such as filamentous cyanobacteria, molds, lichens, many plants, and animals like sponges, acoel flatworms, some annelid worms, and sea stars.

Other species which display this trait include parasitoid wasps, certain flatworms and various aquatic invertebrates.

The parvoviruses have frequently invaded the germ lines of diverse animal species including mammals, fishes, birds, tunicates, arthropods and flatworms.

McConnell had done a series of experiments in which he taught a behavior to flatworms, ground them up, and fed them to other flatworms, which supposedly exhibited the same behavior.

Organisms showing acoelomate formation include the platyhelminthes ( flatworms, tapeworms etc.

The plexus is the characteristic form of nervous system in the coelenterates and persists with modifications in the flatworms.

While at the University of Chicago, Hyman also wrote significant taxonomic papers on such invertebrates as the Turbellaria ( flatworms ) and North American species of the freshwater cnidarian Hydra.

Fausto-Sterling also takes an interest in how flatworms ( more precisely planaria ) manage to reproduce themselves asexually.

Nudibranchs (" sea slugs ") of the genus Trapania and turbellarian flatworms prey on entoprocts.