Aspidogastrea

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Aspidogastrea are a small, ancient group of parasitic flatworms (flukes) within the trematodes, with about 80 species. They are the sister group to the better-known Digenea.

Size and habitat
- They range from about 1 millimeter to a few centimeters long.
- They live as parasites in freshwater and marine molluscs and in vertebrates such as fish, turtles, and other reptiles. Maturation can occur in either the mollusc or the vertebrate host.

Distinctive features
- They have a large ventral adhesive disc with many small attachment structures (suckerlets) or a row of suckers.
- The body surface (tegument) has tiny protrusions called microtubercles.
- Their nervous system is unusually complex for flatworms and includes many sensory receptors.

Life cycle and hosts
- Most Aspidogastreans involve both a mollusc and a vertebrate host.
- Their life cycles are simpler than those of digeneans; there are typically fewer larval stages inside the mollusc, and many species can infect a wide range of hosts (low host specificity). For example, Aspidogaster conchicola can infect many freshwater mussels, snails, fishes, and turtles.

Larvae and development
- Larvae can have patches of cilia in some species, while others lack cilia.
- Some species hatch from eggs, while others do not.

Anatomy and excretion
- Like other flatworms, they use flame cells to excrete waste.
- They have dorsal excretory bladders near the posterior sucker and ducts with cilia to move waste.

Survival outside hosts
- Some species can survive outside a host for days or weeks in water or diluted seawater, suggesting an ancient, less-host-specific lifestyle. Some, such as Lobatostoma manteri, have obligate vertebrate hosts (for example, living in the intestine of certain fish).

Evolution and classification
- Aspidogastreans are viewed as ancient trematodes. They share features with other trematodes, such as a posterior sucker (now an adhesive disc) and life cycles linking molluscs and vertebrates.
- DNA studies support that they are the sister group to Digenea.
- Taxonomic grouping into two orders (Aspidogastrida and Stichocotylida) is debated because the groups are very similar.