These parasites' name refers to the cavities in their holdfasts (Greek hole), which resemble suckers and anchor them within their hosts.
* The skin of all species is a syncitium, a layer of cells that shares a single external membrane.
* Trematodes are divided into two groups, Digenea and Aspidogastrea (also known as Aspodibothrea).
Of about 1,100 species of monogeneans, most are external parasites that require particular host species, mainly fish, but in some cases amphibians or aquatic reptiles.
* However, a few are internal parasites. Adult monogeneans have large attachment organs at the rear, haptors (Greek haptein, means "catch"), which have suckers, clamps, and hooks.
* They often have flattened bodies. In some species, the pharynx secretes enzymes to digest the host's skin, allowing the parasite to feed on blood and cellular debris. Others graze externally on mucus and flakes of the hosts' skins. The name "Monogenea" is based on the fact that these parasites have only one nonlarval generation.
These are often called tapeworms because of their flat, slender but very long bodies – the name "cestode" is derived from the Latin word cestus, which means "tape".
* The adults of all 3,400 cestode species are internal parasites. Cestodes have no mouths or guts, and the syncitial skin absorbs nutrients – mainly carbohydrates and amino acids – from the host, and also disguises it chemically to avoid attacks by the host's immune system.
* Shortage of carbohydrates in the host's diet stunts the growth of the parasites and kills some. Their metabolisms generally use simple but inefficient chemical processes, and they compensate by consuming large amounts of food relative to their size.