The flat worms are named the Platyhelminths, and come in two main types, the monogenea (flukes), or the tapeworm (cestode). These two types of flat worm parasites are similar in some ways, but also have their differences. The monogenea are ectoparsites, which survive on the skin or gills of fish, whilst the tapeworms are endoparasites, which live in the intestines of hosts.
An example of a flat worm monogenean parasite of fish is Entobdella soleae, and this is a parasitic organism which lives on the lower surface of flat fish (Solea solea). Entobdella soleae feeds off the fish epidermis, through attachment via a haptor which contains hooks called sclerites.
In this way the Entobdella soleae is similar to the tapeworm in its attachment to the host, which although hooks are available in both parasites, both use a suction method to attach to their host. This causes minimal damage to the host, and also like the tapeworm, their feeding methods cause minimal damage to the host, as the epidermis regenerates quickly.
Entobdella has some differences, such as a functioning gut, which the tapeworms lack, and rather absorb nutrients through the tegument. The Entobdella parasite has no intermediate host, and it is rather host specific to Solea solea, therefore it relies on behavioural signals of its larva to locate a new host. This is not required in the tapeworm, which can have several hosts, and relies on the passive spread of eggs, and the consumption of cysticerci.
The structure of the egg shells have become adapted to the specific obstacles they are likely to meet, in Entobdella the egg shells are hardened to prevent destruction if they are consumed by a predator, yet in tapeworms the egg shell is less hostile, as the shells need to be digested to enable them to hatch within an intermediate host. There are differences between these flat worms, but their relatedness is seen through how similar their reproductive systems are, similar hooklets, similar outer layers, and both types of parasites can use fish as a host.
There are other types of flat worm flukes which are similar to both the tape worm and Entobdella, such as the human liver fluke. This is a flat worm, but has a life cycle more similar to the tapeworm (endoparasites), with an intermediate host of the snail, and fish. The liver flukes can cause damage to the human host at high levels of infection, which irritate the bile duct epithelium, causing fibrosis, and blockage. These flat worms show just how highly the flukes and the cestode are related.
-Parasitism and the Platyhelminths, by Graham Kearn (1998).