The flatworms have diverged considerably in the structure of their digestive systems and in their attachment organs. The reproductive system has a common basic structure and set of connections, with variations mainly taking the form of different numbers and spatial arrangements of components.
There is no body cavity; the body is covered externally by a tegument, and internal organs are suspended within a parenchyma. The body surface may be smooth or spined. There is usually some modification of the anterior end, in the form of suckers, spines, hooks or glands, to aid in attachment to the host, and other attachment organs may be present at mid- or hind-body. As the common name implies, the body is usually flattened, but some species are nearly round in cross section.
The digestive system in some taxa is incomplete, with a mouth, esophagus, pharynx and blind- ending intestinal ceca. In other taxa, there is no traditional digestive system. Excretory ducts are present, emptying into an excretory bladder in some taxa. A nervous system is present, although it is difficult to visualize. Photoreceptors are present in free-living stages of some taxa.
Typically, platyhelminths are hermaphroditic, with one or more complete sets of reproductive systems per individual. A few taxa are dioecious. Specialized accessory reproductive structures occur in many taxa, in addition to the basic features described below.
The male reproductive system comprises various numbers of testes, connected by vasa efferentia to a common vas deferens, which then enters a cirrus pouch and then a cirrus. The vas deferens may be enlarged for sperm storage outside and inside the cirrus pouch, called external and internal seminal vesicles, respectively. The cirrus, which delivers sperm to the female system, may be retractable and may be covered with spines.
The female reproductive system comprises a number of structures meeting in a region called the ootype. A vagina may be present to receive sperm and conduct it to the ootype. An ovary produces oocytes which are carried to the ootype via an oviduct. Vitelline glands produce nutritive and shell materials that are carried via vitelline ducts to the ootype where they are incorporated into eggs. A Mehli's gland may surround the ootype. Fully-formed eggs exit the ootype and accumulate in a uterus. The uterus may contain a uterine pore. In species lacking a vagina, sperm are received via the uterine pore.
Eggs may be thin or thick-shelled, and may be embryonated or unembryonated when laid. The larva that develops within differs depending on the group of platyhelminth. There may be an operculum to aid the escape of the larva.
The monogenean oncomiracidium is ciliated, has eyespots, a rudimentary digestive system and an opisthaptor to attach to its new host.
The larva of aspidogastreans is ciliated and has a rudimentary digestive system. There is a rudimentary posterior sucking disk.
The digenean miracidium is ciliated, and its prominent internal features include eyespots, various penetration glands, and numerous excretory cells and tubules. Sporocysts are simple, tubular or branched germinal sacs. Rediae are tubular germinal sacs which possess a rudimentary digestive system, a birth pore, and small ambulatory appendages. Cercariae resemble miniature adults, with a similar body shape, digestive system and ornamentation to the adults they will become. Cercariae have only the rudiments of their reproductive systems, but most possess some sort of penetration glands or stylets near the mouth, and some sort of tail (which takes on numerous forms and is useful for identification.) Metacercariae usually resemble the cercariae from which they developed, without the tail, and may be enclosed in a clear cyst.
The cestode oncosphere is undifferentiated except for the presence of three pairs of small hooks. The coracidium is simply an oncosphere surrounded by a layer of ciliated cells. The procercoid is ellipsoid in shape with penetration glands at one end and a cercomer at the other end. Plerocercoids are long and undifferentiated except for the scolex, which now has its adult form. Cysticercoids are solid-bodied larvae with a fully-developed scolex enclosed in a series of tissue layers, and an elongate cercomer. Bladder larvae have one or more scoleces developed within a fluid-filled capsule. The cysticercus contains a single, invaginated scolex. The coenurus has multiple invaginated scoleces. The strobilocercus has a single scolex attached to the bladder by a long section of strobila. Hydatids have a complex cyst internal (and sometimes external) brood capsules lined with a germinal epithelium, and numerous small protoscoleces.