In ectoparasitic forms there is a vagina, opening to the exterior by a pore quite distinct from that of the uterus, or there may be a pair of such structures. The uterus, in the Trematodes, is a continuation of the oviduct, and is a more or less convoluted tube usually opening side by side with the male duct, of ten into a common atrium. In forms without a separate vagina it func tions as a fertilization canal as well as a reservoir for eggs and a duct for their expulsion. In some of the ectoparasitic Trema todes a "genito-intestinal canal" connects the oviduct with the intestine. In the endoparasitic forms (Digenea) this connection is lost, but a duct, known as Laurer's canal, springing from the oviduct and either opening on the surface of the body or ending blindly below the skin, is usually present. The eggs, of ter being fertilized and supplied with yolk, are enclosed in a chitinoid shell of variable form, sometimes stalked or with terminal filaments, and very commonly provided with an operculum at one end.
The order Monogenea comprises forms of almost exclusively ectoparasitic habit, without an alternation of generations in their life-history, and having the following structural peculiarities, among others. The mouth is simple and is not surrounded by an oral sucker, though paired accessory suckers may be present in its neighbourhood. The posterior sucker may be single (but in this case is usually of complex structure and often provided with accessory chitinoid organs of attachment) or there may be a number of paired posterior suckers, which also have chitinoid armatures. There is a vagina, or a pair of vaginae, distinct from the uterus, and there are usually paired excretory pores situated dorsally near the anterior end of the body.
These forms live mainly on the external surface or on the gills of fishes and other cold-blooded aquatic animals, and feed on mucus and other matter, or occasionally on the blood of the host. The order includes 13 families, among which are the Tris tomatidae, Gyrodactylidae, Polystomatidae and Octocotylidae.
The order Digenea includes all the forms which live as endo parasites within the bodies of other animals. These have a com plex life-history involving an alternation between a sexual phase and an asexual or parthenogenetic phase of multiplication. They
have an oral sucker surrounding the mouth, and usually a posterior sucker which is a simple muscular organ without chitinoid armature. There is no vagina, as distinct from the uterus, and the excretory pore is usually single and posterior. The adult forms occur in all classes of vertebrates, the larval forms in both vertebrates and invertebrates.
The suborder Gasterostomata includes a single family (Bu cephalidae), in which the mouth is placed towards the middle of the ventral surface. All the other families (of which there are about 6o) are placed in a second suborder, Prosostomata, in which the mouth occupies its normal anterior position. The great majority of the forms in this suborder are placed in the tribe Fascioloidae, and have a general' resemblance in form to the well known liver-fluke (Fasciola). Among the best known families may be mentioned the Fasciolidae, Dicrocoeliidae, Lepo dermatidae, Opisthorchidae, Cyclocoelidae (Monostomidae), Echinostomatidae, Schistosomatidae and Paramphistomidae. The remarkable genus Aspidogaster, parasitic in molluscs, differs from all other Prosostomata in the form of the posterior sucker, which occupies the greater part of the ventral surface, and is divided up into a number of partitions. It was formerly placed in a separate order, but is regarded by Poche as forming a tribe, Aspidogastroidae of the Prosos tomata.
Development and Life-his tories.—The eggs of the Mono genea are either deposited in wa ter or attached to the host by means of stalk-like processes. In some cases (Gyrodactylus, a parasite of the gills of minnows and other fishes) but a single egg is produced at a time, and this develops within the body of the parent into an embryo, within which another embryo is formed before its birth. There may be a ciliated free-swimming larval stage, as in Polystomum (a form inhabiting the bladder of frogs and toads). The larva of this form invades the gill chamber of tadpoles, and migrates by way of the alimentary canal to its definitive habitat. Diplozoon (found on the skin of the minnow) also has a free-swimming stage, and is remarkable for the fact that two larvae (called Diporpae) unite, each holding the other by its ventral sucker, and fuse into a single X-shaped organism, the two original sets of genitalia becoming permanently interconnected. In other cases there appears to be no f ree swimming stage, and the larvae hatching from the eggs develop directly into the adult form.