The former group contains two families of unsegmented forms, parasitic in fishes : (I) Amphilinidae. These are leaf-shaped or ribbon-shaped forms, with a more or less well developed proboscis-like organ at the "anterior" extremity, and with the openings of the male and female ducts near the "posterior" end. The long, tubular uterus has an independent pore near the anterior end. These worms inhabit the body-cavity of certain fresh-water fishes.
(2) Gyrocotylidae. These are remarkable forms with frilled margins and with a rosette-like organ at the "posterior" extremity, and having all three genital openings near the "anterior" end. The adults inhabit the intestine of certain marine fishes (the Holo cephali), and the intermediate hosts appear to be lamellibranch molluscs.
The Taenioinei are a large assemblage of forms, including about 3o families. They are divided by Poche into four orders : I. Bothriocephalidea. This order includes one family of un segmented forms (the Caryophyllaeidae) and several families of strobilate forms, the most important of which are the Diphyllo bothriidae and the Bothriocephalidae. In the most typical mem bers of this order the scolex is provided with a pair of longitudinal slit-like bothria, situated dorsally and ventrally, and the genital apertures are on the ventral surface.
II. Echinobothriidea, containing only the family Echinoboth riidae, parasitic in elasmobranch fishes.
III. Tetrarhynchidea, containing three families, of which the most typical is the Tentaculariidae (or Tetrarhynchidae). These are forms in which the scolex, in addition to two or four bothria, bears four retractile proboscides armed throughout with hooks or spines. The adults are intestinal parasites of elasmobranch fishes.
IV. Taeniidea. This is the largest and most important order, and may be divided into two suborders, Phyllobothriinea and Taeniinea, and some 17 families. The Phyllobothriinea, in the adult stage, are chiefly parasites of cold-blooded, the Taeniinea of warm-blooded vertebrates. Of the families contained in the former suborder the best-known are the Phyllobothriidae and Proteo cephalidae, while dr Taeniinea include several large and important families, such as the Anoplocephalidae, Davaineidae, Dilepididae, Hymenolepididae, and Taeniidae.
certain Amphilinoinei, however, the embryo has ten hooks. The embryonic hooks are used as levers, chiefly in burrowing among the tissues of the intermediate host.
In some of the Bothriocephalidea the embryo is provided with an external ciliated envelope and hatches in water, where it swims about by means of its cilia. In the human parasite Diphylloboth riumlatum, for example, the ciliated embryo is at first free in fresh water and is then swallowed by a small Copepod (Cyclops or Diap tomus). Shedding its ciliated coat, the embryo penetrates into the body-cavity of the Copepod, and develops into a more elongate form called the procercoid. If the Copepod is swallowed by a suit able fish, such as a pike, perch or trout, the procercoid develops further among its tissues into a plerocercoid, and is then infective for the final host, infection being acquired by eating the fish in a raw or imperfectly cooked state. For the development of this species, therefore, and of others related to it, two successive changes of host are required. The larval forms known as Sparga num, which occur in many land vertebrates, including man, have been shown to be the plerocercoids of Diphyllobothrium.
The larval forms of Tetrarhynchidea, resembling the scolex of the adult, without the strobila, or with only an unsegmented ap pendage, occur encysted among the tissues of various marine animals, chiefly teleostean fishes.
Among the best known of the Taeniidea the developmental his tory is quite different. The human parasite Taenia solium may be taken as typical. If an egg of this worm be swallowed by the proper intermediate host (in this case the pig) the hexacanth em bryo is liberated in the intestine and proceeds to pierce the intes tinal wall, wandering about among the tissues and usually coming to rest among the muscles. Here it grows into a small bladder, in whose wall a depression or invagination appears on the side opposite to the embryonic hooks. At the bottom of this invagina tion five thickenings appear, destined to become the four suckers and the median rostellum. Finally the hooks are developed on the rostellum, and the bladder-worm, or cysticercus, is fully formed. Should this be ingested by a human being with raw or imperfectly cooked pork, the "head" is evaginated and attaches itself to the lining of the intestine, and the "neck" joining it to the bladder begins to form a series of proglottides. Another human parasite, Taenia saginata, has a similar development, but is without hooks, and makes use of the ox as intermediate host.