The Cercaria has a body shaped like that of a sexual Trematode, though its proportions may undergo change in subsequent growth. It possesses the rudiments of suckers, digestive tract, and the excretory system, and is furnished with a tail, or in Bucephalus with two tails, attached to the body posteriorly, by means of which it swims when it is set free from its parent and its parent's host. The tail is sometimes of great length, and may be bifid at its extremity. It is in some marine forms armed with spines set in rings or tufts. Occasionally it is nearly obsolete, or even quite so, as in Leucochloridium, and then the Cercaria is probably always quiescent and only passes into its final host when the latter happens to devour its parent, as e.g. in the case named. The Cercaria is sometimes armed (except it is said when derived from Rediae) with a boring spine, close to which glands open on either side. The glands have been supposed to secrete either a poison or a cystogenous substance. The free Cercaria becomes after a time quiescent, and either encysts itself in the same host that contains its parent, in another intermediate host, usually non-Vertebrate, sometimes Vertebrate, e. g. the tadpole of frogs1 or on some foreign object, e. g. on stems of grass, etc, in the case of Fasciola hepatica2. Encysted Cer-cariae have been found in very various aquatic non-Vertebrates. The cyst is formed by the Cercaria itself, and its material is derived from the secretion of gland-cells. In some forms, e. g.
F. hepatica, the gland-cells in question contain rods. These rods may be found afterwards in the walls of the cyst. The tail is cast off during the process of encystment 3. The Cercaria now grows in size and becomes a sexually immature fluke. A period of rest for this purpose seems necessary, and then as soon as the encysted fluke is swallowed by its final host, a Vertebrate, it attains a sexual condition in the intestine of the latter. It is very rare for it to do so in the encysted condition and to produce ova, as do D. cirrigerum from Astacus and D. agamos from Gammarus pulex. It has been stated that the same Cercaria may give origin to different sexual forms in different hosts (Ercolani), but as yet these statements have not been confirmed. A difference in size between specimens of D. macrostomum from the intestines of Songsters and Water-rails has been noted.
1 The non-sexual forms ( = Diplostomum; Tetracotyle) of Holostomum are frequently met with in various Vertebrata.
2 D. nodulosum is found in the sexual form in the intestines of various fresh-water fishes, e. g. of the Pope, Aerina cernua. Its Cercaria sometimes occurs encysted on the outer surface of the intestine of this fish. It probably enters it at an immature stage in this case, i. e. before undergoing encystation in the Paludina, which is the first host. See von Linstow, A. N. H. (4), xii.
3 It has been stated by several authorities, as by Pagenstecher and Ercolani, that the tail of a Cercaria may in some instances become a Sporocyst and produce germs. The structure of the tail does not favour this supposition. According to Schwarze, it consists of a 'contractile substance occupying the axis and periphery with large vesicular cells between.' Compare Ziegler on the tail of Bucephalus, Z. W. Z. xxxix. 1883, pp. 558-562. For instances to the point, see the work of Ercolani, Dell' adattemento, etc. cited below.
The genus Holostomum, usually classed with the digenetic Trematoda, has perhaps a direct development. The sexual worm inhabits the digestive tract of various carnivorous birds. The organisms known as Diplostomum and Tetracotyle, which occur encysted in various Vertebrata, are believed by Von Linstow and others to be the non-sexual states of this genus. The ova are largev The larva of H. comucopiae is ciliated, except anteriorly, and has two eye-specks.
Digenetic Trematoda are found entoparasitic in members of all classes of Chordata. They usually inhabit various parts of the digestive tract. Man may be infested by Fasciola hepatica (ramifications of bile ducts); by Distoma lanceolatum (gall-bladder and bile ducts); by D. crassum (duodenum) in India and China: by D. Ringeri in the lungs, causing the endemic haemoptysis of Formosa; and by several Distomata of rare occurrence, together with the formidable Bilharzia haematobia, which lives in the blood-vessels of the urinary bladder, mesentery and portal system, and is widely distributed over the African continent. The ova of the last-named escape with the urine. Ruminants are principally infested by Fasciola hepatica and by D. lanceolatum, which cause great and widespread destruction. The former is the source of the disease known in this country as rot or liver-rot. Its embryo enters the small amphibious Pulmonate, Limnaeus truncatulus, in which it becomes a Sporocyst. The latter in turn gives origin to several generations of Rediae. The Cercaria is set free and encysts itself on some foreign object, - the stems of water-plants and grass, which are eventually devoured by sheep, cows, etc.
A very large number of species of digenetic Trematoda are known which can be arranged under relatively few genera.
The Class Trematoda may be divided as follows
1. Monogenetic Trematoda; development direct.
(a) Tristomeae: body roundish or elongate; posterior extremity of body never specially developed. Two adoral suckers often present; a large ventral sucker often armed with chitinoid structures. Sexual apertures on the left side or admedian. Laurer-Stieda canal single or double. Ova with a filament at one pole only.
Three sub-families: Tristomidae, with the single genus Tristomum-, Mono-cotylidae, with Calicotyle, Pseudocotylei and Monocotyle; Udoneltidae, with the single genus Udonella.
(b) Polystomeae: body elongate, pointed and narrow anteriorly; broad behind, and generally provided with special organs of adhesion in the shape of suckers or chitinoid hooks, of suckers or claspers with chitinoid structures. Two adoral suckers in some instances. Sexual apertures median. Laurer-Stieda canal single or double. Male sexual aperture often armed with chitinoid hooks. Ova frequently provided with two long appendages.