The Rediæ and Sporocysts rapidly grow in size and sometimes increase by transverse division. In the course of their further development one of two things may happen. They may either (1) develop fresh Rediæ or Sporocysts by a process of internal budding ([fig. 95] C); or else (2) there may be formed in them, by an analogous process, larvæ with long tails known as Cercariæ ([fig. 95] D.) The direct development of Cercariæ is the usual course, though in Distomum globiparum the reverse is true; but where this does not take place the Rediæ or Sporocysts of the second generation give rise to Cercariæ.
The Cercariæ are developed from spherical masses of cells found in the body cavity of the Sporocyst or Redia. The exact origin of these masses is still somewhat obscure, but they are stated by Wagener (No. [212]) to be derived from the body wall. They are probably to be regarded as internal buds.
The spherical bodies grow rapidly in size, their posterior extremity is prolonged into a process which forms the tail, while the anterior part forms the trunk. When fully formed ([fig. 95] E), the trunk has very much the organization of an adult Distomum. There is an anterior and a ventral sucker, the former of which contains the opening of the mouth, and is often provided with a special chitinous armature. The mouth leads into a muscular pharynx, and this into a bilobed cæcal alimentary tract. An excretory system of the ordinary type is present, consisting of longitudinal contractile trunks continuous anteriorly with branched ciliated canals, which, as has recently been shewn by Bütschli, may be provided with funnel-shaped ciliated internal openings[94]. The contractile trunks unite posteriorly, but instead of opening directly to the exterior are prolonged into a vessel which traverses the substance of the tail, and after a longer or shorter course bifurcates into two branches which open laterally.
The tail is provided with an axial rod of hyaline connective tissue, like the notochord of the tail of a larval Ascidian, and is frequently provided with membranous expansions. It is used as a swimming organ. Beneath the epidermis are layers of circular and longitudinal muscular fibres, the latter arranged in the tail as two bands.
The Cercariæ when fully developed leave the Sporocyst or Redia, and then their host, and become free. In most Rediæ there is a special opening, not far from the mouth, by which they pass out. There is no such opening in Sporocysts, but the Cercariæ bore their way through the walls.
After leaving their parent the Cercariæ pass into the external medium, and for a short period have a free existence. They soon however enter a new host, making their way into its body by a process of boring, which is effected by the head (especially when armed with chitinous processes) assisted by movements of the tail.
The second host is usually some Invertebrate (Mollusc, Worm, Crustacean, Insect larva, &c.), but occasionally a Fish or Amphibian or even a vegetable. The tail is very often lost as the Cercaria bores its way into its host, but whether it is so or not, the Cercaria, after it has once reached a suitable post in its new host, assumes a quiescent condition, and surrounds itself with a many-layered capsule. The cephalic armature and tail (if still present) are then exuviated, and the generative organs gradually become apparent though very small. In other respects the organization is not much altered.
Though an encysted Cercaria may remain some months without further change, it eventually dies unless it be introduced into its permanent vertebrate host, an act which is usually effected by the host in which it is encysted being devoured. It then becomes freed from its capsule as a fully formed Trematode, in which the generative organs rapidly complete their development.
In some cases the Rediæ or Sporocysts do not give rise to tailed Cercariæ, but to tailless forms. In such cases, as a rule, the encystment takes place in the host of the Redia or Sporocyst, but the tailless larvæ sometimes pass through a free stage like the Cercariæ. In the case of Distomum cygnoides, parasitic in the bladder of the Frog, the Cercaria passes directly into the adult host without the intervention of an intermediate host.
The life history of a typical entoparasitic Trematode is shortly as follows: