The free-swimming or creeping embryos make their way into or on to the body of some invertebrate (occasionally vertebrate) form, usually a Mollusc, to undergo the first stage in their metamorphosis. They may either do this on the gills of their host, or very frequently they bore their way into the interior of the body. Soon after the larvæ have reached a satisfactory position the epidermis becomes stripped off, and there emerges a second larval form developed in the interior of the first larva, much as a Nemertine is developed within the larva of Desor. In the case of Monostomum mutabile the new worm is, as stated above, fully formed within the ciliated larva at the time of hatching. The worm which proceeds from the above metamorphosis has different characters corresponding with those of the larva from which it proceeded. If the original larva had an alimentary canal it has one also, and then grows into the form known as a Redia (Fig. 95, B and C). The Redia has anteriorly a mouth leading into a muscular pharynx and thence into a cæcal stomach. Posteriorly the body is prolonged into a kind of blunt caudal process, at the commencement of which are a pair of lateral papillæ. There is a perivisceral cavity, and the body walls are traversed by excretory tubes. If the original larva is without an alimentary tract, the second form becomes what is known as a Sporocyst. The Sporocyst is a simple elongated sack with a central body cavity; when derived from the metamorphosis of a ciliated embryo its walls are provided with excretory tubes, but such tubes are absent in Sporocysts developed from non-ciliated larvæ. Some Sporocysts send out numerous branches amongst the viscera of their hosts. [FIG. 95. VARIOUS STAGES IN THE METAMORPHOSIS OF THE DISTOMEÆ (from Huxley.) _A._ Ciliated larva of Monostomum mutabile. _a._ larval skin. _b._ Redia developed within it. _B._ Redia of Monostomum mutabile. _C._ Redia of Distomum pacificum, with germs of a second brood of Rediæ. _D._ Redia containing Cercariæ. _E._ Cercaria. _F._ Full-grown Distomum.] 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.