The monogenetic Trematoda frequent a single host and are ecto-parasitic, i. e. do not as a rule inhabit the internal organs of this host. The exceptions are Calicotyle, which has been found in the cloaca of Rays; Aspidogaster conckicola, which inhabits the nephridia of the Lamellibranch Anodonta, and A. limacoides from the intestines of the Teleosteans, Leu-ciscus idus and L. dobula; Cotylaspis from the American Anodonta; Poly-stomum integerrinium from the bladder of Rana temporaria, R. escidenta and Bufo viridis, and P. ocellatum from the pharynx of the Chelonian Emys europaea. The other genera are found for the most part attached to the gills of various fishes, or on the surface of their bodies, e. g. Tristomum Molae. Udonella Caligorum lives on the various species of the Crustacean Caligus. The majority are marine. P. integerrirnum, Diplozoon and Gyrodactylus are severally peculiar. The first-named attaches itself as an embryo to the internal gills of frog-tadpoles. When the gills atrophy at the time that the tadpole changes into a frog, the young parasite migrates through the digestive tract into the bladder, where it becomes sexually mature in three years, but does not attain its full size for five or six years.

But if the embryo attaches itself to the gills of a very young tadpole, it undergoes a premature sexual development, does not migrate, and dies when the tadpole undergoes metamorphosis (cf. note 1, p. 648). Diplozoon paradoxum consists of two individuals fused together. The embryo known as Diporpa is at first free-swimming; it soon loses its cilia, and settles on the gills of a Minnow; loses its eyes, but lives in a single condition for weeks or months; but finally one individual attaches itself by its ventral sucker to a conical eminence on the back of a second individual, which thereupon so twists itself as to fix the first individual in the same manner. The cones and suckers fuse completely; in other respects, however, the two Diporpae which make up a single Diplozoon are independent of one another. Gyrodactylus elegans, found on the gills and fins of various freshwater fish, is viviparous, but the embryo before it is extruded, itself contains an embryo, and this in turn another, so that three generations of embryoes are represented simultaneously.

The digenetic Trematoda have at least one intermediate generation, but as a rule very many before the sexual organism reappears. The embryo, whether it is or is not ciliated, enters first a host which is very rarely some Fish, but usually a Mollusc, either a Gastropod or Lamellibranch, but not necessarily always the same Mollusc for the same species of fluke 1. The egg-shell with the embryo may be swallowed by a snail and the embryo set free in the alimentary canal, a mode by which land Molluscs especially may be liable to infection. After its entry it becomes in some instances a Sporocyst, in others a Redia, which must be regarded as dimorphic forms, and are sometimes spoken of as 'nurses.' Both possess a cuticle, a delicate layer of circular and longitudinal muscle fibres, and a layer of cells covering the latter internally and bounding a cavity sometimes crossed by trabeculae of connective tissue in which lie developing germs in all stages 1. Flame-cells have been detected in the body-walls. A Redia also possesses a digestive tract, i. e. a muscular pharynx (? = oral sucker 2) and a caecal intestine, as well as a special opening for the birth of the germs rarely present in Sporocysts. It has also two short processes, one on either side of the posterior extremity of the body, as well as a projecting ring near its anterior extremity3. The Sporocyst possesses the power, at least in some instances, of multiplying either by transverse fission, as e. g. in F. hepatica, and the Cercariaeum Limacis not uncommon in the slug Avion ater; or by gemmation, producing branched structures, as in the genera known as Bucephalus (= G asterostomum) inhabiting Anodonta and the Oyster, and Leucochloridium (= D. macrostomum) inhabiting the Gastropod Succinea amphibia.

The germs to which a Sporocyst gives origin may develope in some instances into Sporocysts, in others into Rediae or into Cercariae; those which originate from a Redia may develope into either Rediae or into Cercariae. And it does not seem certain that there is any limit to the possible number of successive generations of Rediae. Both Cercariae and Rediae may occur side by side in the same nurse. The last term in the series is, however, invariably a Cercaria. The germs which give origin to these various generations arise from two sources: the first, cells which occupy the central region of the young Sporocyst or Redia; the second, the epithelium lining the body-walls, single cells of which enlarge, divide, form morulae, and drop into the cavity of the body to undergo further development4. An invagination without a lumen to form a digestive canal has been observed in the germs of F. hepatica; and a delicate pellicle has been seen surrounding the germ and young Cercaria.

1Sporocysts are occasionally met with in Fish. A free swimming Sporocyst has been observed by Ramsay Wright (American Naturalist, xix. 1885, p. 57).

1The Sporocyst may become enveloped in an adventitious epithelial coat or 'paletot,' formed from the blood corpuscles of its host (Biehringer).

2A rudimentary oral sucker is present in some Sporocysts (Biehringer).

3For the structure of this ring and the mistakes that older observers have fallen into respecting it, see Thomas, Q. J. M. xxiii. pp. 121-2, and note.

4Leuckart restricts (A. N. 48, 1, 1882, p. 95, p. 100) the origin of germs to cells with large nuclei (germinal cells) occupying the central region of the 'nurse'. The cells in question have no share in the growth of the nurse itself. His view appears to be adopted by Schwarze. But as Biehringer points out (I) there are great differences of size in the developing germs within the same 'nurse,' too great to be explained by a temporary arrest of development of some of them; and (2) it is scarcely possible to extend the view to such cases as the branched Sporocyst of Bucephalus, and he might have added, to Leucochloridium and such Sporocysts as multiply by repeated fission, e. g. Cercariaeum Limacis. The origin of germs from the epithelium lining the body-cavity has been established beyond doubt by Biehringer himself in the Sporocysts of Cercaria macrocerca from the gills of Cyclas, and by Thomas in the 'nurse' forms of F. hepatica.

It is probably derived from the surface of the germ, as in the case of the embryo (ante, p. 649).