Thus there are two kinds of distomes: the first live in couples in a cyst, the second in couples joined together, but at liberty; and in each case only one individual produces eggs. These are distomes which act really like dioecious worms. We find, however, a more remarkable instance in the _Monostomum bijugum_ of Miescher. In the tumours which are formed in the beak of the grosbeak (_Fringilla_), he has constantly found two individuals; and in many cases he has surprised them with the penis of one engaged in the sexual organ of its companion. These worms, while they live in couples, resemble each other like snails and leeches; they are mutually fecundated, and both lay eggs. Leuckart recognized these sexual distomes in their cyst, in the larvæ of ephemerides; and Linstow noticed a distome thus sexual and encysted in the _Gammarus pulex_. The name of Monostoma has been given to some of these trematodes which have no abdominal sucker. One of the most curious worms of this group is the _Monostomum mutabile_. It lives in the sub-orbitary sinus of several aquatic birds; that is to say, in the nasal fossæ, especially of water-rails and moorhens. We give a slightly magnified representation of them. It is a worm resembling an elongated leaf. By compressing it slightly 202 on the stage of the microscope, we easily discover the ovary, the matrix, and oviduct full of eggs. By isolating some of the eggs, and crushing them gently to break the shell, we set free the worm (Fig. 44), quite different from the mother (Fig. 43). The former has two eyes surrounded by a ciliated mantle, and by means of this ciliated envelope, the monostome swims freely in the water. If we compress it slightly, we see that in the interior of the ciliated covering, there is still another animal, without eyes, without ciliæ, and of an entirely different form, which in its turn encloses a whole progeny. [Illustration: Fig. 43.--Monostomum mutabile (adult).] [Illustration: Fig. 44.--Monostomum mutabile. Ciliated embryo with sporocyst and young cercariæ, greatly magnified.] The embryo, having long ciliæ in front, and in the interior a sporocyst already full of young cercariæ, is shown in Fig. 44. It is this latter creature which the ciliated embryo must confide to the care of others; this she puts out to nurse with some mollusc or other, until it is fit to provide for itself in its turn. We have still to discover the train by which the parasite must travel, in order to 203 arrive again at the nasal fossæ which are the first cradle of the family.