We may also mention among the distomes a species from fish, which has a great affinity with the singular distome observed by Bilharz, of which we have spoken above. This distome inhabits the “castagnole,” or Brama raii. Under the opercula of this fish, the skin is folded, and forms one or more pouches, in each of which lives a
coupled distome, that is to say, by the side of each large and fat individual, full of eggs, there is one which is slender. It is the Distomum filicolle, to which the name of Monostomum was at first given. We should be correct in supposing that of these two hermaphrodite worms one acts rather as a female, the other as a male. It is doubtless in this sense that Steenstrup maintained his assertion, that there are in nature no hermaphrodites.
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 diœcious 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 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.
Fig. 43.—Monostomum mutabile (adult).