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 arrive again at the nasal fossæ which are the first cradle of the family.
We find occasionally between the feathers of some birds tubercles of the size of a pea, and when we open them we see in each two similar worms, placed so that the stomach of one is applied to that of the other; this is the monostome of which we have spoken above. These worms are from three to four millimètres in length (about ·13 in.), and are found in the titmouse, the siskin, the sparrow, the canary, and some other birds.
Fig. 45.—Cercaria of Amphistomum sub-clavatum.