Each of these animalcules develops a sac in its interior. The sac you may notice in the engraving. Having managed to get into the body of the water-snail, the animalcule’s part in the drama is at an end. It dies, and in dying liberates the sac, which is very comfortably housed and fed by the snail. If you examine this sac (Fig. 5), you will observe that it has a mouth and digestive tube, and is, therefore, very far from being, what its name imports, a mere receptacle; it is an independent animal, and lives an independent life. It feeds generously on the juices of the snail, and having fed, thinks generously of the coming generations. It was born inside the animalcule; why should it not in turn give birth to children of its own? To found a dynasty, to scatter progeny over the bounteous earth, is a worthy ambition. The mysterious agency of Reproduction begins in this sac-animal; and in a short while a brood of _Cercariæ_ move within it. The sac bursts, and the brood escapes. But how is this? The children are by no means the “very image” of their parent. They are not sacs, nor in the least resembling sacs, as you see. (Fig. 6.) [Illustration: Fig. 4. A EMBRYO OF MONOSTOMUM MUTABILE. B Cercaria sac, just set free. _a_ Mouth; _b_ Pigment spots; _c_ Sac.--Magnified.] They have tails, and suckers, and sharp boring instruments, with other organs which their parent was without. To look at them you would as soon suspect a shrimp to be the progeny of an oyster, as these to be the progeny of the sac-animal. And what makes the paradox more paradoxical is, that not only are the _Cercariæ_ unlike their parent, but their parent was equally unlike its parent the embryo of _Monostomum_ (compare fig. 4). However, if we pursue this family history, we shall find the genealogy rights itself at last, and that this Cercaria will develop in the body of some bird into a _Monostomum mutabile_ like its ancestor. Thus the worm produces an animalcule, which produces a sac-animal, which produces a Cercaria, which becomes a worm exactly resembling its great-grandfather. [Illustration: Fig. 5. CERCARIA SAC. A Mouth; B Digestive tube; C A Cercaria newly formed. Four others are seen in different stages.--Magnified.] [Illustration: Fig. 6. CERCARIA DEVELOPED. A Mouth; B, B, B Excretory organ; C Pigment spots; D Tail. ]