This completes their life history as observed by Haeckel, who found it easy to retain them in his glasses in perfect health, and who watched them closely.
As another illustration I may take the Magosphæra planula, discovered by Haeckel on the coast of Norway.
In one stage of its existence (Pl. [V], Fig. 10) it is a minute mass of gelatinous matter, which continually alters its form, moves about, feeds, and in fact behaves altogether like the Amœba just described. It does not, however, remain always in this condition. After a while it contracts into a spherical form (Pl. [V], Fig. ii), and secretes round itself a structureless envelope, which, with the nucleus, gives it a very close resemblance to a minute egg.
Gradually the nucleus divides, and the protoplasm also separates into two spherules (Pl. [V], Fig. 12); these two subdivide into four (Pl. [V], Fig. 13), and so on (Pl. 5, Fig 14), until at length thirty-two are present, compressed into a more or less polygonal form (Pl. [V], Fig. 15). Here this process ends. The separate spherules now begin to lose their smooth outline, to throw out processes, and to show amœboid movements like those of the creatures just described. The processes or pseudopods grow gradually longer, thinner, and more pointed. Their movements become more active, until at length they take the form of ciliæ. The spherical Magosphæra, the upper surface of which has thus become covered with ciliæ, now begins to rotate within the cyst or envelope, which at length gives way and sets free the contained sphere, which then swims about freely in the water (Pl. [V], Fig. 16), thus closely resembling Synura, or one of the Volvocineæ. After swimming about in this condition for a certain time, the sphere breaks up into the separate cells of which it is composed (Pl. [V], Fig. 17). As long as the individual cells remained together, they had undergone no changes of form, but after separating they show considerable contractility, and gradually alter their form, until they become undistinguishable from true Amœbæ (Pl. [V], Fig. 18). Finally, according to Haeckel, these amœboid bodies, after living for a certain time in this condition, return to a state of rest, again contract into a spherical form, and secrete round themselves a structureless envelope. The life history of some other low organisms, as for instance Gregarina, is of a similar character.
It may be said, and said truly, that the difference between such beings as these and the Campodea, or Tardigrade, is immense. But if it be considered incredible that even during the long lapse of geological time such great changes should have taken place as are implied in the belief that there is genetic connection between them and these lower groups, let us consider what happens under our eyes in the development of each one of these little creatures in the proverbially short space of their individual life.
I will take for instance the first stages, and for the sake of brevity only the first stages, of the life-history of a Tardigrade.[77] As shown in Fig. [60], the egg is at first a round body or cell, with a clear central nucleus—the germinal vesicle; it increases in size, and after a while the yolk and the germinal vesicle divide into two (Fig. [61]), then into four (Fig. [62]), and so on, just as we have seen to be the case in Magosphæra. From the minute cells (Fig. [63]) arising through this process of yolk-segmentation, the body of the Tardigrade is then built up.[78]
Fig. 60, Egg of Tardigrade, Kaufmann, Zeit f. Wiss. Zool. 1851, Pl. 1. 61, Egg of Tardigrade after the yolk has subdivided. 62, Egg of Tardigrade in the next stage. 63, Egg of Tardigrade more advanced.