We may take, as an illustration of the lower worms, the genus Prorhynchus (Fig. [59]), which consists of a hollow cylindrical body, containing a straight simple tube, the digestive organ.
But however simple such a creature as this may be, there are others which are far less complex, far less differentiated; which therefore, on Mr. Darwin’s principles, may be considered still more closely to represent the primæval ancestor from which these more highly-developed types have been derived, and which, in spite of their great antiquity—in spite of, or perhaps in consequence of, their simplicity, still maintain themselves almost unaltered.
Thus the form which Haeckel has described[74] under the name Protamœba primitiva, Pl. [V], Fig. 1-5, consists of a homogeneous and structureless substance, which continually alters its form; putting out and drawing in again more or less elongated processes, and creeping about like a true Amœba, from which, however, Protamœba differs, in the absence of a nucleus. It seems difficult to imagine anything simpler; indeed, as described, it appears to be an illustration of properties without structure. It takes into itself any suitable particle with which it comes in contact, absorbs that which is nutritious, and rejects the rest. From time to time a constriction appears at the centre (Pl. [V], Fig. 2), its form approximates more and more to that of an hour-glass (Pl. [V], Fig. 3), and at length the two halves separate, and each commences an independent existence (Pl. [V], Fig. 5).
PLATE V.
Figs. 1-5, Protamœba 6-9, Protamyxa aurantiaca, Haeckel, Beit. zur Monog. der Moneren, pl. 1; 10-18, Magosphœra planula, Haeckel, loc. cit. pl. 5.]
In the true Amœbas, on the contrary, we find a differentiation between the exterior and the interior: the body being more or less distinctly divisible into an outer layer and an inner parenchyme. In the Amœbas, as in Protamœba, multiplication takes place by self-division, and nothing corresponding to sexual reproduction has yet been discovered.
Somewhat more advanced, but still of great simplicity, is the Protomyxa aurantiaca (Pl. [V], Fig. 8), discovered by Haeckel[76] on dead shells of Spirula, where it appears as a minute orange speck, which shows well against the clear white of the Spirula. Examined with a microscope, the speck is seen to be a spherical mass of orange-coloured, homogeneous, albuminous matter, surrounded by a delicate, structureless membrane. It is obvious from this description that these bodies closely resemble eggs, for which indeed Haeckel at first mistook them. Gradually, however, the yellow sphere broke itself up into smaller spherules (Pl. [V], Fig. 9), after which the containing membrane burst, and the separate spherules, losing their globular form, crept out as small Amœbæ (Pl. [V], Fig. 6), or amœboid bodies. These little bodies moved about, assimilated the minute particles of organic matter, with which they came in contact, and gradually increased in size (Pl. [V], Fig. 7) with more or less rapidity according to the amount of nourishment they were able to obtain. They threw out arms in various directions, and if divided each section maintained its individual existence. After a while their movements ceased, they contracted into a ball, and again secreted round themselves a clear structureless envelope.