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. 5, 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. 5, Fig. 2), its form approximates more and more to that of an hour-glass (Pl. 5, Fig. 3), and at length the two halves separate, and each commences an independent existence (Pl. 5, Fig. 5). [Illustration: FIG. 59, _Prorhynchus stagnaus_.[75]] [Illustration: PLATE V. FIGS. 1-5, _Protamœba_; 6-9, _Protamyxa Aurantiaca_, Haeckel, Beit. zur Monog. der Moneren, pl. 1; 10-18, _Magosphæra 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.