I, Heteronereid or Heterosyllid stage. A, Non-sexual; A', sexual region of the body, with modified parapodia.
II, Syllis. The hinder sexual region, B, is similarly modified, and will separate from the parent zooid, A, and become an independent zooid.
III, Autolytus. The hinder zooid, B, develops a head by budding before separation.
IV, Autolytus, etc. A zone of budding (z) makes its appearance in front of the head of B, and by its growth will give rise to a series of new segments in the middle of the body.
V, Myrianida, Autolytus, etc. From this zone of budding a very large number of segments have been formed, which have, further, become grouped so as to form three individuals, C, D, E; B is the hindmost zooid, which is either formed from the hinder segments of the parent zooid or is produced by budding, like C, D, E.
One original "stock," or asexual zooid, thus produces several sexual zooids, but these are only of one sex for a given stock. The males differ in several important characters from the females; so different, indeed, are the two sexes that before their history was worked out by Agassiz[[335]] they were placed in different genera. The male zooid has thus come to be known as Polybostrichus (Fig. 149, B). It has three tentacles and two bifid palps; there are two pairs of peristomial cirri; the testes are confined to the four anterior segments, which are without natatory chaetae. The female is termed Sacconereis, owing to the possession of a great ventral brood sac; its head possesses no separate palps; the peristomium carries only one cirrus on each side; ova occur in every segment of the body, and may even extend into the hinder segments of the asexual zooid (Fig. 149, C).
Fig. 149.—Myrianida fasciata. (From Malaquin.) The bright red markings of the living animal are here represented black. A, An asexual individual which has produced by budding from the zone (z) a chain of twenty-nine zooids, the oldest being labelled 1, the youngest 29. B, A ripe male zooid (Polybostrichus), with three tentacles and a pair of forked palps (p). There are five unaltered anterior segments. C, A ripe female zooid (Sacconereis) with the palps fused with the prostomium; s, the ventral brood pouch projecting on each side; t, tentacles.
A further development of this process of gemmiparity is exhibited by Myrianida. Here, there is no conversion of the hinder segments, but the normal preanal zone of proliferation gives rise to a large number of new segments. After a time the most anterior of these becomes a head, and thus a new zooid is marked out. The zone of proliferation immediately in front of the new head now proceeds to form new segments, and a second zooid results. This process goes on till a considerable number of new worms have been formed at the tail of the original one, the oldest of these new ones being the most posterior, the youngest next the original "stock." In each zooid there is a zone of activity which adds to its number of segments, so that as we pass backwards the zooids increase in size. As many as twenty-nine such zooids may be formed in this way entirely by gemmation; and as each zooid becomes completed, genital organs make their appearance, and when these are ripe the zooid separates from the "colony" and leads an independent life. Here, as in Autolytus, the sexes are dimorphic, the male and female resembling those of that genus.
The process of gemmation, as seen in Autolytus, closely resembles that exhibited by certain Oligochaeta (Naididae), where there exists a definite alternation of generations; the production of new individuals by gemmation occurring throughout the greater part of the year, and sexual reproduction recurring only at certain intervals. In the Polychaeta such alternation exists in Myrianida; but it is only the terminal link of a series, which takes its starting-point in the process exhibited by the majority of Annelids, where no sexual character marks maturity. The next stage is presented by "epigamous" forms like Heteronereis and Heterosyllis; then "schizogamy" makes its appearance in certain Syllidae, resulting in the formation of two morphologically and physiologically distinct individuals which lead independent lives. The appearance of a head and of a zone of proliferation leading to the formation of a chain of sexual zooids is accompanied by a delay in the appearance of the genital organs, for in Autolytus these arise during the formation of the new individuals, as part of the general process of new formation; whilst in Myrianida the delay is prolonged, and the generative elements do not make their appearance till after the new individuals have reached some size.