But in other genera, such as _Autolytus_, the regeneration of segments may commence _before_ the separation of the sexual zooid; and the head of the sexual zooid becomes budded out _before_ separation from the asexual portion. So that the animal now consists of two worms, each with its own head, separated by {279}a region or zone of proliferation (Fig. 148, IV). Moreover, in some species not only is the hinder part of the body _converted_ into a sexual zooid, but the zone of proliferation becomes very active, and produces by gemmation a large number of segments, which become marked out, by the appearance of heads at intervals, into a number of zooids, in which genital organs will later make their appearance. A chain of as many as sixteen zooids may be formed in _Autolytus_ (Fig. 148, V)—the hindermost by _conversion_ of the hinder part of the body of the original "stock," the intervening zooids by _gemmation_. [Illustration: FIG. 148.—Diagrams illustrating the various stages in the asexual formation of a chain of zooids. (Modified from Malaquin.) 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 {280}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).