y by budding. There
is a further resemblance between the two orders of Chaetopoda in that
this budding is not a general phenomenon, but confined to a few forms
only. Budding, in fact, among the Polychaetes is limited to the family
_Syllidae_. In the Oligochaetes it is only the families
_Aeolosomatidae_ and _Naididae_ that show the same phenomenon. It has
been mentioned that in the Nereids a sexual form occurs which differs
structurally from the asexual worms, and was originally placed in a
separate genus, _Heteronereis_; hence the name "Heteronereid" for the
sexual worm. In _Syllis_ there is also a "Heterosyllid" form in which
the gonads are limited to a posterior region of the body which is
further marked off from the anterior non-sexual segments by the
oak-like setae. In some Syllids this posterior region separates off
from the rest, producing a new head; thus a process of fission occurs
which has been termed schizogamy. A similar life history distinguishes
certain Sabellid worms, e.g. _Filigrana_. Among the Syllids this
simple state of affairs is further complicated. In _Autolytus_ there
is, to begin with, a conversion of the posterior half of the body to
form a sexual zooid. But before this separates off a number of other
zooids are formed from a zone of budding which appears between the two
first-formed individuals. Ultimately, a chain of sexual zooids is thus
formed. A given stock only produces zooids of one sex. In _Myrianida_
there is a further development of this process. The conversion of the
posterior end of the simple individual into a sexual region is
dispensed with; but from a preanal budding segment a series of sexual
buds are produced. The well-known Syllid, discovered during the voyage
of the "Challenger," shows a modification of this form of budding.
Here, however, the buds are lateral, though produced from a budding
zone, and they themselves produce other buds, so that a ramifying
colony is created.

Quite recently, another mode of budding has been described in
_Trypanosyllis gemmipara_, where a crowd of some fifty buds arising
symmetrically are produced at the tail end of the worm. In some
Syllids, such as _Pionosyllis gestans_, the ova are attached to the
body of the parent in a regular line, and develop in situ; this
process, which has been attributed to budding, is an "external
gestation," and occurs in a number of species.