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PAUL BRIEN
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N
FIG. 6. Diagram showing the succession of continuous precocious scissiparity in
one segment, N, as in Nais. Z—parent zooid; BI, B2, B3—isolated blastozooids; N—
the segment where successive scissiparities occur, each time separating a posterior
part, b, which takes part in the formation of the zooids, from an anterior pygidial
part, a, which restores the segment N for a new scissiparity. (L. Dehorne, 1916.)
especially by Dehorne, Stolte, and Herlant-Meewis. In 1916, Dehorne
clearly defined the details of scissiparity in the oligochaetes. Furthermore, they correspond very well to conditions shown by stolon-forming
schizogonia of the Syllidae among the polychaetes.
In the naidimorphs, the posterior extremity, the pygidium, is always
growing. Scissiparity is the result of this.
In the scissiparity called "slow" by Dehorne (1916) and "paratomie"
by Herlant-Meewis (1933), and as observed in Dero, the zone of scissiparity appears in a specific segment: the thirty-eighth in Dero
digitata
and the thirty-fifth in Ophidonais. It is nearer the anterior septum of the
segment than the posterior septum; it appears as an annular thickening
of the ectoderm. The new zooid or blastozooid becomes detached only
after its head is completely formed and it is capable of moving freely.
The segment containing the zone of scissiparity is not absolutely fixed
but is located between two precise limits. The budding segment, N,
is separated into two parts, a and b. The anterior part a remains ata