REGENERATION IN ANNELIDS
189
segments for caudal regeneration to occur. In sedentary polychaetes
the fragment can be much smaller. For example, in Salmacina (Cresp,
1956) fragments containing only two anterior thoracic segments can
elaborate a pygidial regeneration bud. In Hydroides norvegica (Cresp,
1952), regeneration is possible if the worm is transected behind the first
thoracic segment, but is complete only when the posterior half of the
thorax is involved. In Magalia perarmata (Abeloos, 1950a) a fragment
containing five setigerous segments with the proboscis and a piece of
gut regenerates posteriorly. Caudal regeneration occurs backward of
segment 7 in Saccocirrus papillocercus (Gay, 1956) and Clymene oerstedii
(Abeloos, 1953) and backward of segment 8 in Polydora flava (Abeloos
and Thouveny, 1957). At the other extreme there are some species, such
as Polydora ciliata and Terebella lapidaria, in which regeneration is not
possible forward of segment 30.
In the Oligochaetae, we have shown for Lumbricillus lineatus (Herlant-Meewis, 1946) that under certain conditions a fragment consisting
of the eight anterior segments can reconstruct the worm integrally. In
Eisenia foetida, Gates (1950a) and Avel (1959) point out that posterior
regeneration proceeds normally starting from segment 40. In some
terricolous oligochaetes of the Allolobophora group posterior regeneration is restricted to the diapause which takes place annually (Abeloos
and Avel, 1928; Michon, 1954; Saussey, 1960; Boulot and Gallissian,
1960; Gallissian, 1963). Finally, Avel (1959), Gates (1950b) and Michon
(1954) point out that it does not occur in the genus Lumbricus.
3. Morphallaxis
When caudal regeneration is hypomeric, it may be accompanied by
morphallactic changes at the site of the old parts ; several examples of
such changes have been described. Recently, Gay (1956) mentioned one
case in Saccocirrus papillocercus. In this species the gut is made up of
two different zones, a glandular anterior region and an absorbent
posterior one. When the worm is transected in the region of the glandular
gut, reconstruction of a relatively small number of segments is concomitant with the transformation of the gut at the base of the regeneration
bud : over a certain length, changes take place in the epithelium which
takes on the features of an absorbent gut.
B. Cephalic Regeneration
1. Morphogenesis
During morphogenesis of the cephalic region, no embryonic zone
persists. In each of the blastemal layers, cell multiplication is immediately followed by differentiation. Thus, few segments of cephalic type
189
segments for caudal regeneration to occur. In sedentary polychaetes
the fragment can be much smaller. For example, in Salmacina (Cresp,
1956) fragments containing only two anterior thoracic segments can
elaborate a pygidial regeneration bud. In Hydroides norvegica (Cresp,
1952), regeneration is possible if the worm is transected behind the first
thoracic segment, but is complete only when the posterior half of the
thorax is involved. In Magalia perarmata (Abeloos, 1950a) a fragment
containing five setigerous segments with the proboscis and a piece of
gut regenerates posteriorly. Caudal regeneration occurs backward of
segment 7 in Saccocirrus papillocercus (Gay, 1956) and Clymene oerstedii
(Abeloos, 1953) and backward of segment 8 in Polydora flava (Abeloos
and Thouveny, 1957). At the other extreme there are some species, such
as Polydora ciliata and Terebella lapidaria, in which regeneration is not
possible forward of segment 30.
In the Oligochaetae, we have shown for Lumbricillus lineatus (Herlant-Meewis, 1946) that under certain conditions a fragment consisting
of the eight anterior segments can reconstruct the worm integrally. In
Eisenia foetida, Gates (1950a) and Avel (1959) point out that posterior
regeneration proceeds normally starting from segment 40. In some
terricolous oligochaetes of the Allolobophora group posterior regeneration is restricted to the diapause which takes place annually (Abeloos
and Avel, 1928; Michon, 1954; Saussey, 1960; Boulot and Gallissian,
1960; Gallissian, 1963). Finally, Avel (1959), Gates (1950b) and Michon
(1954) point out that it does not occur in the genus Lumbricus.
3. Morphallaxis
When caudal regeneration is hypomeric, it may be accompanied by
morphallactic changes at the site of the old parts ; several examples of
such changes have been described. Recently, Gay (1956) mentioned one
case in Saccocirrus papillocercus. In this species the gut is made up of
two different zones, a glandular anterior region and an absorbent
posterior one. When the worm is transected in the region of the glandular
gut, reconstruction of a relatively small number of segments is concomitant with the transformation of the gut at the base of the regeneration
bud : over a certain length, changes take place in the epithelium which
takes on the features of an absorbent gut.
B. Cephalic Regeneration
1. Morphogenesis
During morphogenesis of the cephalic region, no embryonic zone
persists. In each of the blastemal layers, cell multiplication is immediately followed by differentiation. Thus, few segments of cephalic type