REGENERATION IN ANNELIDS
193
activity involved in the reconstruction of a head is particularly important in the budding zone. Abeloos concluded from his observations
that in sedentary polychaetes, the concepts of Cephalic territory' and
'cephalic region' are not generally applicable.
3. Morphallaxis
When the cephalic regeneration bud is hypomeric, the new anterior
region becomes abnormally connected with older segments belonging to
more posterior regions. This condition can cause morphallaxis phenomena, particularly in sedentary polychaetes, whose bodies are divided
into two distinct morphological regions—a thoracic and an abdominal
region. This phenomenon has been recently studied by Abeloos (1954)
in Sabellids and Spionids. When Sabella pavonima is transected in the
abdominal region, the posterior fragment, lacking both a head and a
thorax, regenerates by epimorphosis the branchial tuft, the prothoracic and the first thoracic segments only. The following abdominal
segments are then transformed into thoracic ones. Such morphallaxis
essentially deals with the form of the parapodia. I t is triggered off at the
time of transection, continues throughout the elaboration of the cephalic
regeneration bud and ends with regeneration of the first thoracic segment. Following a second decapitation, morphallaxis spreads posteriorly
to further segments and may not affect the same number of metameres
on the right and left sides. This summary indicates that the influence of
morphallaxis gradually spreads from the transformed segments and that
the two sides can react independently. The number of segments undergoing this metamorphosis is independent of the total number of segments present in the fragment. By successive transection of segments
undergoing transformation Abeloos showed that metamorphosis, once
triggered off in a setigerous segment, proceeds regardless of new conditions. After destroying the anterior regeneration buds many times, in
order to eliminate the influence of new cephalic structures, Abeloos
noted a definite delay in the spreading of morphallaxis and thought this
might be due to the influence of the regeneration bud. Similar phenomena
have also been observed by Abeloos in Polydora flava where regeneration of the eight cephalic segments is accompanied by transformation
of a number of abdominal metameres ; these acquire gills and take on
the aspect of thoracic segments. These processes do not necessarily
reproduce the initial form of the worm ; thus, in Salmacina incrustans,
Cresp (1953) has shown that eighteen abdominal segments can be
changed into thoracic segments, whereas normally in this species the
thorax is made up of seven to nine segments.
Morphallaxis can also affect internal organs. During anterior regeneration in Polydora flava, transformation of abdominal segments into
193
activity involved in the reconstruction of a head is particularly important in the budding zone. Abeloos concluded from his observations
that in sedentary polychaetes, the concepts of Cephalic territory' and
'cephalic region' are not generally applicable.
3. Morphallaxis
When the cephalic regeneration bud is hypomeric, the new anterior
region becomes abnormally connected with older segments belonging to
more posterior regions. This condition can cause morphallaxis phenomena, particularly in sedentary polychaetes, whose bodies are divided
into two distinct morphological regions—a thoracic and an abdominal
region. This phenomenon has been recently studied by Abeloos (1954)
in Sabellids and Spionids. When Sabella pavonima is transected in the
abdominal region, the posterior fragment, lacking both a head and a
thorax, regenerates by epimorphosis the branchial tuft, the prothoracic and the first thoracic segments only. The following abdominal
segments are then transformed into thoracic ones. Such morphallaxis
essentially deals with the form of the parapodia. I t is triggered off at the
time of transection, continues throughout the elaboration of the cephalic
regeneration bud and ends with regeneration of the first thoracic segment. Following a second decapitation, morphallaxis spreads posteriorly
to further segments and may not affect the same number of metameres
on the right and left sides. This summary indicates that the influence of
morphallaxis gradually spreads from the transformed segments and that
the two sides can react independently. The number of segments undergoing this metamorphosis is independent of the total number of segments present in the fragment. By successive transection of segments
undergoing transformation Abeloos showed that metamorphosis, once
triggered off in a setigerous segment, proceeds regardless of new conditions. After destroying the anterior regeneration buds many times, in
order to eliminate the influence of new cephalic structures, Abeloos
noted a definite delay in the spreading of morphallaxis and thought this
might be due to the influence of the regeneration bud. Similar phenomena
have also been observed by Abeloos in Polydora flava where regeneration of the eight cephalic segments is accompanied by transformation
of a number of abdominal metameres ; these acquire gills and take on
the aspect of thoracic segments. These processes do not necessarily
reproduce the initial form of the worm ; thus, in Salmacina incrustans,
Cresp (1953) has shown that eighteen abdominal segments can be
changed into thoracic segments, whereas normally in this species the
thorax is made up of seven to nine segments.
Morphallaxis can also affect internal organs. During anterior regeneration in Polydora flava, transformation of abdominal segments into