SOME ASPECTS O F THE BIOLOGY O F THE CHAETOGNATHS
355
may even see the eyes. I n this case, however, there is always doubt as
to whether it is true regeneration or a more or less perfect repair of an
extended lesion of the cephalic region. Specimens with such features
are alas quite exceptiona,l and are usually to be found in plankton
samples which are generally fixed in such a manner that no careful
histological researches are possible. On the other hand, Sagitta beheaded
for experimental purposes do not live in the laboratory longer than a
few hours, thus making any further observation impossible. The
subject is, however, very interesting and it deserves further study.
I think, however, that instances of regeneration, if any, should be
considered exceptional, otherwise it ought to be easy to find specimens
in all phases of “ regeneration ” so as to form more complete series
than those of Pierce and my own.
Further, as we have said before, the deficiency of regenerative
power of the Chaetognatha is part of a more general picture (Ghirardelli, 1959b, 1965b). In fact, not only the chaetognaths but also other
animals, whose eggs contain a germinal determinant rich in RNA, lack
extensive regenerative power, irrespective of their systematic position.
A germinal determinant, associated with poor regenerative power and
an absence of asexual reproduction, must therefore be a result of an
analogous evolution in animals very different from each other (e.g.
Chaetognatha, Cladocera, Copepoda, Rotifera, anuran Amphibia, etc.),
involving the disappearance of those totipotent or at least pluripotent
cells which constitute the so-called “ embryonic reserve ”. To the lack
of regenerative power therefore, no discriminating character can be
ascribed, supporting the relationship of the chaetognaths to any specific
zoological group (Ghirardelli, 1965b).
V. AFFINITIES AND SYSTEMATIC POSITION
After Slabber’s discovery ill 1769, several authors have tried to
establish the affinities and the systematic position of the Chaetognatha
(Ghirardelli, 196313). Two excellent reviews of works dealing with the
affinities of Chaetognatha are to be found in Kuhl’s monograph (1938)
and in Hyman’s treatise on zoology (1959) ; the majority of the data
which follow are taken from these works.
Darwin ( 1 844), referring to Xagittn, said the species of this genus are
to be remcmbered for the “ obscurity of their affinities ” ; in the same
year Krohn refuted the conclusions of some previous authors, among
whom was Darwin himself, who supported the existence of affinities
between chaetognaths and molluscs. Krohn believed the chaetognaths
to be rather nearer the anellids, whereas Huxley (1851) presumed
they were related to the Arthropoda. Gegenbaur (1853) also denied the
355
may even see the eyes. I n this case, however, there is always doubt as
to whether it is true regeneration or a more or less perfect repair of an
extended lesion of the cephalic region. Specimens with such features
are alas quite exceptiona,l and are usually to be found in plankton
samples which are generally fixed in such a manner that no careful
histological researches are possible. On the other hand, Sagitta beheaded
for experimental purposes do not live in the laboratory longer than a
few hours, thus making any further observation impossible. The
subject is, however, very interesting and it deserves further study.
I think, however, that instances of regeneration, if any, should be
considered exceptional, otherwise it ought to be easy to find specimens
in all phases of “ regeneration ” so as to form more complete series
than those of Pierce and my own.
Further, as we have said before, the deficiency of regenerative
power of the Chaetognatha is part of a more general picture (Ghirardelli, 1959b, 1965b). In fact, not only the chaetognaths but also other
animals, whose eggs contain a germinal determinant rich in RNA, lack
extensive regenerative power, irrespective of their systematic position.
A germinal determinant, associated with poor regenerative power and
an absence of asexual reproduction, must therefore be a result of an
analogous evolution in animals very different from each other (e.g.
Chaetognatha, Cladocera, Copepoda, Rotifera, anuran Amphibia, etc.),
involving the disappearance of those totipotent or at least pluripotent
cells which constitute the so-called “ embryonic reserve ”. To the lack
of regenerative power therefore, no discriminating character can be
ascribed, supporting the relationship of the chaetognaths to any specific
zoological group (Ghirardelli, 1965b).
V. AFFINITIES AND SYSTEMATIC POSITION
After Slabber’s discovery ill 1769, several authors have tried to
establish the affinities and the systematic position of the Chaetognatha
(Ghirardelli, 196313). Two excellent reviews of works dealing with the
affinities of Chaetognatha are to be found in Kuhl’s monograph (1938)
and in Hyman’s treatise on zoology (1959) ; the majority of the data
which follow are taken from these works.
Darwin ( 1 844), referring to Xagittn, said the species of this genus are
to be remcmbered for the “ obscurity of their affinities ” ; in the same
year Krohn refuted the conclusions of some previous authors, among
whom was Darwin himself, who supported the existence of affinities
between chaetognaths and molluscs. Krohn believed the chaetognaths
to be rather nearer the anellids, whereas Huxley (1851) presumed
they were related to the Arthropoda. Gegenbaur (1853) also denied the