SOME ASPECTS O F THE BIOLOGY O F THE CHAETOGNATHS
357
nervous system. They have been considered as related to the Vermes
Oligomera six times and placed near the ncrnatodes five times, twice
near the annelids and once they have been connected to cach of the
following groups : gordiaceans, crustaceans, arachnids, hemichordates
and chordates. Finally, Hyman notes some likeness between Chaetognatha and Aschelminthes or pseudocoelomote groups based on the
features of the adults : viz. Kinorhyncha, Priapulida, Rotifera, Gastrotricha, Nematoda and Gordiacea or Nematomorpha. However, on
considering the larval characters and their manner of development,
Hyman herself doubts whether the affinities between Chaetognatha
and Aschelminthes are real, and is rather inclined to believe that they
constitute an independent branch of the Deuterostomia ; this also
because there " is the possibility that the chaetognaths are remotely
related to the dipleurula ancestor of the other Dcutcrostomia ".
MacBridc (1914) and Burfield (1927) considered the chaetognaths
as a non-modified branch of Protocoelomata from which all other
coelomata would derive. Lately de Beauchamp (1960) agrees with the
conclusions of Claus et al. (1932) and of Hyman, in considering chaetognaths as a completely isolated phylum ; according to him, this could
be even the most isolated group in the animal kingdom. Incidentally,
Grassi had also come to similar conclusions when writing :
From the comparisons made, it is clear and apparent that as far as is now
known, between Chaetognatha and any other type no relation exists which
might be firmly believed to be close. Some resemblances which taken by
themselves might seem t o be the expression of such a relation, when compared-as it is logical-with the relevant differences, are reduced t o simple
analogies, or secondary convergences or perhaps even t o very far affinities.
(Transl. )
I n my opinion, the fact that chaetognaths may show affinities or
resemblances to such widely separated groups may actually prove that
there is no real affinity with any of those groups. Indeed, all living
organisms arc built according to a limited number of models of organizations, which selection has often established as the fittest for a certain
habitat or manner of life ; consequently some of them are repeated in
more or less identical form even in very widely separated groups. I n
the majority of cases, these are simple analogies or instances of convergence, without any true homology.
At this point one may ask whether-at the present state of our
knowledge and considering also the most recent researches-there are
still positive reasons for considering the chaetognaths to be related to
one of the existing zoological groups and whether it is still worth while
following the principles of the major authors of the nineteenth and
357
nervous system. They have been considered as related to the Vermes
Oligomera six times and placed near the ncrnatodes five times, twice
near the annelids and once they have been connected to cach of the
following groups : gordiaceans, crustaceans, arachnids, hemichordates
and chordates. Finally, Hyman notes some likeness between Chaetognatha and Aschelminthes or pseudocoelomote groups based on the
features of the adults : viz. Kinorhyncha, Priapulida, Rotifera, Gastrotricha, Nematoda and Gordiacea or Nematomorpha. However, on
considering the larval characters and their manner of development,
Hyman herself doubts whether the affinities between Chaetognatha
and Aschelminthes are real, and is rather inclined to believe that they
constitute an independent branch of the Deuterostomia ; this also
because there " is the possibility that the chaetognaths are remotely
related to the dipleurula ancestor of the other Dcutcrostomia ".
MacBridc (1914) and Burfield (1927) considered the chaetognaths
as a non-modified branch of Protocoelomata from which all other
coelomata would derive. Lately de Beauchamp (1960) agrees with the
conclusions of Claus et al. (1932) and of Hyman, in considering chaetognaths as a completely isolated phylum ; according to him, this could
be even the most isolated group in the animal kingdom. Incidentally,
Grassi had also come to similar conclusions when writing :
From the comparisons made, it is clear and apparent that as far as is now
known, between Chaetognatha and any other type no relation exists which
might be firmly believed to be close. Some resemblances which taken by
themselves might seem t o be the expression of such a relation, when compared-as it is logical-with the relevant differences, are reduced t o simple
analogies, or secondary convergences or perhaps even t o very far affinities.
(Transl. )
I n my opinion, the fact that chaetognaths may show affinities or
resemblances to such widely separated groups may actually prove that
there is no real affinity with any of those groups. Indeed, all living
organisms arc built according to a limited number of models of organizations, which selection has often established as the fittest for a certain
habitat or manner of life ; consequently some of them are repeated in
more or less identical form even in very widely separated groups. I n
the majority of cases, these are simple analogies or instances of convergence, without any true homology.
At this point one may ask whether-at the present state of our
knowledge and considering also the most recent researches-there are
still positive reasons for considering the chaetognaths to be related to
one of the existing zoological groups and whether it is still worth while
following the principles of the major authors of the nineteenth and