SOME ASPECTS OF THE BIOLOGY OF THE CHAETOGNATHS
359
between the coelenteric cavity and the coelom of the chaetognaths.
Apart from the different origin of the two cavities, the Chaetognatha
are typically triblastic animals, much more differentiated than
coelenterates.
The affinities with the Echinodermata are limited to the very first
phases of embryonic development. Chaetognaths, however, have only
two primary divisions in the coelom and no true larvae, while there are
three primary divisions in the coelom of echinoderms and hemichordates,
which have larvae that can be referred to the dipleurula.
MacBride’s (1914) consideratioiis which refer the chaetognaths to a
very early blind offshoot of the Protocoelomata, which should be
represented today by the Ctenophora and during the ontogenesis of the
Bilateria by the dipleurula and the trochophores, are but interesting
speculations, though in this way they allow the connexion of chaetop a t h s to the other bilateral Metazoa. On the other hand, though it be
true that embryology may often supply elements for the Iocation of
affinities between groups which at first sight seem far apart, it is also
a fact that the ways of segmentation of the eggs are few, depending
on the type of egg, and often depending on the character of the environment in which the organism lives.
The relations with brachiopods, based on the comparison made by
Lameere (1931) between the hooks of the chaetognaths and the
lophophore, are not supported by the data obtained from the morphology of the adults nor by those obtained by studying the development.
Lately the problem has been again resumed by Hadii (1963). I n
his opinion the Chaetognatha derive “ b y way of neoteny from the
planktonic larvae of Brachiopoda ”. They are archicoelomata and also
constitute an actual aberrant group because, among all the Oligomera
where Hadii places them, they are the only ones which can lead a
pelagic life ; their adaptation could just be explained by their origin.
Affinities with the arthropods could be based only on the structure
of their nervous system and the presence, if any, of chitin in the
tegument and in the tegumentary formations, though it is opportune
to recall that chitin is not to be found in Arthropoda alone. There is,
on the contrary, no sign of metamerism and the development shows
quite a different pattern.
I do not see how affinities with arachnids can be supported, as
Huxley (1851) did, or with crustaceans, even with strongly modified
forms like some parasitic copepods (Lernaea) or with cirripedes.
Crustaceans, aIso, always have typical larvae which have allowed
workers to establish with certainty that even the aberrant forms we
have mentioned are crustaceans.
359
between the coelenteric cavity and the coelom of the chaetognaths.
Apart from the different origin of the two cavities, the Chaetognatha
are typically triblastic animals, much more differentiated than
coelenterates.
The affinities with the Echinodermata are limited to the very first
phases of embryonic development. Chaetognaths, however, have only
two primary divisions in the coelom and no true larvae, while there are
three primary divisions in the coelom of echinoderms and hemichordates,
which have larvae that can be referred to the dipleurula.
MacBride’s (1914) consideratioiis which refer the chaetognaths to a
very early blind offshoot of the Protocoelomata, which should be
represented today by the Ctenophora and during the ontogenesis of the
Bilateria by the dipleurula and the trochophores, are but interesting
speculations, though in this way they allow the connexion of chaetop a t h s to the other bilateral Metazoa. On the other hand, though it be
true that embryology may often supply elements for the Iocation of
affinities between groups which at first sight seem far apart, it is also
a fact that the ways of segmentation of the eggs are few, depending
on the type of egg, and often depending on the character of the environment in which the organism lives.
The relations with brachiopods, based on the comparison made by
Lameere (1931) between the hooks of the chaetognaths and the
lophophore, are not supported by the data obtained from the morphology of the adults nor by those obtained by studying the development.
Lately the problem has been again resumed by Hadii (1963). I n
his opinion the Chaetognatha derive “ b y way of neoteny from the
planktonic larvae of Brachiopoda ”. They are archicoelomata and also
constitute an actual aberrant group because, among all the Oligomera
where Hadii places them, they are the only ones which can lead a
pelagic life ; their adaptation could just be explained by their origin.
Affinities with the arthropods could be based only on the structure
of their nervous system and the presence, if any, of chitin in the
tegument and in the tegumentary formations, though it is opportune
to recall that chitin is not to be found in Arthropoda alone. There is,
on the contrary, no sign of metamerism and the development shows
quite a different pattern.
I do not see how affinities with arachnids can be supported, as
Huxley (1851) did, or with crustaceans, even with strongly modified
forms like some parasitic copepods (Lernaea) or with cirripedes.
Crustaceans, aIso, always have typical larvae which have allowed
workers to establish with certainty that even the aberrant forms we
have mentioned are crustaceans.