356
ELVEZIO GHIRARDELLI
affinity with molluscs, since he did not find any likeness between the
embryonic development of chaetognaths and molluscs. Leuckart
(1854) classified Xagitta among the worms, but since he was unable to
give it a precise systematic position within the known forms, he instituted the group of chaetognaths, which he placed between nematodes
and annelids. Gegenbaur (1859), after further researches about Xagitta,
instituted the new class of the Oestelminthes placed between Nemathclminthes and Annulates, all under Vermes. The name of Oestelminthes
was later dropped by Gegenbaur (1870) himself and replaced by the
former name of Chaetognatha, without, however, modifying the
previous systematic position. Schneider (1 866) allied Chaetognatha
with Nematoda in the phylum Nemathelminthes, based on the identical
arrangement of the muscular bands in the two groups. Metschnikov
(1867) stressed the affinities of chaetognaths with somc marine nematodes (i.e. Draconematid and Epsilonematid). Kowalewski (1871),
aftcr a careful study of the development of the mouth and of the
coelomic sacs, was unable to decide whether the affinities with molluscs
or those with worms were more relevant. Hertwig (1880) dealt with
the affinities of chaetogiiaths from the aspect of the theory of the
germ-layer and first believed them to he near the gordiaceaiis for some
features and near the annelids for others ; he did not, however, reach a
definite conclusion, since he lacked the elements to decide whether the
coelom is originated by enterocoelia or schyzocoelia. The cmbryology
of chaetognaths still presents in our day several blank spots, especially
about the origin of the body cavity and of some organs.
Grassi ( 1883) examined particularly the affinities of chaetognaths
with brschiopods, echinoderms and hemichordates. The differences he
found, however, especially on the origin of the coelom and of the
mesoderm, were such that in his own opinion it was not possible t o
believe that t-he chaetognaths were connected with these groups.
Affinities have also been suggested with coelenterates (Hertwig, 1880)
and even with the vertebrates, as Meissner (1857) described a dorsal
chord under the brains which, howcver, does not exist at all.
Butschli (1910) placed Chaetognatha among the worms and particularly among the Oligomera, which have the coelom divided into
three cavities. Butschli’s classification, though it was apparently
artificial, has been incliplicably accepted, says Hyman, in a treatise as
important as ‘‘ Handbuch der Zoologie ” (Kukenthal and Krumbach,
1923-63).
I n order to conclude this list, we may note that according to Kuhl
the Chaetognatha have been considered as related to the Mollusca
eight times, mainly on account of their form and the structure of their
ELVEZIO GHIRARDELLI
affinity with molluscs, since he did not find any likeness between the
embryonic development of chaetognaths and molluscs. Leuckart
(1854) classified Xagitta among the worms, but since he was unable to
give it a precise systematic position within the known forms, he instituted the group of chaetognaths, which he placed between nematodes
and annelids. Gegenbaur (1859), after further researches about Xagitta,
instituted the new class of the Oestelminthes placed between Nemathclminthes and Annulates, all under Vermes. The name of Oestelminthes
was later dropped by Gegenbaur (1870) himself and replaced by the
former name of Chaetognatha, without, however, modifying the
previous systematic position. Schneider (1 866) allied Chaetognatha
with Nematoda in the phylum Nemathelminthes, based on the identical
arrangement of the muscular bands in the two groups. Metschnikov
(1867) stressed the affinities of chaetognaths with somc marine nematodes (i.e. Draconematid and Epsilonematid). Kowalewski (1871),
aftcr a careful study of the development of the mouth and of the
coelomic sacs, was unable to decide whether the affinities with molluscs
or those with worms were more relevant. Hertwig (1880) dealt with
the affinities of chaetogiiaths from the aspect of the theory of the
germ-layer and first believed them to he near the gordiaceaiis for some
features and near the annelids for others ; he did not, however, reach a
definite conclusion, since he lacked the elements to decide whether the
coelom is originated by enterocoelia or schyzocoelia. The cmbryology
of chaetognaths still presents in our day several blank spots, especially
about the origin of the body cavity and of some organs.
Grassi ( 1883) examined particularly the affinities of chaetognaths
with brschiopods, echinoderms and hemichordates. The differences he
found, however, especially on the origin of the coelom and of the
mesoderm, were such that in his own opinion it was not possible t o
believe that t-he chaetognaths were connected with these groups.
Affinities have also been suggested with coelenterates (Hertwig, 1880)
and even with the vertebrates, as Meissner (1857) described a dorsal
chord under the brains which, howcver, does not exist at all.
Butschli (1910) placed Chaetognatha among the worms and particularly among the Oligomera, which have the coelom divided into
three cavities. Butschli’s classification, though it was apparently
artificial, has been incliplicably accepted, says Hyman, in a treatise as
important as ‘‘ Handbuch der Zoologie ” (Kukenthal and Krumbach,
1923-63).
I n order to conclude this list, we may note that according to Kuhl
the Chaetognatha have been considered as related to the Mollusca
eight times, mainly on account of their form and the structure of their