360
ELVEZIO GHIRARDELLI
The form and transparency of the body make Chaetognatha look
somewhat like heteropod molluscs. The large species of Xagitta
undoubtedly recall Pirola and similar species, but it is certainly a case
of convergence. The nervous system does not show signs of chiastoneuria, even if the general organization may present some resemblance
to that of molluscs, mainly as regards the position of the ventral
ganglion which might be homologized with the pedal one of the
Molluscs. There is no sign of a foot, however, and the supposed
homologies between the radula and the mouth armature of the chaetognaths are certainly rather forced.
There remain then still to be seen only the affinities between
chaetognaths and annelids and nematodes, the groups which they seem
more t o resemble.
As far as annelids are concerned, both they and the chaetognaths
have four longitudinal muscles which in section show a characteristic
herring-bone disposition ; but in chaetognaths there is no circular
musculature or any oblique muscle. The ventral ganglion of the
Chaetognatha could be considered as a derivative of the ventral chain
of the annelids, due to a concentration of ganglia. There is also some
likeness between the hooks of the chaetognaths and the bristles of the
annelids as far as their constitution is concerned. The hooks (Schmidt,
1951, 1952; Hyman, 1958) and the bristles seem actually to be of a
chitinous nature, in both groups ciliated cells are prrsent and the
ciliated funnel by which the male genital orifice opens to the outside,
recalls the nephrostoma of the annelids. The study of the embryonic
development, on the other hand, offers really few elements in support
of affinities between chaetognaths and annelids. The ways of segmentation are different, the mesoderma in the Chaetognatha forms from the
diverticula of the archenteron, in the annelids from the coelomic sacs.
The adult annelids have a true coelom while the Chaetognatha have the
appearance of pseudocoelomate animals. The only typical coelomate
structures are the ciliated funnels of the sperm ducts (Hyman, 1959).
There is no metamerism in the Chaetognatha, neither during the
development of the ventral ganglion-which as we saw originates
(Ghirardelli, 1958b) from a common anlage together with the nervous
part of the ciliary loop-nor during the development of the gonoducts.
Furthermore, Chaetognatha seem to lack excretory organs. The ciliary
loop which Reisinger (1 934) considered a solenocytic protonephridium
would seem to have mainly sensory functions, and in its glandular
region a secretory but not excretory function (Ghirardelli, 1959e).
Reisinger (1968 in press) proposes again some affinities between ehaetognaths and the Chordata. He notes an analogy between the endostyle
ELVEZIO GHIRARDELLI
The form and transparency of the body make Chaetognatha look
somewhat like heteropod molluscs. The large species of Xagitta
undoubtedly recall Pirola and similar species, but it is certainly a case
of convergence. The nervous system does not show signs of chiastoneuria, even if the general organization may present some resemblance
to that of molluscs, mainly as regards the position of the ventral
ganglion which might be homologized with the pedal one of the
Molluscs. There is no sign of a foot, however, and the supposed
homologies between the radula and the mouth armature of the chaetognaths are certainly rather forced.
There remain then still to be seen only the affinities between
chaetognaths and annelids and nematodes, the groups which they seem
more t o resemble.
As far as annelids are concerned, both they and the chaetognaths
have four longitudinal muscles which in section show a characteristic
herring-bone disposition ; but in chaetognaths there is no circular
musculature or any oblique muscle. The ventral ganglion of the
Chaetognatha could be considered as a derivative of the ventral chain
of the annelids, due to a concentration of ganglia. There is also some
likeness between the hooks of the chaetognaths and the bristles of the
annelids as far as their constitution is concerned. The hooks (Schmidt,
1951, 1952; Hyman, 1958) and the bristles seem actually to be of a
chitinous nature, in both groups ciliated cells are prrsent and the
ciliated funnel by which the male genital orifice opens to the outside,
recalls the nephrostoma of the annelids. The study of the embryonic
development, on the other hand, offers really few elements in support
of affinities between chaetognaths and annelids. The ways of segmentation are different, the mesoderma in the Chaetognatha forms from the
diverticula of the archenteron, in the annelids from the coelomic sacs.
The adult annelids have a true coelom while the Chaetognatha have the
appearance of pseudocoelomate animals. The only typical coelomate
structures are the ciliated funnels of the sperm ducts (Hyman, 1959).
There is no metamerism in the Chaetognatha, neither during the
development of the ventral ganglion-which as we saw originates
(Ghirardelli, 1958b) from a common anlage together with the nervous
part of the ciliary loop-nor during the development of the gonoducts.
Furthermore, Chaetognatha seem to lack excretory organs. The ciliary
loop which Reisinger (1 934) considered a solenocytic protonephridium
would seem to have mainly sensory functions, and in its glandular
region a secretory but not excretory function (Ghirardelli, 1959e).
Reisinger (1968 in press) proposes again some affinities between ehaetognaths and the Chordata. He notes an analogy between the endostyle