288
ELVEZIO GIIIRARDELLI
similar to those of the sensory prominences. Also in Xayitta bipunctata
the cells of the ring form a sort of groove as described in Pterosagitta
and here it is often the inner edge that folds in such a manner that the
cells of the inner ring end by being more numerous than those of the
outer ring. The disposition is, however, not constant in this species
either and the aspect of the inner and outer corona may vary in the
same animal.
All the cells of the corona have one part richer in cytoplasm
inclining towards the middlc line where a vacuole may almost always
be observed. It is probable that contiguous vacuoli contribute to form
that sort of annular canal which Reisinger has described in Xpadella ;
in Sugitta bipunctata it does not seem to be a continuous formation.
X. bipunctata also lacks a well-differentiated glandular portion of the
corona, since all cells have a sensory character (Ghirardelli, 1959e).
The ciliary loop of X. in$ata is placed entirely on the head and has
two lateral lobes that are generally well marked. I n this species, in
contrast to what has been found in X. bipunctata and Xpadella mentioned above, I have observed that there are now and then some
differences between the inner and the outer ring. I n the majority of
cases the two regions of the ciliated ring are exactly similar to each
other, and so is the horse-shoe disposition of the cells and the usually
pycnotic nuclei, but in some specimens only the cells of the outer ring
have pycnotic nuclei and are certainly provided with cilia, while the
nuclei of the inner ring cells are more voluminous and roundish and
have a very evident chromatic reticule. They remind one of the inner
corona cells in Xpadella, but they never show in their cytoplasm those
characters of glandular elements which are so apparent in Spadella.
It has not been possible as yet to ascertain whether the two different
sorts of cells may be connected with the physiological condition or the
age of the specimens considered (Ghirardelli, 1959e).
It would seem that in the pelagic chaetognaths the ciliary loop has
different features from those found in the benthic form Xpadella
cephaloptera. Actually in all the pelagic species considered up to now
(Pterosagitta draco, Xagitta bipunctuta, X. i7;jZata) the glandular part of
the corona is indeed missing.
Summing up, one may state that in pelagic chaetognaths the ciliated
ring is made up of cells that have all more or less the characters of the
sensory elements of the tactile prominences or of the portion of the
corona in Spadella which is deemed t o be sensitive. These cells are
also distinguishable, apart from the character of their nuclei, by the
presence of cilia. As far as pelagic chaetognaths are concerned, the
question whether the corona is a secretory organ or a sense organ does
ELVEZIO GIIIRARDELLI
similar to those of the sensory prominences. Also in Xayitta bipunctata
the cells of the ring form a sort of groove as described in Pterosagitta
and here it is often the inner edge that folds in such a manner that the
cells of the inner ring end by being more numerous than those of the
outer ring. The disposition is, however, not constant in this species
either and the aspect of the inner and outer corona may vary in the
same animal.
All the cells of the corona have one part richer in cytoplasm
inclining towards the middlc line where a vacuole may almost always
be observed. It is probable that contiguous vacuoli contribute to form
that sort of annular canal which Reisinger has described in Xpadella ;
in Sugitta bipunctata it does not seem to be a continuous formation.
X. bipunctata also lacks a well-differentiated glandular portion of the
corona, since all cells have a sensory character (Ghirardelli, 1959e).
The ciliary loop of X. in$ata is placed entirely on the head and has
two lateral lobes that are generally well marked. I n this species, in
contrast to what has been found in X. bipunctata and Xpadella mentioned above, I have observed that there are now and then some
differences between the inner and the outer ring. I n the majority of
cases the two regions of the ciliated ring are exactly similar to each
other, and so is the horse-shoe disposition of the cells and the usually
pycnotic nuclei, but in some specimens only the cells of the outer ring
have pycnotic nuclei and are certainly provided with cilia, while the
nuclei of the inner ring cells are more voluminous and roundish and
have a very evident chromatic reticule. They remind one of the inner
corona cells in Xpadella, but they never show in their cytoplasm those
characters of glandular elements which are so apparent in Spadella.
It has not been possible as yet to ascertain whether the two different
sorts of cells may be connected with the physiological condition or the
age of the specimens considered (Ghirardelli, 1959e).
It would seem that in the pelagic chaetognaths the ciliary loop has
different features from those found in the benthic form Xpadella
cephaloptera. Actually in all the pelagic species considered up to now
(Pterosagitta draco, Xagitta bipunctuta, X. i7;jZata) the glandular part of
the corona is indeed missing.
Summing up, one may state that in pelagic chaetognaths the ciliated
ring is made up of cells that have all more or less the characters of the
sensory elements of the tactile prominences or of the portion of the
corona in Spadella which is deemed t o be sensitive. These cells are
also distinguishable, apart from the character of their nuclei, by the
presence of cilia. As far as pelagic chaetognaths are concerned, the
question whether the corona is a secretory organ or a sense organ does