286
ELVEZIO GHIRARDELLI
enough to carry off even large portions of the ciliary loop. It is, furthermore, certainly not a fact in support of the supposed excretory functions
that in reared specimens of Xpadella the ring sooner or later begins
degenerating without causing the death of the animal ; beheaded
Xpadella deprived also of their corona survive for several weeks. This
shows that the ciliary loop is not essential for excretion, unless its
functions are duplicated by other glandular structures, which in
Spadella a t least, have a quite different morphological aspect and in
some cases even well identifiable functions, as is the case of the adhesive
glands or of the ciliated sensory tactile bodies. The beheaded specimens
can survive because in them sensitivity is supplied by the ventral
ganglion and by the aforementioned sensory formations of the epidermis, and because probably the material contained in the ovaries
and in the intestinal walls may constitute an effective alimentary
reserve.
The corona ciliata has also glandular functions, as we have seen,
but these are secretory and Reisinger himself has pointed out the
presence of a secretion within the glandular cells of the ring. The
structure of the corona, its origin, the rich innervation and the presence
of cilia, seem t o confirm the hypothesis that it may have sensory
purposes (Ghirardelli, 1958b) ; nevertheless Horridge and Boulton
(1967) state that the electron microscope does not reveal the cells of
the ciliary loop as nerve cells. It is then clear that it would be very
useful on all grounds that the corona in Xpadella be studied
again. As t o the opinion that the corona might be a phylogenetic
precursor of a solenocytic protonephridium, Hyman (1959) finds this
idea rather difficult to accept. Actually " as the invertebrate Deuterostomia do not have protonephridia, this would mean that
Chaetognatha are looking a long way forward t o Amphioxus ".
I n few other chaetognaths has the corona been studied histologically. I n Pterosagitta draco which is a bathypelagic species, the
epidermis shows features quite similar to, even if more remarkable than,
those found in Spadella cephaloptera. Here too the lateral expansions
of the epidermis are well developed and are constituted by typical
vacuolized cells. The " corona ciliata " in Pterosagitta is partially sunk
into the epidermis, as in Xpadella, and is likewise formed by two
concentric strips or bands. I n sections, the two regions of the corona,
though clearly recognizable, do not show constant differentiations. The
cells are always disposed in such a way as to form a sort of groove whose
margins may bend on themselves towards the inside, that is one
towards the other. Often only the outer edge bends, so that the cells
of the outer ring assume a disposition similar to a horse-shoe with the
ELVEZIO GHIRARDELLI
enough to carry off even large portions of the ciliary loop. It is, furthermore, certainly not a fact in support of the supposed excretory functions
that in reared specimens of Xpadella the ring sooner or later begins
degenerating without causing the death of the animal ; beheaded
Xpadella deprived also of their corona survive for several weeks. This
shows that the ciliary loop is not essential for excretion, unless its
functions are duplicated by other glandular structures, which in
Spadella a t least, have a quite different morphological aspect and in
some cases even well identifiable functions, as is the case of the adhesive
glands or of the ciliated sensory tactile bodies. The beheaded specimens
can survive because in them sensitivity is supplied by the ventral
ganglion and by the aforementioned sensory formations of the epidermis, and because probably the material contained in the ovaries
and in the intestinal walls may constitute an effective alimentary
reserve.
The corona ciliata has also glandular functions, as we have seen,
but these are secretory and Reisinger himself has pointed out the
presence of a secretion within the glandular cells of the ring. The
structure of the corona, its origin, the rich innervation and the presence
of cilia, seem t o confirm the hypothesis that it may have sensory
purposes (Ghirardelli, 1958b) ; nevertheless Horridge and Boulton
(1967) state that the electron microscope does not reveal the cells of
the ciliary loop as nerve cells. It is then clear that it would be very
useful on all grounds that the corona in Xpadella be studied
again. As t o the opinion that the corona might be a phylogenetic
precursor of a solenocytic protonephridium, Hyman (1959) finds this
idea rather difficult to accept. Actually " as the invertebrate Deuterostomia do not have protonephridia, this would mean that
Chaetognatha are looking a long way forward t o Amphioxus ".
I n few other chaetognaths has the corona been studied histologically. I n Pterosagitta draco which is a bathypelagic species, the
epidermis shows features quite similar to, even if more remarkable than,
those found in Spadella cephaloptera. Here too the lateral expansions
of the epidermis are well developed and are constituted by typical
vacuolized cells. The " corona ciliata " in Pterosagitta is partially sunk
into the epidermis, as in Xpadella, and is likewise formed by two
concentric strips or bands. I n sections, the two regions of the corona,
though clearly recognizable, do not show constant differentiations. The
cells are always disposed in such a way as to form a sort of groove whose
margins may bend on themselves towards the inside, that is one
towards the other. Often only the outer edge bends, so that the cells
of the outer ring assume a disposition similar to a horse-shoe with the