364
ELVEZIO GHIRARDELLI
dircction of the cilium, and in some respects it recalls those of the
Hydrozoa. Furthermore, observations made on some Protostoma
reopen the whole problem, making a deeper analysis of photoreceptive
cells necessary, in connexion with the problems of phylogenesis. In
fact, in the polychaete annelid Branchiomma vesiculosum the cilia
transformed into photoreceptive elements, have lost the two inner
tubules, and have therefore the symmetry 9 + 0 typical of Deuterostomia. Afzelius (1963) has, however, found in the flagellum of mature
sperms of Sagitta elegans some cases in which the filaments were 9 + 4
or 10 + 4. Some considerations on the phylogenesis of neritic chaetognaths are reported in Alvariflo (1963) who stresses the great similarity
between neritic species, and by Tokioka (1Y65b). According to the
latter, the neritic forms-which for swimming should not need a
transversal musculature-should have derived from the species of the
subclass Phragmophora, with transversal musculature which is useful
for movements on the bottom. From the neritic forms would derive
the oceanic species and from these in turn the meso- and bathypelagic
species of Aphragmophora. I n Tokioka's opinion, primitive characters
would be the corona present only on the head and the pronounced
length of the caudal section, as can be observed in Spadella and
Pterosagitta. I n conclusion, I do not think that any of the elements
which have been considered can actually determine the position of
Chaetognatha in a natural classification. They are in any case animals
of a rather elevated organization, perfectly adapted to their habitat,
though these remarks are no great help in clarifying their affinities a i d
their systematic position.
There is in my opinion, at the present state of our knowledge,
nothing else to do but to accept the conclusions reached by the majority
of the authors quoted above and say with Grassi (1883) : '' Credo
insomma che le relazioni dei Chetognati rimangano oscure e sepolte, nB
vedesi indizio che siano per essere scoperte fra breve " [" I think after
all that the relations of Chaetognatha remain obscure and buried ; nor
is any sign to be seen that they might be discovered in a short while "1.
Let us hope t,hat Grassi's pessimistic views may be dispelled in a
shorter time than has passed since he expressed them.
VI. LABORATORY EXPERIMENTS
In concluding this review on the biology of chaetognaths, it is perhaps
appropriate to quote observations made in the laboratory on living
specimens by Stevens (1505, 1910), Kulmatycki (1918), Vasiljev (192Fi),
,John (1933), Reisinger (1934), Sanzo (1937), Jagersten (1940), Parry
(1944), Ghirardelli (1950a,b, 1952, 1953d, 1954a, 1955, 1956a,b, 1958a,b,
ELVEZIO GHIRARDELLI
dircction of the cilium, and in some respects it recalls those of the
Hydrozoa. Furthermore, observations made on some Protostoma
reopen the whole problem, making a deeper analysis of photoreceptive
cells necessary, in connexion with the problems of phylogenesis. In
fact, in the polychaete annelid Branchiomma vesiculosum the cilia
transformed into photoreceptive elements, have lost the two inner
tubules, and have therefore the symmetry 9 + 0 typical of Deuterostomia. Afzelius (1963) has, however, found in the flagellum of mature
sperms of Sagitta elegans some cases in which the filaments were 9 + 4
or 10 + 4. Some considerations on the phylogenesis of neritic chaetognaths are reported in Alvariflo (1963) who stresses the great similarity
between neritic species, and by Tokioka (1Y65b). According to the
latter, the neritic forms-which for swimming should not need a
transversal musculature-should have derived from the species of the
subclass Phragmophora, with transversal musculature which is useful
for movements on the bottom. From the neritic forms would derive
the oceanic species and from these in turn the meso- and bathypelagic
species of Aphragmophora. I n Tokioka's opinion, primitive characters
would be the corona present only on the head and the pronounced
length of the caudal section, as can be observed in Spadella and
Pterosagitta. I n conclusion, I do not think that any of the elements
which have been considered can actually determine the position of
Chaetognatha in a natural classification. They are in any case animals
of a rather elevated organization, perfectly adapted to their habitat,
though these remarks are no great help in clarifying their affinities a i d
their systematic position.
There is in my opinion, at the present state of our knowledge,
nothing else to do but to accept the conclusions reached by the majority
of the authors quoted above and say with Grassi (1883) : '' Credo
insomma che le relazioni dei Chetognati rimangano oscure e sepolte, nB
vedesi indizio che siano per essere scoperte fra breve " [" I think after
all that the relations of Chaetognatha remain obscure and buried ; nor
is any sign to be seen that they might be discovered in a short while "1.
Let us hope t,hat Grassi's pessimistic views may be dispelled in a
shorter time than has passed since he expressed them.
VI. LABORATORY EXPERIMENTS
In concluding this review on the biology of chaetognaths, it is perhaps
appropriate to quote observations made in the laboratory on living
specimens by Stevens (1505, 1910), Kulmatycki (1918), Vasiljev (192Fi),
,John (1933), Reisinger (1934), Sanzo (1937), Jagersten (1940), Parry
(1944), Ghirardelli (1950a,b, 1952, 1953d, 1954a, 1955, 1956a,b, 1958a,b,