358
ELVEZIO GHIRARDELLI
beginning of the twentieth centuries in looking for unity in the animal
world ; or whether i t is nearer the truth to believe the chaetognaths to
be a group completely isolated from any other. We should therefore
reconsider, under the light of these remarks and taking into account the
most recent researches, the affinities that have been proposed between
Chaetognatha and other phyla.
As to their origin, the very few fossils known tell very little: they
all belong to the same period and the same species. We only know that
Amiskwia sagittiformis Walcott of the middle Cambrian in British
Columbia, had no caudal septum (Walcott, 1911), the origin for which
Grassi (1883) thought could be due to the necessity of separating the
female gonads placed in the trunk coelom from the male ones, placed in
the tail coelom, thus avoiding self-fertilization.
It seems to me,
however, more like Spadella both in the general shape of the body,
because it has only one pair of lateral fins, and because it has tentacles
on the head: all these are typical features of Spadella rather than
Xagitta. The size is, however, larger in Amiskwia (ca 20 mm against
5 or 6 mm at the most in Spadella cephaloptera). Furthermore in
Amiskwia, contrary to what can be seen in all chaetognaths, the
intestine ends a t the caudal end of the body and not at the caudal
septum : this is quite a significant difference from the present-day
chaetognaths. On the other hand, this fossil tells little of the evolution
of the group, because it is the only one and also because no other organ
is visible except the intestine.
Owre and Bayer (1962) also believe that there are resemblances
between Spadella and Amiskwia, but they conclude that the latter is
not a chaetognath but rather a nemertine, closely related to Nectonermes rnirabilis Verrill, mainly because of the structure of the
intestine, which is longer than in Chaetognatha, and the absence in
Amiskzuia of the gonads, which in the opinion of these authors should
have been preserved, if present, in the same way as the intestine was
fossilized. The lack of a caudal septum also supports the attribution of
Amiskwia to the nemertines. Thus, all phylogenetic considerations
based on the study of this fossil fail.
We have seen that Tokioka (1965a) though feeling some doubts as
t o the actual affinity betwccii Amiskwia sagittiformis Walcott and
chaetognaths, establishes for this species the Class of extinct chaetognaths Archisagittoidea. Some further affinities with other groups have
now to be examined, even though they appear in some instances t o be
rather weak.
As for coelenterates, there remains the possibility of a comparison
Amiskwia is considered to be related to Sagitta.
ELVEZIO GHIRARDELLI
beginning of the twentieth centuries in looking for unity in the animal
world ; or whether i t is nearer the truth to believe the chaetognaths to
be a group completely isolated from any other. We should therefore
reconsider, under the light of these remarks and taking into account the
most recent researches, the affinities that have been proposed between
Chaetognatha and other phyla.
As to their origin, the very few fossils known tell very little: they
all belong to the same period and the same species. We only know that
Amiskwia sagittiformis Walcott of the middle Cambrian in British
Columbia, had no caudal septum (Walcott, 1911), the origin for which
Grassi (1883) thought could be due to the necessity of separating the
female gonads placed in the trunk coelom from the male ones, placed in
the tail coelom, thus avoiding self-fertilization.
It seems to me,
however, more like Spadella both in the general shape of the body,
because it has only one pair of lateral fins, and because it has tentacles
on the head: all these are typical features of Spadella rather than
Xagitta. The size is, however, larger in Amiskwia (ca 20 mm against
5 or 6 mm at the most in Spadella cephaloptera). Furthermore in
Amiskwia, contrary to what can be seen in all chaetognaths, the
intestine ends a t the caudal end of the body and not at the caudal
septum : this is quite a significant difference from the present-day
chaetognaths. On the other hand, this fossil tells little of the evolution
of the group, because it is the only one and also because no other organ
is visible except the intestine.
Owre and Bayer (1962) also believe that there are resemblances
between Spadella and Amiskwia, but they conclude that the latter is
not a chaetognath but rather a nemertine, closely related to Nectonermes rnirabilis Verrill, mainly because of the structure of the
intestine, which is longer than in Chaetognatha, and the absence in
Amiskzuia of the gonads, which in the opinion of these authors should
have been preserved, if present, in the same way as the intestine was
fossilized. The lack of a caudal septum also supports the attribution of
Amiskwia to the nemertines. Thus, all phylogenetic considerations
based on the study of this fossil fail.
We have seen that Tokioka (1965a) though feeling some doubts as
t o the actual affinity betwccii Amiskwia sagittiformis Walcott and
chaetognaths, establishes for this species the Class of extinct chaetognaths Archisagittoidea. Some further affinities with other groups have
now to be examined, even though they appear in some instances t o be
rather weak.
As for coelenterates, there remains the possibility of a comparison
Amiskwia is considered to be related to Sagitta.