SOME ASPECTS O F THE BIOLOGY OF THE CHAETOGNATHS
329
migrating stream when it reaches the caudal septum. Gabe himself,
who does not believe the secretion of the corona to be of a neurosecretory nature (see p. 285), thinks that it is not unlikely that the
product itself-probably a mucopolysaccharide or a mucoprotein-may
be concerned in the migration of the spermatozoa.
2. Fertilization in Sagitta and other genera
Things appear somewhat more complicated as far as Sagitta and
the other genera of Chaetognatha are concerned. First of all it is much
more difficult to make laboratory observations, due to their different
behaviour and to the fact that it is very difficult to keep Sagitta in
culture conditions long enough. It can also be presumed that the
reproductive behaviour of pelagic organisms may be different from that
of benthic species, even if belonging to the same systematic group. Also
few authors have dealt with the subject in pelagic Chaetognatha.
We have quoted Grassi who believed that the fertilization occurred
after a reciprocal mating. Stevens (1910) and Bord&s (1920) deemed
self-fertilization probable, because the sperms, coming out of the
vesiculae seminales placed on the posterior end of the body, could
reach the receptacula by climbing along the tail and entering them.
Van Oye (1931) observed that the contents of a mature vesicle in
Sagitta is expelled in the form of a spermatophore more or less sphereshaped and wrapped in a mucous substance whose origin is unknown.
The expulsion occurs as the consequence of a series of strong movements
of trunk and tail. The spermatophore seems then to be pushed towards
the end of the caudal fin which bends so as to form a sort of gutter: it
is not clear how this bending may occur in view of the absence of
muscles in the caudal fin. Kuhl (1938) thinks that the phenomenon
may be due to purely mechanical causes, i.e. to the leaning on the
spermatophore of the terminal part of the fin, which in adult animals
may be more or less damaged and is in any case very flexible. Mating
according to van Oye occurs when two individuals, each carrying a
spermatophore on the caudal fin, happen to meet; they then dispose
themselves with the heads iii opposite directions and, since the caudal
regions of the Sagitta are of about the same length, the spermatophore
carried by the fin of one of them comes to face the genital orifice of the
other, which in the meantime has extended. The spermatophores are
then held by the latcral fins of the individuals which have been fertilized
and the sperms actively penetrate the seminal receptacle. Mating is
very quick; it can be repeated several times and occurs a t night.
The exchange of the spermatophores is, according to van Oye, mutual,
and self-fertilization must be excluded. The lateral fins may also play
329
migrating stream when it reaches the caudal septum. Gabe himself,
who does not believe the secretion of the corona to be of a neurosecretory nature (see p. 285), thinks that it is not unlikely that the
product itself-probably a mucopolysaccharide or a mucoprotein-may
be concerned in the migration of the spermatozoa.
2. Fertilization in Sagitta and other genera
Things appear somewhat more complicated as far as Sagitta and
the other genera of Chaetognatha are concerned. First of all it is much
more difficult to make laboratory observations, due to their different
behaviour and to the fact that it is very difficult to keep Sagitta in
culture conditions long enough. It can also be presumed that the
reproductive behaviour of pelagic organisms may be different from that
of benthic species, even if belonging to the same systematic group. Also
few authors have dealt with the subject in pelagic Chaetognatha.
We have quoted Grassi who believed that the fertilization occurred
after a reciprocal mating. Stevens (1910) and Bord&s (1920) deemed
self-fertilization probable, because the sperms, coming out of the
vesiculae seminales placed on the posterior end of the body, could
reach the receptacula by climbing along the tail and entering them.
Van Oye (1931) observed that the contents of a mature vesicle in
Sagitta is expelled in the form of a spermatophore more or less sphereshaped and wrapped in a mucous substance whose origin is unknown.
The expulsion occurs as the consequence of a series of strong movements
of trunk and tail. The spermatophore seems then to be pushed towards
the end of the caudal fin which bends so as to form a sort of gutter: it
is not clear how this bending may occur in view of the absence of
muscles in the caudal fin. Kuhl (1938) thinks that the phenomenon
may be due to purely mechanical causes, i.e. to the leaning on the
spermatophore of the terminal part of the fin, which in adult animals
may be more or less damaged and is in any case very flexible. Mating
according to van Oye occurs when two individuals, each carrying a
spermatophore on the caudal fin, happen to meet; they then dispose
themselves with the heads iii opposite directions and, since the caudal
regions of the Sagitta are of about the same length, the spermatophore
carried by the fin of one of them comes to face the genital orifice of the
other, which in the meantime has extended. The spermatophores are
then held by the latcral fins of the individuals which have been fertilized
and the sperms actively penetrate the seminal receptacle. Mating is
very quick; it can be repeated several times and occurs a t night.
The exchange of the spermatophores is, according to van Oye, mutual,
and self-fertilization must be excluded. The lateral fins may also play