SOME ASPECTS O F THE BIOLOGY OF THE CHAETOGNATHS
323
(1959) and in Spadella cephaloptera by Vasiljev (1925), and Ghirardelli
(1953d, 1954a). Some authors believe that cross-fertilization is the
rule (Ghirardelli and van Oye), while others think that one of the
partners acts only as male and the other only as female, so that there
should not be a cross-fertilization (John, 1933).
My work has been carried out on specimens of Xpadella cephaloptera
from the prairies of Posidonia near Capo Posillipo (Naples). The
Spadella concerned were always mature male, with the seminal vesicles
at the completely filled stage ; the ovaries, on the other hand, were a t
different stages of maturity.
Before mating, two specimens approach, and touch each other
repeatedly with their heads, necks and perhaps the corona ciliata: so
quick are the movements that it is often impossible t o follow their
succession exactly. Then the two specimens place themselves with
their heads in opposite directions and their bodies almost touching.
Immediately afterwards, they make a sort of scissors-like movement
which is repeated several times, following which the neck and trunk
region of one crosses with the caudal region of the other. Similar
observations were made in 1966 a t the Helsingcar Marine Biological
Laboratory by a group of students (Thorson, personal communication)
(Figs. 22 and 23). After this series of very quick movements, which may
last even less than 1 sec, each individual has one of its seminal vesicles
empty, i.e. the one which was turned towards the partner. The contents
of the vesiculae appear on the neck of each of the two individuals in
the form of a spermatophore whose dimensions are the same as those
of the vesiculae seminalis. The spermatophores are always laid on the
dorsal median line directly behind the ciliary loop. According t o Vasiljev
(1925), on the other hand, the two individuals place themselves with
their heads pointing in opposite directions but in such a manner that
the seminal vesicle of each is in contact with the orifice of the vesicula
seminalis of the other. I n this way the spermatozoa would pass direct'ly
into the receptaculus seminalis from the vesiculae which could have
been broken by some stick-shaped inclusions contained in the cells
surrounding the genital papilla.
I n the specimens I have observed, the exchange of the spermatophores is, apart from some rare exceptions, mutual and simultaneous.
Only in some cases does the exchange occur at successive intervals, one
individual giving its spermatophore to the other without receiving
another at the same time. The two individuals, however, remain in
contact until the second spermatophore has also been placed on the
head of the partner. Furthermore, in other rare cases, as John
observed, only one of the two individuals acts as a male, while the
323
(1959) and in Spadella cephaloptera by Vasiljev (1925), and Ghirardelli
(1953d, 1954a). Some authors believe that cross-fertilization is the
rule (Ghirardelli and van Oye), while others think that one of the
partners acts only as male and the other only as female, so that there
should not be a cross-fertilization (John, 1933).
My work has been carried out on specimens of Xpadella cephaloptera
from the prairies of Posidonia near Capo Posillipo (Naples). The
Spadella concerned were always mature male, with the seminal vesicles
at the completely filled stage ; the ovaries, on the other hand, were a t
different stages of maturity.
Before mating, two specimens approach, and touch each other
repeatedly with their heads, necks and perhaps the corona ciliata: so
quick are the movements that it is often impossible t o follow their
succession exactly. Then the two specimens place themselves with
their heads in opposite directions and their bodies almost touching.
Immediately afterwards, they make a sort of scissors-like movement
which is repeated several times, following which the neck and trunk
region of one crosses with the caudal region of the other. Similar
observations were made in 1966 a t the Helsingcar Marine Biological
Laboratory by a group of students (Thorson, personal communication)
(Figs. 22 and 23). After this series of very quick movements, which may
last even less than 1 sec, each individual has one of its seminal vesicles
empty, i.e. the one which was turned towards the partner. The contents
of the vesiculae appear on the neck of each of the two individuals in
the form of a spermatophore whose dimensions are the same as those
of the vesiculae seminalis. The spermatophores are always laid on the
dorsal median line directly behind the ciliary loop. According t o Vasiljev
(1925), on the other hand, the two individuals place themselves with
their heads pointing in opposite directions but in such a manner that
the seminal vesicle of each is in contact with the orifice of the vesicula
seminalis of the other. I n this way the spermatozoa would pass direct'ly
into the receptaculus seminalis from the vesiculae which could have
been broken by some stick-shaped inclusions contained in the cells
surrounding the genital papilla.
I n the specimens I have observed, the exchange of the spermatophores is, apart from some rare exceptions, mutual and simultaneous.
Only in some cases does the exchange occur at successive intervals, one
individual giving its spermatophore to the other without receiving
another at the same time. The two individuals, however, remain in
contact until the second spermatophore has also been placed on the
head of the partner. Furthermore, in other rare cases, as John
observed, only one of the two individuals acts as a male, while the