324
ELVEZIO GHIRAEDELLI
other acts as female. Also according to the observations made at
Helsingm the exchange is not reciprocal; in fact the artist has drawn
the spermatophore placed on the caudal region of the individual acting
as female, and not near the corona. In the specimens I have observed,
on the contrary, the only position chosen for the laying of the spermatophores was the one behind the corona.
FIG. 22. Preliminary phases of the meeting and the mating in Spadella cephaloplera
(Drawn by Kaj
according to the observations made in Helsingm Laboratory.
Olsen.)
Some seconds after the spermatophores have been placed on the neck,
their posterior regions appear to melt (i.e. those directed towards the
tail) and they give rise to a stream of spermatozoa heading towards the
caudal septum. When the stream has reached a certain distance from
the caudal septum, it forks and each of the two branches heads towards
the orifice and enters the receptaculus seminalis (Figs 23 and 24).
Not more than 2 or 4 min have elapsed from the beginning of the
migration of the spermatozoa. The penetration of all spermatozoa
through the orifice of the seminal receptacle takes a little longer. When
mating occurs between individuals whose receptacles are already more
or less full, it may even take 20 min before all spermatozoa can enter
ELVEZIO GHIRAEDELLI
other acts as female. Also according to the observations made at
Helsingm the exchange is not reciprocal; in fact the artist has drawn
the spermatophore placed on the caudal region of the individual acting
as female, and not near the corona. In the specimens I have observed,
on the contrary, the only position chosen for the laying of the spermatophores was the one behind the corona.
FIG. 22. Preliminary phases of the meeting and the mating in Spadella cephaloplera
(Drawn by Kaj
according to the observations made in Helsingm Laboratory.
Olsen.)
Some seconds after the spermatophores have been placed on the neck,
their posterior regions appear to melt (i.e. those directed towards the
tail) and they give rise to a stream of spermatozoa heading towards the
caudal septum. When the stream has reached a certain distance from
the caudal septum, it forks and each of the two branches heads towards
the orifice and enters the receptaculus seminalis (Figs 23 and 24).
Not more than 2 or 4 min have elapsed from the beginning of the
migration of the spermatozoa. The penetration of all spermatozoa
through the orifice of the seminal receptacle takes a little longer. When
mating occurs between individuals whose receptacles are already more
or less full, it may even take 20 min before all spermatozoa can enter