Die Embryonalentwicklung der Paradiesschnecke ... - TOBIAS-lib

Kapitel 4
P2 does not curve backwards but pushes the organs in the head forwards.
This effect is amplified by a protrusion of the visceral sac (not shown in Fig.
3E,F, see histological section in Fig. 1C, protrustion marked by asterisk).
Osphradium and osphradial ganglion are again positioned on the ventral side
of the visceral sac like in P1, in this case even beneath the digestive gland
(Fig. 1D).
Comparing Fig. 4A, depicting the control situation, and Fig. 4B, depicting
the nervous system in the untorted individual, the difference between
the two arrangements of this organ is quite striking. The nervous system of
the sluggish snail is compressed along the middle axis in a way that visceral
ganglion and supraintestinal ganglion are located in the same plane. However,
a closer inspection reveals that the general bauplan of the two nervous
systems is essentially the same. During embryogenesis of M. cornuarietis
two intestinal ganglia develop on either side of the larval stomach (details
will be explained in the following section of this paper). During torsion,
these ganglia change sides and the one which has moved to the right side
of the body, the subintestinal ganglion, fuses with the right pleuro-pedal
ganglionic mass, later only called pedal ganglion. In Ampullariidae, this fusion
leads to a loss of the connective between right pleuro-pedal ganglion
and subintestinal ganglion, thus obscuring the crossing of the pleuro-visceral
connectives and, therefore, the classical form of streptoneury even though
streptoneury in its broadest sense has been established in this snail family
(for a detailed description see Demian and Yousif, 1975). Because of this,
only the supraintestinal nerve can be clearly seen, the subintestinal nerve is
too closely associated with the pedal commissure. In both PtCl 2 -exposed
and control adult snails, P2 and C1, the seperate intestinal ganglia can be
seen on the left side of the body and in both cases the connection between
supraintestinal ganglion and right pedal ganglion, the supraintestinal nerve
is present. The nervous system of P2 is in a “post-torsional” arrangement
even though ontogenetic torsion was experimentally prevented. Views (from
caudal to cranial) of the nervous systems of P1 and P2 are shown in Figs.
4C and D. Also here, the “post-torsional” and thus streptoneurous condition
can be observed in both untorted individuals. Nevertheless, some individual
117