THE PHYSIOT.OUY OF ASCIDIANB
127
many of the the electrical characteristks of the myocardial cells. It now
seems likely that the reversal of heart beat, which is common to all
ascidians, is thr result of fluctuations in the activity of the visceral
pacemaker, but, we are still totally ignorantz of thc functional significance
of such behaviour. I t is suggestd in this review that i t may bc associated in some way with the blood syqtcni of' thci test and blind ending
vascular ampullae a t the periphcry.
Advances in our understanding of the blood itself have been confined
to the morula cells and their precrursors and little new information has
appeared on other aspects of blood and its funciion. It has been established that the morula cell is derived from amoebocytes through a
series of steps including signet-ring and compartment cells and recent
work implies that this is associated with the synthesis of the organometal compound haemovanadin or in some ascidians a similar compound of iron. These cells are intimately concerned with the production
of the test and migrate through the mantJe or test vessels into the substance of the tunic where they cytolyse. The organo-metal compound
is a reducing agent held in the reduced state inside the cell by the
presence of strong acid. A t cytolysis the acid is dispersed and the
reducing action of the compound becomes available but the precise
way in which this activity is used has not been established. The
association of those cells with polysaccharide fibres suggests that they
may play some significant role in fibre production.
The simplicity of life in ascidians is matched by the apparent
simplicity of their nervous system. Ascidian nerves are unresponsive to
normal nerve stains and they are difficult to trace, but experimental
work has thrown considerable light on their function. The primary role
of the nervous system is in the maintenance of the water current. This
seems to be achieved through vincercll control of the branchial cilia and
somatic control of siphons and mantle. While niueh is now known about
separate facets of the nervous mechanism we are still far from an
understanding of the ovemll control of water movement. Siphonal
apertures are controlled by a cross reflex mediated through the
ganglion but the sensory mechanisms providing for minor adjustments
are not fully understood. The precise role of such structures as ocelli
and cupula organs remains to be elucidated. The ocelli are not photoreceptors but might be mechano-receptors for monitoring the position
of the siphonal lobes. Cupula organs in the atrial cavity may be pressure
receptors. Ciliary activity fs known to be controlled, at least in part,
through the visceral fibre complex and perhaps also through an
innervation from the ventral side of the brunchial sac, but the relationship between this and the activity of the somatic. Bystem is unknown.
127
many of the the electrical characteristks of the myocardial cells. It now
seems likely that the reversal of heart beat, which is common to all
ascidians, is thr result of fluctuations in the activity of the visceral
pacemaker, but, we are still totally ignorantz of thc functional significance
of such behaviour. I t is suggestd in this review that i t may bc associated in some way with the blood syqtcni of' thci test and blind ending
vascular ampullae a t the periphcry.
Advances in our understanding of the blood itself have been confined
to the morula cells and their precrursors and little new information has
appeared on other aspects of blood and its funciion. It has been established that the morula cell is derived from amoebocytes through a
series of steps including signet-ring and compartment cells and recent
work implies that this is associated with the synthesis of the organometal compound haemovanadin or in some ascidians a similar compound of iron. These cells are intimately concerned with the production
of the test and migrate through the mantJe or test vessels into the substance of the tunic where they cytolyse. The organo-metal compound
is a reducing agent held in the reduced state inside the cell by the
presence of strong acid. A t cytolysis the acid is dispersed and the
reducing action of the compound becomes available but the precise
way in which this activity is used has not been established. The
association of those cells with polysaccharide fibres suggests that they
may play some significant role in fibre production.
The simplicity of life in ascidians is matched by the apparent
simplicity of their nervous system. Ascidian nerves are unresponsive to
normal nerve stains and they are difficult to trace, but experimental
work has thrown considerable light on their function. The primary role
of the nervous system is in the maintenance of the water current. This
seems to be achieved through vincercll control of the branchial cilia and
somatic control of siphons and mantle. While niueh is now known about
separate facets of the nervous mechanism we are still far from an
understanding of the ovemll control of water movement. Siphonal
apertures are controlled by a cross reflex mediated through the
ganglion but the sensory mechanisms providing for minor adjustments
are not fully understood. The precise role of such structures as ocelli
and cupula organs remains to be elucidated. The ocelli are not photoreceptors but might be mechano-receptors for monitoring the position
of the siphonal lobes. Cupula organs in the atrial cavity may be pressure
receptors. Ciliary activity fs known to be controlled, at least in part,
through the visceral fibre complex and perhaps also through an
innervation from the ventral side of the brunchial sac, but the relationship between this and the activity of the somatic. Bystem is unknown.