probably protective in f'iinction. I t eiisures that at the moment of
contraction the cilia lire relaxcd tind the stigmatra are " wide open " thus
permitting 11 free flow of water tmd pressure equalization between the
titrial cavity wid the brcinehiiil sac without any risk of' damaging the
ciliary mechanism.
Although Alcxandrowicz ( 1; ) 13) reported finding nerve fibres, from
bundles on tho pericardial raplie, innervating the heart muscle in Ciona
intestinalis it now seems certain that this is incorrect and the ascidian
heart musrle is totally devoid of nervous innervation (see Bacq, 1935a ;
Krijgsman, 1966; Bone and Whitear, l!t5H; Kalk, 1970). According to
Bone and Whitear who studied living hearts with phase microscopy
there is a plexus of nerve fibres in the wall of the pericardium as well as
a fine plcxus on the surrounding epicardium. This plexus arises from
bundles of fibres which pass through the connective tissue layer between
epicardium and pwicnrdium and thereafter branch into single fibres
across the pericardial wall. Nerve bundles, similar to t,hose seen by
Alcxandrowicz, were sometimes seen on the raphe above the heart but
no fibrcs go from thesc to the heart. Around the pericardial wall there is
a band of Hmooth muscle, but Bone and Whitear were unable to locate
any nervous innervation of it and they concluded that the nerve plexus
in tho wall must be sensory. SkramIik (1929) found that a pressure of
2 cm of water in the pericardirtm is necessary for proper functioning of
the heart and Hone and Whitear suggested that possibly the nerve
plexus is part of a sensory-motor reflex involving the pericardial muscle
which might control this pressure. They further suggested that since no
ganglion cells tire visible in this region the system is probably connected
to the centml ganglion. Markman (1 968) stated that the pericardium
has a double innervation, one end iniiervsted from the visceral nerve and
the other from a branch of one of the anterior nerves. Markman also
illustrates elongate structures in the raphe of the pericardium which he
considered may be sensory. Hunter ( 1898) described similar structures
which he also helieved to be sensory.
The gonad appears to be innervated directly from the visceral nerve
but is also intitnately associated with the dorsal strand arising from the
neural gland (p. 123).
In summary, the existing evidence for a visceral innervation in
ttscidians is still incomplete and fragmentary and until further critical
microscopical and electro-physiological techniques can be brought to
bear upon this system our undcrstanding must remain limited. The
main visceral nerve emanating from the posterior part of the ganglion
i H supplemented by a system of fibres and cells forming a meshwork
wound the dorsal strand. It is propoeed here that this meshwork around
contraction the cilia lire relaxcd tind the stigmatra are " wide open " thus
permitting 11 free flow of water tmd pressure equalization between the
titrial cavity wid the brcinehiiil sac without any risk of' damaging the
ciliary mechanism.
Although Alcxandrowicz ( 1; ) 13) reported finding nerve fibres, from
bundles on tho pericardial raplie, innervating the heart muscle in Ciona
intestinalis it now seems certain that this is incorrect and the ascidian
heart musrle is totally devoid of nervous innervation (see Bacq, 1935a ;
Krijgsman, 1966; Bone and Whitear, l!t5H; Kalk, 1970). According to
Bone and Whitear who studied living hearts with phase microscopy
there is a plexus of nerve fibres in the wall of the pericardium as well as
a fine plcxus on the surrounding epicardium. This plexus arises from
bundles of fibres which pass through the connective tissue layer between
epicardium and pwicnrdium and thereafter branch into single fibres
across the pericardial wall. Nerve bundles, similar to t,hose seen by
Alcxandrowicz, were sometimes seen on the raphe above the heart but
no fibrcs go from thesc to the heart. Around the pericardial wall there is
a band of Hmooth muscle, but Bone and Whitear were unable to locate
any nervous innervation of it and they concluded that the nerve plexus
in tho wall must be sensory. SkramIik (1929) found that a pressure of
2 cm of water in the pericardirtm is necessary for proper functioning of
the heart and Hone and Whitear suggested that possibly the nerve
plexus is part of a sensory-motor reflex involving the pericardial muscle
which might control this pressure. They further suggested that since no
ganglion cells tire visible in this region the system is probably connected
to the centml ganglion. Markman (1 968) stated that the pericardium
has a double innervation, one end iniiervsted from the visceral nerve and
the other from a branch of one of the anterior nerves. Markman also
illustrates elongate structures in the raphe of the pericardium which he
considered may be sensory. Hunter ( 1898) described similar structures
which he also helieved to be sensory.
The gonad appears to be innervated directly from the visceral nerve
but is also intitnately associated with the dorsal strand arising from the
neural gland (p. 123).
In summary, the existing evidence for a visceral innervation in
ttscidians is still incomplete and fragmentary and until further critical
microscopical and electro-physiological techniques can be brought to
bear upon this system our undcrstanding must remain limited. The
main visceral nerve emanating from the posterior part of the ganglion
i H supplemented by a system of fibres and cells forming a meshwork
wound the dorsal strand. It is propoeed here that this meshwork around