synaptic vesivles it is irnpossible to do more than speculate on their
function.
Apart from the tine iierve fibres arising peripherally and reported by
Hilton and Millar there lire five main nerves arising from the ganglion.
A t the anterior end there are t w o iiervcs each of which gives rise to four
branches as follow8 : ( 1 ) To the dorsal side of the branchial siphon,
(2) to the ventral side of'the brim-hid siphon, (3) and ( 4 ) to the musculature of the mantle. Posteriorly there are two similar nerves which
innervate thc atrial siphon as well UH the mantle musculature and
arising rnedially between these two nerves t h w e is an unpaired visceral
nerve which enters the roof of thc brunchial Nac and extends pogteriorly
toward t h e m i n visceral mam. Miller dates that while the branchial
siphon is wholly innervated from thc anterior region of the ganglion and
the atrial siphon from the posterior end, the mantle musculature is
innervated from both ends although the nerves of the anterior end
contribute more to the ventral side than do the posterior nerves which
contribute more to dorsal musculature.
B. The visceral innervation
From the generalized account of the ascidian nervous system it
would appear that the sensory motor innervation is entirely associated
with the paired anterior and posterior nerves while the visceral innervation is confined to the single unpaired posterior visceral nerve. This is
probably true for the sensory motor system but there is considerable
doubt and uncertainty about the nature of the visceral innervation.
Some of the viscerd functions which may be expected to be under
nervous control are ;LS foliows :
(a) Ciliary activity in the branchial m c . The beating of the cilia is not
continuous but intermittent. This is difficult to dembnstrate on a large
solitary ascidian ljiit can be obscrvect readily in small zooids from
colonial forms or in young uscidians with only a few rows of stigmata.
Observation of these reveal^ that at intervals all of the cilia etop heating
momentarily and simultaneously. This co-ordinated activity and inhibition of the cilia is under nervous control (Mackie et al., 1974).
(b) The activity of th,e heart a d pericurdium. The peristaltic activity
of the heart is reversible and thc heart beats for a certain period in one
direction then stops and recommences beating in the opposite direction
(see p. 48). Although this type of activity could be under neurogenic
control it has now been established that, this is not the case, that it is
entirely myogenic and there is no direct innervation of the heart (see
p. 64). Nevertheless there is an innervation of thc pericardium which is
further discussed below.
function.
Apart from the tine iierve fibres arising peripherally and reported by
Hilton and Millar there lire five main nerves arising from the ganglion.
A t the anterior end there are t w o iiervcs each of which gives rise to four
branches as follow8 : ( 1 ) To the dorsal side of the branchial siphon,
(2) to the ventral side of'the brim-hid siphon, (3) and ( 4 ) to the musculature of the mantle. Posteriorly there are two similar nerves which
innervate thc atrial siphon as well UH the mantle musculature and
arising rnedially between these two nerves t h w e is an unpaired visceral
nerve which enters the roof of thc brunchial Nac and extends pogteriorly
toward t h e m i n visceral mam. Miller dates that while the branchial
siphon is wholly innervated from thc anterior region of the ganglion and
the atrial siphon from the posterior end, the mantle musculature is
innervated from both ends although the nerves of the anterior end
contribute more to the ventral side than do the posterior nerves which
contribute more to dorsal musculature.
B. The visceral innervation
From the generalized account of the ascidian nervous system it
would appear that the sensory motor innervation is entirely associated
with the paired anterior and posterior nerves while the visceral innervation is confined to the single unpaired posterior visceral nerve. This is
probably true for the sensory motor system but there is considerable
doubt and uncertainty about the nature of the visceral innervation.
Some of the viscerd functions which may be expected to be under
nervous control are ;LS foliows :
(a) Ciliary activity in the branchial m c . The beating of the cilia is not
continuous but intermittent. This is difficult to dembnstrate on a large
solitary ascidian ljiit can be obscrvect readily in small zooids from
colonial forms or in young uscidians with only a few rows of stigmata.
Observation of these reveal^ that at intervals all of the cilia etop heating
momentarily and simultaneously. This co-ordinated activity and inhibition of the cilia is under nervous control (Mackie et al., 1974).
(b) The activity of th,e heart a d pericurdium. The peristaltic activity
of the heart is reversible and thc heart beats for a certain period in one
direction then stops and recommences beating in the opposite direction
(see p. 48). Although this type of activity could be under neurogenic
control it has now been established that, this is not the case, that it is
entirely myogenic and there is no direct innervation of the heart (see
p. 64). Nevertheless there is an innervation of thc pericardium which is
further discussed below.