hus n i c t L s i i n d tho trt~iirj-epil helid resistance in Ciom intestinalis, the
intercellular spacr between adjuccnt cells must be about 160 A and
the maximum gap at the apical nexus is only 0.12 A. Following Waltman (1966) who studied a similar Hituation in the Ampullae of Lorenzini, Kriebel suggests, in thc light of these data, that the apical nexus
forms a barrier to the transepithelial passage of ions. (For a general
account of the nexus in cells see Farquhar and Palade (1963) and
Dewey and Barr (1964).) In addition to the nexus Kalk has described
peg and Rocket cytoplasmic junctions between adjacent myoendothelial
cells.
Kawaguti and lkemoto ( 1958), Schulze (1964) and K d k (1970) have
all described cellular processes or cytoplasmic lobes projecting from the
apical cell surface into the lumcn of the heart and according to Kalk
each lobe contains 1-3 multivesiculate vacuoles which she considers
may be extruded into the heart lumen during contraction. Schulze and
Kalk describe a fibrous matrix lining the heart and into which the
cytoplasmic lobes protrude. Further detail of the ultrastructure of the
myoendothelial cell in Corella, ioillmerianu Herdman is given by Oliphant
and Cloney ( 1 972).
Apart from the myoendothelial cells described above there is a line
of Undifferentiated cells, about, 2-5 cells wide, running the length of the
heart, on the side opposite to the raphe and a ring of similar cells
connects the heart at either end to the main blood sinuses. Millar
(1 953a) considers that these undifferentiated cells give rise to new
muscle cells and he dcsoribes the processes whereby older muscle cells
degencrate and are cast off into the pericardial cavity to form the
pericardial body.
The nervous innervation of the heart and pericardium irr discuwed
later (p. 92) and it is concluded that the heart itself is dcvoid of innervation, but there does appear to be a fine innervation of the pericardium
which may be a, sensory motor reflex controlling pericardial preseure.
R. Contraction of the heart
In iLn active undisturbed ascidian a series of peristaltic contractions
will pass from one end of the heart to another pushing the blood
forwards as it does so. After a period of time the wave of contraction
will slow up and stop and either immediately or after a pause the heart
commences beating again but in the opposite direction. The series of
beats in any one direction is known as a pulsation series and the period
during which a pulsation lasts and until reverd occurs is known as a
revmml period (Kriebc*l, 1968~). Thcrc. is no Iwgulsrity about reverwl
wid suc(1essivc rwcrsd periods nir~y 1~ of diffcrrnt lengths, but the
intercellular spacr between adjuccnt cells must be about 160 A and
the maximum gap at the apical nexus is only 0.12 A. Following Waltman (1966) who studied a similar Hituation in the Ampullae of Lorenzini, Kriebel suggests, in thc light of these data, that the apical nexus
forms a barrier to the transepithelial passage of ions. (For a general
account of the nexus in cells see Farquhar and Palade (1963) and
Dewey and Barr (1964).) In addition to the nexus Kalk has described
peg and Rocket cytoplasmic junctions between adjacent myoendothelial
cells.
Kawaguti and lkemoto ( 1958), Schulze (1964) and K d k (1970) have
all described cellular processes or cytoplasmic lobes projecting from the
apical cell surface into the lumcn of the heart and according to Kalk
each lobe contains 1-3 multivesiculate vacuoles which she considers
may be extruded into the heart lumen during contraction. Schulze and
Kalk describe a fibrous matrix lining the heart and into which the
cytoplasmic lobes protrude. Further detail of the ultrastructure of the
myoendothelial cell in Corella, ioillmerianu Herdman is given by Oliphant
and Cloney ( 1 972).
Apart from the myoendothelial cells described above there is a line
of Undifferentiated cells, about, 2-5 cells wide, running the length of the
heart, on the side opposite to the raphe and a ring of similar cells
connects the heart at either end to the main blood sinuses. Millar
(1 953a) considers that these undifferentiated cells give rise to new
muscle cells and he dcsoribes the processes whereby older muscle cells
degencrate and are cast off into the pericardial cavity to form the
pericardial body.
The nervous innervation of the heart and pericardium irr discuwed
later (p. 92) and it is concluded that the heart itself is dcvoid of innervation, but there does appear to be a fine innervation of the pericardium
which may be a, sensory motor reflex controlling pericardial preseure.
R. Contraction of the heart
In iLn active undisturbed ascidian a series of peristaltic contractions
will pass from one end of the heart to another pushing the blood
forwards as it does so. After a period of time the wave of contraction
will slow up and stop and either immediately or after a pause the heart
commences beating again but in the opposite direction. The series of
beats in any one direction is known as a pulsation series and the period
during which a pulsation lasts and until reverd occurs is known as a
revmml period (Kriebc*l, 1968~). Thcrc. is no Iwgulsrity about reverwl
wid suc(1essivc rwcrsd periods nir~y 1~ of diffcrrnt lengths, but the