‘I’HE PllYS101~0(1Y O r AXCIDIANS
45
tissue lining on t he luni(~nd wall. I’or iL tiillcr clcscriptio~~ of the ascidiun
blood system sci’ Millw ( 1 !h53a).
The henrt aiid circwlatory Rysteiii havr no valves and the blood is
pumped around the body by a series of peristaltic contractions passing
from one end o f t h e heart to ariothcr a t II frequency of about 25 to 50
heats per niinutc acc*ording to the species and size of the animal conclerned. The direction of the peristaltic wave changes at fairly regular
intervals so that a t one time the wave is advisceral (i.e. passes from the
hypobranchial ond toward the viscerd end) while at other times it is
iibviscerd (passing from the visc.cral end to the hypobranchial). This
reversal of heart beat is found universally throughout the Tunicata but
is not unique t o this group of animals. It is well known in certain groups
of insects (Jones, 1966) and has been recorded in several other groups
of animals (Azariah, 1965 ; Beklemishev, 1969 ; Mislin, 1969).
The ascidian heart poses a number of interesting questions namely :
(1) How is the heart beat controlled? Is it neurogenic (under nervous
control) or is it myogenic (under the intrinsic control of its
own musculature)?
(2) How is diastolic filling of tha heart effmted in the absence of valves
or extrinsic musculature?
(3) What controls the reversal of peristulsis and what is its functional
significance ?
The literutirre on the physiology of the ascidian heart is taxtcnsive.
Most, of the carly work has been reviewed hy Skramlik (1938). Krijgsman
(1956), Krijgsmttn and Krijgsmi~n ( 1 !)57) rtnd Millar ( 1953%).
A. h’trur./ure of the hmrt
The tubular heart is enclosed dong its whole length within a fluid
filled pericardium. In dcvelopnient the lieurt is formed as a longitudinal
invagination of thc poricardiuni the lips of which approximate together
und close off t~ tubular fold. The hoart remains attached to the pericardium nlong the length of the original suture, this dorsal attachment
bcing referred to ns the raphe. Thc walls of the pericardium are formed
of a flattened basement epithelium in which the membranes of adjacent
cells are folded in such a way as to interdigitate closely with one another
(Kalk, 1970) (Fig. i 3). The interlocking of cells eoupled with B thickened
basement membrane probably provides rigidity to the wall and enables
it to exert pressure upon the fluid. A few strands of smooth muscle
surround the pericardial wall (Miller, 1953a) and presumably provide a
mechanism for rtltering tension in thcl fluid.
45
tissue lining on t he luni(~nd wall. I’or iL tiillcr clcscriptio~~ of the ascidiun
blood system sci’ Millw ( 1 !h53a).
The henrt aiid circwlatory Rysteiii havr no valves and the blood is
pumped around the body by a series of peristaltic contractions passing
from one end o f t h e heart to ariothcr a t II frequency of about 25 to 50
heats per niinutc acc*ording to the species and size of the animal conclerned. The direction of the peristaltic wave changes at fairly regular
intervals so that a t one time the wave is advisceral (i.e. passes from the
hypobranchial ond toward the viscerd end) while at other times it is
iibviscerd (passing from the visc.cral end to the hypobranchial). This
reversal of heart beat is found universally throughout the Tunicata but
is not unique t o this group of animals. It is well known in certain groups
of insects (Jones, 1966) and has been recorded in several other groups
of animals (Azariah, 1965 ; Beklemishev, 1969 ; Mislin, 1969).
The ascidian heart poses a number of interesting questions namely :
(1) How is the heart beat controlled? Is it neurogenic (under nervous
control) or is it myogenic (under the intrinsic control of its
own musculature)?
(2) How is diastolic filling of tha heart effmted in the absence of valves
or extrinsic musculature?
(3) What controls the reversal of peristulsis and what is its functional
significance ?
The literutirre on the physiology of the ascidian heart is taxtcnsive.
Most, of the carly work has been reviewed hy Skramlik (1938). Krijgsman
(1956), Krijgsmttn and Krijgsmi~n ( 1 !)57) rtnd Millar ( 1953%).
A. h’trur./ure of the hmrt
The tubular heart is enclosed dong its whole length within a fluid
filled pericardium. In dcvelopnient the lieurt is formed as a longitudinal
invagination of thc poricardiuni the lips of which approximate together
und close off t~ tubular fold. The hoart remains attached to the pericardium nlong the length of the original suture, this dorsal attachment
bcing referred to ns the raphe. Thc walls of the pericardium are formed
of a flattened basement epithelium in which the membranes of adjacent
cells are folded in such a way as to interdigitate closely with one another
(Kalk, 1970) (Fig. i 3). The interlocking of cells eoupled with B thickened
basement membrane probably provides rigidity to the wall and enables
it to exert pressure upon the fluid. A few strands of smooth muscle
surround the pericardial wall (Miller, 1953a) and presumably provide a
mechanism for rtltering tension in thcl fluid.