ment has not been investigated. In the tnajority of phlebobranchs other
tharl Ciona the alimentary caiiul has been reflected forward to one side
of the pharyllx accompanied by a reduction of mantle musculature on
that side. This reduction reaches its highest degree in the genus A8cilia
where mttiitle muscultiture is idmost entirely absent on the left-hand
side. As i l consequence of thisthere has usually been a modification of the
musculature on the mantle of the right side and while longitudinal and
circular components are still apparent there is u tendency to develop a
~ ~ e t w o r k
of crossing fibrcs. At the same time the test has become thicker
and more rigid than in Ciona and, while developing elaaticity, haa
imposed considerable limitations on body movement. C i o m is capable
of longitudinal retractioti snd body shortening to a degree quite impossible in the heavier framework of Aacidia. Goodhody and Trueman
(1969) have likened the movement of the mantle in Aecidia to a diaphragm which contracts the mantle cavity but there is no opposing
niuscle system to enable it to chxpand again. Hecht (1918a) has further
shown that in Ascidia the test on the right side is thinnkr than on the
left. In this way the test bends easily on the right during muscle
contraction while on the left side it forms an elaatic framework to assist
return of body shape. The situation in the stolidobranchs, where the
gut is reflected forwards and in most cases the test is tough but the
musculature complete 011 both sides, does not appear to have been
examined.
Within this framework the types of movement available to aecidians
are limited and restricted to : (1) opening and closing of the siphons and,
under strong stimulation, retraction of the siphons ; (2) expansion and
Contraction of the mantle wall ; contraction of the wall results in expulsion of water from the brmchial and atrial cavities through the siphons ;
(3) longitudinal contractiqn of the whole body which in Ciona results in
uniform shortening of the body. In Ascidia it tendti to produce a
bending of the anterior end of the animal towsrd the right-hand side ;
this is entirely due to the unequal distribution of muscle and the rigidity
of the test.
Two principal types of' activity and response in ascidians can be
identified, namely direct responses and crossed reflexes. Direct responee8
are the result of external stirnulation of the animal either on the general
surface of the test or around the siphons, and result in siphon closure,
contraction of the mantle musculature and expulsion of water from the
branchial and atrial cavities. Jf the external surface is stimulated near
one of the siphons, that siphon will close first and the other siphon a
moment later. This type of response is primarily elicited by mechanical
stimulation of the body surface.
tharl Ciona the alimentary caiiul has been reflected forward to one side
of the pharyllx accompanied by a reduction of mantle musculature on
that side. This reduction reaches its highest degree in the genus A8cilia
where mttiitle muscultiture is idmost entirely absent on the left-hand
side. As i l consequence of thisthere has usually been a modification of the
musculature on the mantle of the right side and while longitudinal and
circular components are still apparent there is u tendency to develop a
~ ~ e t w o r k
of crossing fibrcs. At the same time the test has become thicker
and more rigid than in Ciona and, while developing elaaticity, haa
imposed considerable limitations on body movement. C i o m is capable
of longitudinal retractioti snd body shortening to a degree quite impossible in the heavier framework of Aacidia. Goodhody and Trueman
(1969) have likened the movement of the mantle in Aecidia to a diaphragm which contracts the mantle cavity but there is no opposing
niuscle system to enable it to chxpand again. Hecht (1918a) has further
shown that in Ascidia the test on the right side is thinnkr than on the
left. In this way the test bends easily on the right during muscle
contraction while on the left side it forms an elaatic framework to assist
return of body shape. The situation in the stolidobranchs, where the
gut is reflected forwards and in most cases the test is tough but the
musculature complete 011 both sides, does not appear to have been
examined.
Within this framework the types of movement available to aecidians
are limited and restricted to : (1) opening and closing of the siphons and,
under strong stimulation, retraction of the siphons ; (2) expansion and
Contraction of the mantle wall ; contraction of the wall results in expulsion of water from the brmchial and atrial cavities through the siphons ;
(3) longitudinal contractiqn of the whole body which in Ciona results in
uniform shortening of the body. In Ascidia it tendti to produce a
bending of the anterior end of the animal towsrd the right-hand side ;
this is entirely due to the unequal distribution of muscle and the rigidity
of the test.
Two principal types of' activity and response in ascidians can be
identified, namely direct responses and crossed reflexes. Direct responee8
are the result of external stirnulation of the animal either on the general
surface of the test or around the siphons, and result in siphon closure,
contraction of the mantle musculature and expulsion of water from the
branchial and atrial cavities. Jf the external surface is stimulated near
one of the siphons, that siphon will close first and the other siphon a
moment later. This type of response is primarily elicited by mechanical
stimulation of the body surface.