The cross reflex type of response is elicited by stimulation 9f the
interior of either siphon. the brltnchiltl tentacles and the interior of the
mantle cavity adjacent to the atrial siphon. Stimulation of the interior
of either siphon results in the following series of events, (1) closure of the
opposite siphon. (2) contraction of the mantle wall and (3) closure of
thr stinirrlated siphon. The iinport,tmt fcrtturo of this ictivity i A that if
the branchid siphon is etimiiluted it, is tho cLtriel siphon which closes
first followed by mantle contraction ctiusing expulsion of water through
the branchid siphon and thcreby “ fluehing out ” any foreign matter
which may bc the sourcc of irritation. Only then does the branchial
siphon cloee. If, on the other hand, it is the atrial siphon or mantle
cavity which i H stimulated the branchial siphon closes first so that water
is ejected through the atrial siphon. This type of reflex is primarily
elicited by mechanical stirnulibtion (Hecht, l918b ; Day, 1919) but Hecht
also reports that ascidians are sensitive to chemical, thermal and
osmotic stitnulation as well as to light.
The nervous mechanisms underlying these responses and reflexes
have been studied by a number of workers of whom the most important
are Jordan (1908), Yolimanti (1911), Kinoshita (1910, 1911), Hecht
(1918b), Fedele (1923b, 1937b), Ten Cate (1928), Bacq (1935a) and
Hoyle (1952). The earlier work is reviewed by Hoyle who also carried
out a series of very precise experiments on Phcsllusia mammillata and
Ascidiella aspupersa ainicd at clarifying the nature of the integrative
mechanism involved in direct responses. He stimulated the animals by
means of electrodes placed in the test and recorded the responses of the
animals by a mechanical monitoring of siphonal activity. This method
of stimulation may be criticized on the grounds that sense cells are not
common in the test except immediately around the siphons and hence
considerable resistance muet be overcome in the test material. ThiH is
emphasized by the fact that in Phallusin a stimulus strength of the
order of 100 V was required in order to elicit a response and 25 V
in Ascidiella nspersa. Nevertheless thc nature of the results obtained
suggests that the techniques were not at fault. From these investigations Hoyle found that the type of response initiated in an animal varied
considerably and was dependent on the previous history of the animal
and the stage it has rewhetl in the cycle of spontaneous squirting.
Spontaneous rhythms art’ discussed elsewhere (p. 21) and in Phallusia
contractions of the mantle occur about once every 6 to 9 minutes.
Electrical stitnuli given just before or after a spontaneous contraction
may elicit an uhnormal Reriee of responses and in addition there are
refractory periods when the animal does not respond at all to artificial
stimuli. Providing these variation8 arr taken into consideration it is
interior of either siphon. the brltnchiltl tentacles and the interior of the
mantle cavity adjacent to the atrial siphon. Stimulation of the interior
of either siphon results in the following series of events, (1) closure of the
opposite siphon. (2) contraction of the mantle wall and (3) closure of
thr stinirrlated siphon. The iinport,tmt fcrtturo of this ictivity i A that if
the branchid siphon is etimiiluted it, is tho cLtriel siphon which closes
first followed by mantle contraction ctiusing expulsion of water through
the branchid siphon and thcreby “ fluehing out ” any foreign matter
which may bc the sourcc of irritation. Only then does the branchial
siphon cloee. If, on the other hand, it is the atrial siphon or mantle
cavity which i H stimulated the branchial siphon closes first so that water
is ejected through the atrial siphon. This type of reflex is primarily
elicited by mechanical stirnulibtion (Hecht, l918b ; Day, 1919) but Hecht
also reports that ascidians are sensitive to chemical, thermal and
osmotic stitnulation as well as to light.
The nervous mechanisms underlying these responses and reflexes
have been studied by a number of workers of whom the most important
are Jordan (1908), Yolimanti (1911), Kinoshita (1910, 1911), Hecht
(1918b), Fedele (1923b, 1937b), Ten Cate (1928), Bacq (1935a) and
Hoyle (1952). The earlier work is reviewed by Hoyle who also carried
out a series of very precise experiments on Phcsllusia mammillata and
Ascidiella aspupersa ainicd at clarifying the nature of the integrative
mechanism involved in direct responses. He stimulated the animals by
means of electrodes placed in the test and recorded the responses of the
animals by a mechanical monitoring of siphonal activity. This method
of stimulation may be criticized on the grounds that sense cells are not
common in the test except immediately around the siphons and hence
considerable resistance muet be overcome in the test material. ThiH is
emphasized by the fact that in Phallusin a stimulus strength of the
order of 100 V was required in order to elicit a response and 25 V
in Ascidiella nspersa. Nevertheless thc nature of the results obtained
suggests that the techniques were not at fault. From these investigations Hoyle found that the type of response initiated in an animal varied
considerably and was dependent on the previous history of the animal
and the stage it has rewhetl in the cycle of spontaneous squirting.
Spontaneous rhythms art’ discussed elsewhere (p. 21) and in Phallusia
contractions of the mantle occur about once every 6 to 9 minutes.
Electrical stitnuli given just before or after a spontaneous contraction
may elicit an uhnormal Reriee of responses and in addition there are
refractory periods when the animal does not respond at all to artificial
stimuli. Providing these variation8 arr taken into consideration it is