possible with the techriiqucs uucd by Hoyle to show that the ascidian
nervous system has the following characteristics :
(1) Single stimuli evoke only a minimal response but two successive
shocks produce a strong contraction (Fig. 27a).
(2) Above the threshold the strength of a stimulus is unimportant
and the response is dependent on the frequency of stimulation. With
successive shocks there is increasing contraction of the siphon until it is
fully closed and withdrawn (Fig. 2%).
10s
U
S
' I 27QV
I '
19ov
' 150V
Pro. 27. (a) Hesponcurs (H) of tho branchial niphon of Aaewlielln naperm to a ningle nhock,
to a pair of shocks and to R COrI8oCutive Heriw of h n nhocks (8). (h) bHxponmn (It)
of the branchid Riphon of Pkdhv& mnmnrt&ln to pairn of ntirnuli (N) at varioiia
voltages above the threshold (145 V). (Redrawn from Hoylc, 1952.)
(3) The types of response elicited under these conditionH of' external
stimulation are independent of control by the cerebral ganglion. After
extirpation of the ganglion both branchial and atrial siphons respond
strongly to electrical stimuli.
We cari deduce from this that these ascidianrt exhibit an all or
nothing through conduction system which is subject to facilitation very
much as is found in actiniaritms. There is, however, at the present time
110 conclusive evidence in favour of the existence of a nerve net system
(for a discussion of this see Fedele (1937a,b) and Bullock and Horridge
(1966)). No mrttter what is the nature of the excitable system the
nervous system has the following characteristics :
(1) Single stimuli evoke only a minimal response but two successive
shocks produce a strong contraction (Fig. 27a).
(2) Above the threshold the strength of a stimulus is unimportant
and the response is dependent on the frequency of stimulation. With
successive shocks there is increasing contraction of the siphon until it is
fully closed and withdrawn (Fig. 2%).
10s
U
S
' I 27QV
I '
19ov
' 150V
Pro. 27. (a) Hesponcurs (H) of tho branchial niphon of Aaewlielln naperm to a ningle nhock,
to a pair of shocks and to R COrI8oCutive Heriw of h n nhocks (8). (h) bHxponmn (It)
of the branchid Riphon of Pkdhv& mnmnrt&ln to pairn of ntirnuli (N) at varioiia
voltages above the threshold (145 V). (Redrawn from Hoylc, 1952.)
(3) The types of response elicited under these conditionH of' external
stimulation are independent of control by the cerebral ganglion. After
extirpation of the ganglion both branchial and atrial siphons respond
strongly to electrical stimuli.
We cari deduce from this that these ascidianrt exhibit an all or
nothing through conduction system which is subject to facilitation very
much as is found in actiniaritms. There is, however, at the present time
110 conclusive evidence in favour of the existence of a nerve net system
(for a discussion of this see Fedele (1937a,b) and Bullock and Horridge
(1966)). No mrttter what is the nature of the excitable system the