MR. G. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSAS. 
715 
tation are not perceptibly impaired ; but soon after this stage the intensity of the spasm 
begins to become less, and later still it assumes a local character. It is important, 
also, to notice that at this stage the effect of marginal stimulation is very often that of 
producing a general locomotor contraction, and sometimes a series of two or three such. 
During recovery in normal sea-water all these phases recur in reverse order. 
§ 4. Character of the Contractile Tissues of Aurelia aurita. (A) Contractional Conti- 
nuity. — (a) This year I have been able to collect some additional facts relating to this 
subject ; and of these facts the first that I shall deal with is an important one, namely, 
that exploration of the contractile tissues by graduated stimuli affords direct proof 
that these tissues are in a high degree functionally heterogenous. That is to say, if 
the swimming-bell of Aurelia be paralyzed by removal of its lithocysts, and if its irri- 
table surface be then explored by induction-shocks of graduated intensity, differences 
in the excitability of different parts may thus be rendered clearly apparent. These 
differences are observable in all degrees, from being scarcely perceptible to being 
so pronounced that no strength of stimulation will cause any, except a very local, 
response. Moreover it is of importance for us to note that if the stimuli be adminis- 
tered by means of needle-point electrodes, it may almost always be observed that an 
area of low excitability does not graduate into an adjacent area of high excitability by 
successive stages, but that the one area is separated from the other by an exceedingly 
well-defined line of demarcation. 
(b) Having observed these lines of demarcation between areas of different degrees of 
excitability to be so well defined, I was led to try cutting a spiral strip until the 
contractile waves became blocked, and then exploring the line of blockage by stimulus. 
In all cases I found that if I stimulated a fraction of a millim. on the spiral strip 
side of this line, the spiral strip responded, while the uncut portion of the bell 
remained passive ; and, conversely, that if I stimulated the other side of this line, 
the uncut portion of the bell responded, while the spiral strip remained passive. To 
make the fact in question quite clear I may refer to fig. 9, which represents a portion 
Fig. 9. 
of a broad strip wherein the contractile waves became blocked at the curved line 
ah. On stimulating the tissue at any point as close as possible on one side of this line, 
as at c, a contractile wave would start from that point, and course all the way along the 
strip in the direction c d , but in that direction only ; while if the stimulus were applied 
at any point on the other side of the line a b, as at e, the contractile wave thus originated 
5 H 
MDCCCLXXVII. 
