On the Tjocomotor System o/' Medusca. 249 



tioii, is of a very marked character ; for the spasmodic mo\emeuts 

 of the nectocalyx are as easily aud as certainly excited by irritating 

 any part of the severed strip as they are by irritating the snbstauee 

 of the nectocalyx itself. 



From this description it will readily be seen that a Medusa, 

 when thus operated upon, supplies all the conditions required for 

 conducting expei'iments in electrotonus : the animal in this form 

 is, for all practical purposes, a nerve-muscle preparation. Ac- 

 cordingly I have spent a great deal of labour over this part of my 

 subject, but with no very satisfactory results. In the case of 

 Stcmropliora lacinlata, however, I have sometimes obtained decided 

 indications of kathelectrotonus, but never any of anelectrotonus. I 

 cannot yet speak decidedly with respect to Pfliiger's law. 



(d) a. The excitable tissues of Medusae, although somewhat 

 capricious in the comparative sensitiveness they show to make 

 and break of the current, upon the whole conform to the rules 

 which are followed by the excitable tissues of other animals. 



0. Different species of Medusae manifest differences in the de- 

 gree of their sensitiveness to electrical stimulation. In all cases, 

 however, the degree of sensitiveness is wonderfully high. 



y. When the constant current is passing in a portion of a strip 

 of a severed margin, the nectocalyx sometimes manifests un- 

 easy motions darinfj the time the current is passinrj ; this, however, 

 is perhaps due to variations in the intensity of the current. 



?. When the intrapolar portion of the severed margin of S. 

 laciniata happens to be spontaneously contracting prior to the pas- 

 sage of the constant current, the moment this current is thrown 

 in such spontaneous contractions usually cease, and are seldom 

 resumed until the current is again broken, when they are almost 

 sure to recommence. This inhibitory effect may be produced 

 a great number of times in succession. 



e. Exhaustion of the excitable tissues may be easily shown by 

 the ordinary methods. Exhausted tissue is much less sensitive to 

 stimulation than is fresh tissue, and, so far as the eye can judge, 

 the contractions are slower \^-ith the period of latent stimulation 

 prolonged. 



(. Tetanus produced by Faradaic electricity is not of the nature 

 of an apparently single prolonged contraction (except, of course, 

 such of the naked-eyed Medusae as respond to all kinds of stimuli 

 in this way), but that of a number of contractions I'apidly succeeding 

 one another. There is hence no appearance of summation. 



7j. When the swimming-bell of Sarsia has had its margin re- 

 moved, and so (as proved by hundreds of experiments) has been 

 entirely deprived of its locomotor centres, nevertheless, in response 

 to electrical stimulation, instead of giving a single contraction to 

 make or break, it may begin a highly peculiar motion of a flur- 

 ried, shivering character, which lasts without intermission for 

 periods varying from a few seconds to half an hour. I never but 

 once saw a similar motion in a perfect animal : and this was in the 



Ann. & Mag. N. Hist. Ser. 4. Vol. xvii. 17 



