Papers from the Marine Biological Laboratory at Tortugas. 29 



were returned to sea-water they soon regained most of their normal rate 

 and amplitude of pulsation. 



The stimuli which cause pulsation normally arise in the motor centers 

 called rhopalia, or marginal sense-organs, of which there are usually about 

 16 in Cassiopea xamachana, although they may vary in number from 9 to 23. 

 It appears that this normal stimulation is due to the constant formation of 

 sodium oxalate in the rhopalia, thus precipitating calcium oxalate to form 

 the crystals of the sense-club and setting free ionic sodium, which is a most 

 powerful stimulant.^ 



The pulsation-rate of the normal medusa is therefore the rate at which 

 its motor centers can produce successive stages of stimulation. This is 

 always much slower than the rate at which the nerves can conduct the pulsa- 

 tion-wave around the subumbrella of the medusa, and there is a long pause 

 after each contraction, during which the animal remains unstimulated 

 and hence inactive. 



Cassiopea is, however, an excellent animal for physiological studies 

 upon nerve-conduction, on account of the ease with which we may dis- 

 pense with the motor centers and yet maintain a continuous neurogenic 

 pulsation-wave in the subumbrella tissue. This method was devised and 

 described by Mayer, 1906, p. 22, and it consists in cutting off the rhopalia, 

 thus paralyzing the nervous network of the subumbrella. Then the sub- 

 umbrella tissue is cut into the form of a ring-shaped strip, and a pulsation- 

 wave is started in one direction in this ring, so that it must continue to 

 travel around the ring, being in effect entrapped within the circuit of tissue 

 through which it must continually progress in one direction, a single stimulus 

 often maintaining itself in this manner for days at a time. 



Such entrapped circuit-waves are of course not peculiar to Cassiopea, 

 for they may be started and maintained in ring-shaped strips of the ventricle 

 of the heart of the sea turtle, Caretta caretta, in the hearts of fishes, or in 

 other scyphomedusae. We may thus study the operations of the nervous 

 network as a continuous transmitter of a single pulsation-stimulus. 



As has been said, the nerves transmit this stimulus around the bell 

 much more rapidly than the motor centers can engender it. For example, 

 a medusa, which normally gave 45 pulsations per minute, was deprived of 

 its rhopalia and cut into the form of a ring, and a pulsation-wave then 

 traveled constantly around this ring at the rate of 129 turns per minute. 

 This ring also responded without missing a pulsation to 152 Faradaic shocks 

 per minute, and it followed 190 shocks, responding by a separate con- 

 traction to each one for about 3 minutes, after which it occasionally failed 

 to respond, following alternate stimuli. Many experiments of this sort were 

 made, always with the same general result, and it is evident that the nerves 

 and muscles can react to successive stimuli with greater frequency and at 

 shorter time-intervals than they are called upon to respond in nature. 



It is evident, then, that there is a considerable "factor of safety" in the 

 normal medusa, and that the motor centers initiate pulsation-stimuli at a rate 



1 Mayer, 1908, p. 130. 



