28 Papers from the Marim Biological Laboratory at Tortugas. 



I can not determine the individual effects of the calcium cation owing to 

 its habit of associating itself with the sodium, thus enabling the sodium to 

 offset the effects of magnesium. 



The laws stated above for the rate of nerve-conduction apply also with 

 modifications of detail to the rate at which the motor centers (the rhopalia) 

 generate the stimuli which produce the nerve-impulse. The rhopalia are, 

 however, more readily affected by osmotic and by concentration changes 

 than are the nerves, and I have not observed any increase in rate upon 

 diluting the sea-water with distilled water or dextrose. Any change in 

 concentration either above or below that of normal sea-water produces 

 depression in the action of the rhopalia. 



Attention may be directed to an explanation of the apparent converse 

 relation between the activities of muscles and cilia in trochophores, cteno- 

 phores, and other forms having well-differentiated cilia which move in a 

 coordinated manner. In all these cases the normal muscular tonus of the 

 animal produces a state of tension over the outer skin, thus pressing upon 

 the cilia-bearing cells and reducing or even stopping their movement. When 

 this tonus is relieved, however, the cilia beat rapidly. Thus magnesium 

 reduces the muscular tonus, but its depressant effect upon cilia is not so 

 marked as upon the muscles, and the cilia-bearing cells being relieved of 

 pressure beat with abnormally great activity in such a solution. 



Sodium, on the other hand, contracts the muscles, thus increasing the 

 pressure upon the cilia-bearing cells and stopping the movement of their 

 cilia. Hence this converse relation between neuro-musculai* and ciliary 

 movement is a mechanical, not a chemical, matter.^ 



THE GENERAL NATURE OF THE PULSATION-WAVE. 



Cassiopea xamachana is a very favorable animal for physiological experi- 

 mentation, for it normally lives in semistagnant lagoons and is, therefore, 

 relatively insensitive to changes in osmotic pressure due to concentration 

 or dilution of the sea-water. Moreover, as there is probably a considerable 

 amount of dissolved CO2 in its normal environment, and as the medusa 

 is infested with symbiotic plant-cells, it thrives remarkably well in aquaria, 

 there being practically no deaths due to confinement under laboratory 

 conditions. 



In this series of experiments, whenever the animals were placed in a 

 new solution they were permitted to remain in a large quantity of it for at 

 least an hour with several changes before the effect of the new solution was 

 determined by a kymograph record. In this manner the ultimate condition 

 of penetration and balance between the animal and the surrounding solu- 

 tion was secured. The relative concentrations of the solutions were deter- 

 mined by titration with AgNOs and potassium chromate. 



The effects described are all practically reversible, and fatal or seriously 

 injurious degrees of dilution of ions were avoided, so that when the animals 



^ Mayer (1912), Ctenophores of the Atlantic Coast of North America, p. 25. Cam. Inst. Wash. 

 Pub. No. 162. 



