Law Governing Loss of Weight in Starving Cassiopea. 63 



darkened half of the medusa loses color and fades rapidly, while the fluid 

 surrounding the half of the medusa which is in daylight fades more slowly. 

 This and the lime-water test in magnesium-free sea-water show that the 

 medusa gives off CO2, and that in diffuse daylight less CO2 is given off than 

 in darkness ; the difference being apparently due to photosynthesis on the 

 part of the commensal plant-cells or zooxanthellae which infest the gela- 

 tinuous substance immediately under the external cell-layers of the medusa. 



Tables 3, 11, and 12 show that when the medusae are confined in a small 

 volume of water they starve more rapidly than if kept in large aquaria. 

 They also pulsate slower when confined in small aquaria, and a slight excess 

 of CO2 brings them to rest, but even when not moving they lose weight 

 far more rapidly than if pulsating in normal sea-water, the toxic effect of 

 CO2 being very noticeable. If a few bubbles of CO2 be passed through 

 sea-water the medusae cease to pulsate; plate i, figure A, is derived from a 

 photograph of animals treated in this manner. Upon replacing the medusae 

 in normal sea-water pulsation is immediately renewed, no permanent toxic 

 effects appearing until they have been subjected to the CO2 for about 24 

 hours. 



The influence of CO2 is probably counteracted by the well-known photo- 

 synthesis due to the commensal plant-cells or zooxanthellae. 



When the medusae are starved in diffuse daylight these plant-cells become 

 more and more crowded as the medusa declines in bulk, and after several 

 days of starving they begin to escape early in the morning, when the swarm- 

 ing zooxanthellae penetrate the ectoderm and pass out into the ocean, where 

 they swim actively, and finally settle upon those lighted parts of the 

 cylindrical glass aquarium which are away from the source of light, the 

 swarming cells being entrapped in the lighted meniscus of the aquarium 

 in accordance with the Jennings reaction. When, on the other hand, the 

 medusae are starved in the dark, the algal cells are rendered inactive and do 

 not escape into the water. Medusae starved in the light become dark 

 brown in color after the first 10 days, for the plant-cells remain alive, al- 

 though apparently unhealthy and probably inactive. 



When the medusae are starved in darkness many of the plant-cells die 

 and dissolve in situ in the gelatinous substance. Not all of these cells die, 

 however, for a few still remain alive and apparently healthy after being 

 more than a month in the dark, and if the medusa be replaced in the light 

 these cells propagate and again provide the animal with a normal supply of 

 zooxanthellae. Sections show that when the medusae starve in the dark the 

 gelatinous substance becomes vacuolated and edemic, due possibly to the 

 unreduced excess of CO2 in the tissues. Thus the medusae when starved 

 in the dark appear to suffer from lack of oxygen, being in effect partially 

 suffocated. Yet even when the plant-cells have been greatly reduced 

 through 26 days of starvation in the dark, the medusae can still pulsate for 

 2 or more hours at nearly a normal rate in sea-water that has been boiled 

 to expel its oxygen. It at one time seemed that these plant-cells played 



