SCYPHOMEDUSAE COLLECTED BY STEAMER " ALBATROSS." 183 



as Cassiopea. In the free-floating Pelagia, however, the planula 

 larva never becomes attached, but remains swimming through the 

 water until it develops directly into a jellyfish. Thus it is that these 

 jellyfishes are quite independent of the land and are widely distrib- 

 uted over the tropical and warm oceans; but this is exceptional, for 

 most of the Scyphomedusae must spend their early days attached to 

 some fixed object and usually in relatively shallow water near some 

 coast. 



Some of these coastal medusae are, however, widely distributed 

 over the world, one of these being the large semi-transparent Aurel- 

 lia aurita of our own bays and harbors, which appears so commonly 

 during the summer, and may be recognized by its four horseshoe- 

 shaped, milky or pink-colored genital organs. This form occurs 

 from pole to pole. 



Such adaptability to wide range of temperature is very rare among 

 jellyfishes, and is known only in Aurellia aurita and according to 

 Vanhoffen in Nausithoe puncata among the scyphomedusae, and Sol- 

 mundella among hydromedusae ; these forms occurring in seas of all 

 temperatures. 



Nevertheless, even tropical medusae are much more injuriously 

 affected by a slight rise in temperature than are the jellyfishes of the 

 temperate regions, and we may say that most tropical forms live 

 within 12° C. of their heat-death-temperature, and even tropical forms 

 can withstand cooling better than they can resist heat. To use an en- 

 gineering expression we might say that the medusae of temperate 

 regions have a larger " factor of safety " in respect to temperature. 

 Harvey showed that, upon heating, the rate of conduction of the 

 nervous stimulus which causes pulsation increases in an arithmetical 

 ratio, so that its " curve " is a straight line. At from 34° to 39°, 

 however, the curve makes a sudden bend downward and the rate de- 

 clines sharply. This decline may, in part, be due to the formation 

 of carbon dioxide (C0 2 ) in the tissues, for Winterstein showed that 

 the rate of oxygen consumption in jellyfishes is 3^ times more rapid 

 at 30° to 35° than it is at about 12° C. Indeed, experiments made 

 by the author in 1917 support the idea that high temperature causes 

 acid to accumulate in the tissues and this causes death through 

 acidosis. 



The large, rich rosin-brown colored cyaneas of our New England 

 coast are not found in the Tropics, but closely allied species reappear 

 in the South Temperate Zone, so that somehow they have managed, 

 perhaps in the glacial epoch, to cross the warm zone of the Tropics, 

 or they may have succeeded in crossing the Equator in the cold, deep, 

 underlying drift that moves toward the warm regions over the sea 

 bottom from both the northern and southern polar seas. 



