166 Papers from the Department of Marine Biology. 



little pear-shaped planula, as it is called, settles down head first upon 

 the bottom and fastens itself to some fixed object. Then for the first 

 time the mouth develops at that which was the posterior end of the 

 planula and tentacles grow out so that the mouth is soon surrounded 

 by 16 or more of these organs which serve to capture the minute crus- 

 tacea and other organisms upon which the little polyp feeds. Thus it 

 remains sedentary for a long period, growing all the time and super- 

 ficially resembling a small sea-anemone. Finally a series of constric- 

 tions develop at regular intervals, and the creature appears as if it 

 were composed of a series of disks set one upon the other. The margin 

 of each disk soon develops 8 cleft lobes, and 8 sense-clubs appear in 

 the clefts. Then the uppermost disk, containing the mouth and the 

 crown of tentacles, is cast off and perishes, while the others are set free 

 in succession and swim away as minute jellyfishes, soon to develop ten- 

 tacles and finally to become mature and repeat this peculiar process of 

 development. After the last disk has been cast off, only the stump of 

 the strobila, as it is called, remains, but this may regenerate a new ring 

 of tentacles and continue to grow, possibly to develop more jellyfishes 

 at the succeeding season. 



There are many interesting variations of this typical process of 

 development. Often the strobila, instead of giving off a series of 

 disks, develops only a single constriction and every alternate tentacle 

 changes into a sense-club, while the other tentacles may be wholly 

 absorbed, so that they disappear. In this case only a single ephyra 

 or larval jellyfish is set free. This form of development is especially 

 characteristic of the Rhizostomae or multi-mouthed jellyfishes, such 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 jelly- 

 fishes are quite independent of the land and are widely distributed 

 over the tropical and warm oceans. 



Several other sorts of jellyfishes are widely distributed over the 

 world, one of these being the large semi-transparent Aurellia aurita of 

 our coast, which appears so commonly during the summer and may be 

 recognized by its 4 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 Nausithoe punctata 

 among the scyphomedusse, and Solmundella among hydromedusse, 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 engineer- 



