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 Rhizostome 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 Nausithoé punctata 
among the scyphomeduse, and Solmundella among hydromeduse, these 
forms occurring in seas of all temperatures. 
Nevertheless, even tropical meduse 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- 
