JELLYFISHES AND HYDROIDS 31 
victim is then held in the stomach of the jellyfish for a few hours 
or days and the undigested remnant is ejected through the mouth. 
In common with other low invertebrates, the rate at which jellyfishes 
grow is dependent upon their supply of food. Indeed one can ob- 
serve them enlarge after every meal, and when starved they con- 
tract in size. 
The great majority of the jellyfishes are so small as to escape 
ordinary observation, but are on the other hand so numerous as 
often to cause a brilliant phosphorescence of the sea at night. 
The eggs of jellyfishes do not usually develop directly into 
new jellyfishes. In the large jellyfishes (Scyphomeduse ) the egg 
develops into a little pear-shaped creature whose body is covered 
with vibrating cilia which enable it to swim rapidly. Soon it set- 
tles down, and the narrow end adheres to the bottom. ‘Then a 
mouth and a row of tentacles appear at the upper end. ‘The little 
creature then grows for some months until suddenly it begins to 
constrict at intervals, and finally to split up into a series of thin, flat 
disks, each one of which swims off and grows into a separate 
jellyfish. 
In the smaller jellyfishes (Hydromedusaw) the egg changes 
into a beautiful little tree-shaped animal called a hydroid, and this 
gives rise to many little jellyfishes which bud out from it in various 
ways. Some jellyfishes, however, do not give rise to hydroids and 
many hydroids do not develop jellyfishes. 
Descriptions of the jellyfishes of our Atlantic coast will be 
found in “North American Acalephe,” A. Agassiz, 1865; ‘“ Contri- 
butions to the Natural History of the United States,” Vol. III, 1860, 
by Louis Agassiz; C. W. Hargitt in “The American Naturalist,” 
1901, Vol. XXXV; “ Das System der Medusen,” by Haeckel; 3 vols., 
1879-80, and ‘“‘Meduse from The Tortugas, Florida,” in the Bul- 
letin of the Museum of Comparative Zoology at Harvard, Vol. 
XXXVII, 1900 by A. G. Mayer. 
The Portuguese Man-of-War, (Physalia arethusa, Fig. 8). This 
beautiful animal is sometimes seen floating along our coast late in 
summer, but its home is in the tropical Atlantic and the Gulf Stream. 
The large pear-shaped float is filled with atmospheric air, and beau- 
tiful iridescent blues and pinks play over its surface and along its 
comb-like crest. Attached to the float there is a complex colony of 
