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 jelly fishes 

 grow is dependent upon their supply of food. Indeed one can ob- 

 serve them enlarge after every meal, and when starved tliey con- 

 tract in size. 



The great majority of the jelly fishes 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 fScyplwmedusceJ 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 (Hydnwicdnscc) 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 Acalephre," A. Agassiz, 1865; "Contri- 

 butions to the Natural History of the United States," Vol. Ill, 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 "Medusa 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, /P/«/ysaZm nrethusa, 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 Avith 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 



