JELLYFISHES AND HYDROIDS al 



victim is then held in the stomach of the jelJyfish for a few hours 

 or days and the undigested remnant is ejected throngli the month. 

 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 Avhen starved tliey con- 

 tract in size. 



The great majority of the jellyfishes are so small as to escape 

 ordinary oljservation, 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 (ScyijliomeduscvJ the egg- 

 develops into a little pear-shaped creature Avhose body is covered 

 with vibrating cilia which enable it to swim rapidl)'. 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 (Ilijdromcdiisa;) 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 vVtlantic coast will be 

 found in " North American Acalepliaj," A. Agassiz, 1S05; "Contri- 

 butions to the Natural History of the United States," Vol. Ill, 18G0, 

 by Louis Agassiz ; C. W. Hargitt in " The American Naturalist," 

 I'.IOl, Vol. XXX\^ ; " Das System der Medusen," by Haeckel ; 3 vols., 

 1879-'80, and "Medusaj from The Tortugas, Florida," in the Bul- 

 letin of the Museum of Comparative Zoology at Harvard, A^ol. 

 XXXVII, 1900 by A. G. Mayer. 



The Portuguese Man-of-War, (Phtjsalia arethitsa, Fig. S). 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-sha]ied 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 



