98 ELEMENTARY ZOOLOGY 



colonial je-llyfishes are floating or swimming colonies of 

 polypoid and medusoid individuals in which there is a 

 marked division of labor among the individuals, ac- 

 companied by marked differences in structural charac- 

 ter. The individuals are accordingly polymorphic, 

 that is, appear in various forms, although all belong 

 to the same species. Because these various individuals 

 forming a colony have given up very largely their 

 individuality, combining together and acting together like 

 the organs of a complex animal, they are usually not 

 called individuals, nor on the other hand organs, but 

 zooids, or animal-like structures. The beautiful " Portu- 

 guese man-of-war " (fig. 13) is one of these colonial jelly- 

 fishes. It appears as a delicate bladder-like float, brilliant 

 blue or orange in color, usually about six inches long, 

 and bearing on its upper surface which projects above the 

 water a raised parti-colored crest, and on its under surface 

 a tangle of various appendages, thread-like with grape- 

 like clusters of little bell- or pear-shaped bodies. Each 

 of these parts is a peculiarly modified polyp- or medusa- 

 zooid produced by budding from an original central zooid. 

 The Portuguese man-of-war is very common in tropical 

 oceans, and sometimes vast numbers swimming together 

 make the surface of the ocean look like a splendid flower- 

 garden. 



Usually the central zooid in a Siphonophore to which 

 the other zooids are attached is not a bladder-like float, 

 but is an upright tube of greater or less length. In the 

 Siphonophore shown in figure 14, the compound body is 

 composed of a long central hollow stem with hundreds 

 or thousands of variously shaped parts, each of which is 

 reducible to either a polyp or medusazooid, attached 

 around it. The upper end is enlarged to form an air- 

 filled chamber, a sac-like boat, by means of which the 

 whole colony is kept afloat. Around the uprjer end of the 



