THE PHYLA COELENTERATA AND CTENOPHORA 



191 



but no tentacles, and are used only for feeding. Still others develop as 

 simple stalks that bear a third kind of medusa bud along their sides. 

 These buds produce eggs and sperm, and are the only sexually repro- 

 ductive individuals in the colony. Entire floating colonies of siphono- 

 phores may remain intact, or pieces including all the kinds of individuals 

 may break loose and lead independent lives. 



A famous siphonophore is PhysaUa (Fig. 10.9), the dreaded Portu- 

 guese man-of-war. It has a large purple air float up to 12 cm. long that 

 rides high out of the water and is carried by the wind across the oceans. 

 Swimming medusae are absent. Tentacles of the stinging polyps may 

 trail out 40 feet into the water, and their nematocysts easily penetrate 

 the skin of man. The intense pain and occasional paralysis caused 

 by many stings can result in drowning. 



The three kinds of polyps in PhysaUa occur in groups, one of which 

 is shown in Figure 10.10. Although the mouth of the feeding polyp can 

 open very wide, the polyp is unable to swallow prey unless it is compara- 

 tively small. Larger prey are consumed in an ingenious fashion. Many 

 feeding polyps become attached to the prey, each spreading its mouth 



F(zecLin0 

 polyp 



Meciu-sa. bzid 



Reproductive 

 polyp 



Figure 10.8. A hydroid, Obelia, showing a small portion of the branching colony. 

 One polyp is shown in longitudinal section. (After Parker and Haswell.) 



