TYPE CGELENTEEA. 



91 



-P 



7. Order Siphonophorae. 



The Siphonopbores are free-swimming Hydromedusan 

 colonies, the constituent individuals of which show a high 

 degree of division of labor. The 

 various forms of individuals which 

 are to be found in different members 

 of the order are usually not present 

 in any one colony, some genera pos- 

 sessing some forms which others 

 lack ; it will be convenient, therefore, 

 to consider an ideal form in which the 

 various modifications are present. 

 Each colony (Fig. 47) consists of an 

 axis or stolon on which the various 

 individuals are seated and which 

 places them in connection with each 

 other ; it is usually long and slender, 

 but in some cases, as in Porpita, may 

 be reduced to a disk. At the ex- 

 tremity of the stolon may be found 

 a float or pneumatophore (Fig. 47, p), 

 a double-walled sac containing air in 

 the interior, and which is to be re- 

 garded as a modified medusa form. 

 Next to it come usually several medusa 

 forms lacking manubrium, tentacles, 

 and sense-organs, and possessing a 

 locomotor function ; these are the 

 swim-bells or nedocalyces (n). At in- 

 tervals along the rest of the stolon 

 are situated groups of individuals, 

 each group covered over by one or 

 more scale-like structures (cs), which 

 are again highly modified medusae, and 

 in each group is to be found a tropho- 

 polyp (tr\ a vase-like polyp form with 

 a wide trumpet-shaped mouth, and 

 having near its base a single tentacle 

 (t) which bears along one side a row of numerous secondary 



47. DlAGKAM OF A Sl- 



PHONOPHORE COLONY. 



cs covering scale. 

 n = uectocalyx. 

 p pneumatocyst. 

 r = reproductive polyp. 

 s = sensory polyp. 

 Ir nutritive polyp. 

 t = tentacle. 



