7. HTDROZOA : SIPHONOPHORA. 



243 



si) 



Order VII. Siphonophora. 



The Siphonophora are among the most beautiful of pelagic 

 animals, some transparent, some brightly colored. Each (fig,. 

 187) consists of a colony of individ- 

 uals springing from a common C03- 

 nosarcal tube which is strongly mus- 

 cular and contains a central canal 

 lined with entoderm by which the 

 members of the colony receive their 

 nourishment. At one end the tube 

 is usually closed by a float filled with 

 air, the pneumataphore, which acts 

 as a hydrostatic apparatus, and keeps 

 the colony vertical in the water. 



The individuals, springing from 

 the coenosarcal axis, perform differ- 

 ent functions and hence have differ- 

 ent structures. Close behind the 

 float commonly come several swim- 

 ming bells (nectocalyces) which re- 

 tain of medusal structures only those 

 (bell, velum) necessary for swimming 

 and those (ring and radial canals) 

 for the distribution of nourishment 

 received from the common tube. 

 Then come, scattered through the 

 colony, the covering scales, for pro- 

 tection, firm gelatinous plates which 

 have lost the ring canal, the muscles, 

 and the bell shape of the medusae. 

 Food is taken by wide-mouthed feed- 

 ing tubes (In/} which may be com- 

 pared to polyps (fig. 57) or the m 

 nubrium of a medusa. They digest 

 the food by means of large masses of 

 glands (' liver bands ') and convey it calyx); st ' stalk ' 

 by the central tube to all the members of the colony. At the^ 

 base are long muscular tentacles (t) from which small lateral 

 threads depend, each ending in a brightly colored swelling, the*, 

 nettle head, composed of large, closely packed nettle cells. These- 

 are the cause of the nettling produced by the siphonophores, which, 

 in many is so severe as to be feared by man. The ' feelers ' 



