/. HTBROZOA: SIPIIONOPnORA. 



243 



Order VII. Siphonophora. 



The Siijlionopliora are among the most beautiful of pelagie 

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

 187) consists of a colony of individ- 

 uals springing from a common coe- 

 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 i__ 

 the ccenosarcal axis, perform differ- 

 ent functions and hence have differ- 

 ent structures. Close behind the 

 float commonly come several swim- 

 ming bells [uectocalijces) 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 

 liave lost the ring canal, the muscles, 

 and the l)ell shape of the medusffi. 

 Food is taken by wide-mouthed feed- 

 ing tul)es Utii) which may be com- 



'- T , ,' ,r. -„,',! Fro. IST.-Dianrani iif Siphmiophnre. 



pared to polyps (ng. o?) or the ma- (From Lani,'.) a-h, gnmps dt dif- 



, . p " T '" rm T- , fereiit individuals; dx, coverint,' 



nuLirium of a medusa. i hey digest scales; gi\ gnnophm-es; Im, feeilins,' 

 ,1 jv -, 1 {• -I " p polyps; p. 'tasters' (dit^^estive) ; xh, 



the food by means of large masses ot ji„at'; ',,,/, swimming beii (nccto- 

 glands (' liver bands ') and convey it '^^y^^''^^- ^"^i^. 

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

 base are long muscular tentacles (/) from wliicli 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 liy the siphonophores, which 

 in many is so severe as to be feared In' man. The 'feelers" ( p) 



