I. HYDROZOA: SIPHONOPHORA 



219 



radial canals) for the distribution of nourishment received from the 

 common tube. Then come, scattered through the colony, the covering 

 scales, for protection, firm gelatinous plates which have lost the ring canal, 

 the muscles, and the bell shape of the medusa.'. Food is taken by wide- 

 mouthed feeding tubes (liy) which may be compared to polyps (fig. 58) or 

 the manubrium of a medusa. They digest the food by means of large 

 masses of glands ('liver bands,') and send it by the central tube to all 

 the members of the colony. At the base are long muscular tentacles 

 (/) 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, which in many species is so 



FIG. 182. American siphonophores. A, Nanomia cara (after A. Agassiz). B, 

 Velella meridionalis (.after Fewkes). C, Diph yes pray a (after Fewkes). 



severe as to be feared by man. The 'feelers' (/>) recall mouthless polyps 

 and manubria; they are very sensitive and mobile and, while tactile, ap- 

 parently in some cases are digestive organs. Latest to develop in the 

 colony are the sexiial bells. They are usually brightly colored and re- 

 semble small mouthless Anth.omed.usae without tentacles. They but 

 rarely (Chrysomitra) separate from the colony, but usually persist as 

 more or less reduced sporosacs. From this it follows that the Siphono- 

 phora afford fine examples of division of labor and of the consequent 

 polymorphism of individuals. This can indeed be carried so far that 

 many convey the impression of being individuals with a multiplicity of 

 organs. The Siphonophora are all marine, and occur most abundantly 

 in tropical seas. 



Sub Order I. PHYSOPHOR/E (Physonectse). Float present, small; next a 

 large series of swimming bells; then the other members of the colony, f'liy- 

 sophora, Agalmia, Nanomia* (fig. 182). Sub Order II. CALYCOPHOR^E 

 (Calyconectae) . Float lacking; one or two large swimming bells; the other in- 



