138 



COELENTERATA. 



that many organs of the colony which on the old colonial theory 

 are modified polyps are on this view nothing more than parts of 



medusiform indi- 

 viduals which 

 have shifted their 

 attachment, and 

 are therefore really 

 organs. For in- 

 stance, the struc- 

 tures called pal- 

 pons (hydrocysts, 

 dactylozooids) are 

 to be looked upon 

 as mouthless ma- 

 nubria of medu- 

 soids, the um- 

 brellas of which 

 have become modi- 

 fied as bracts, or 

 are entirely degen- 

 erate. The siphons 

 (trumpet - shaped 

 polyps, nutritive 

 polyps) are the 

 manubria of medu- 

 soids, of which the 

 umbrella is a bract, 

 or a nectocalyx or 

 degenerate. The 

 tentacle, on the 

 other hand, is to 

 be looked upon 

 as the only sur- 

 viving marginal 

 tentacle of the 

 medusoid of the 

 siphon, which has 

 shifted so as to be 

 attached to the base of its manubrium. This theory then agrees 

 with the second theory in asserting the colonial nature of the 

 Siphonophora, but admits that there has been that vegetative 



FIG. 116. Diagram of a colony of Siphonanthae. St coenosome 

 or stem ; Ek ectoderm ; En endoderm ; Pn pi*eumatophore ; Sk 

 budding nectocalyx ; S nectocalyx ; T palpon (hydrocyst, dacty- 

 lozooid); Sf tentacle and pal pacle ; P siphon (polyp) ; mouth 

 of siphon ; Nk battery of nematocysts ; D hydrophyllium ; G 

 gonophore. 



