v] LATER PROGRESS 119 



are special polyps that defend 'the colony as well as 

 those that nourish it 1 . 



All these species are sedentary for most of their 

 lives ; history again repeats itself, for once more it 

 is in the free- swimming forms that the members of 

 the colony have been most modified, most subordinated 

 to a higher individuality. 



The Siphonophora, close relations of the hydroids, 

 are a group of beautiful pelagic creatures, which 

 slowly drive their trailing length through the water 

 by an array of pulsating bells. Besides these loco- 

 motor organs, there are in the colony organs for 

 feeding, for reproduction, for defence, for offence, 

 and for flotation (see Fig. 11). Most of these 

 apparent organs are really modified individuals, 

 either of the polyp or medusa type. The reproductive 

 " organs ' are sometimes detached as perfect jelly- 

 fish ; the swimming-bells and the protective bracts 

 often show unmistakeable vestiges of medusoid 

 structures. The nutritive "organs' 1 are very like 

 an ordinary polyp, but without tentacles, and the 



1 It is interesting to note that in the Polyzoa, another group of 

 colonial animals, there has been a different kind of division of labour. 

 The ordinary animals both feed and reproduce the colony, and defence 

 is undertaken by much modified persons called Avicularia (from their 

 resejnblance to birds' heads with snapping beaks). Here the 

 differentiation is between most of the somatic and all the germinal 

 functions on one side, and a single somatic function on the other. In 

 some forms there are no Avicularia, the colonies then consisting of 

 only one kind of person. 



