194 



THE EVOLUTION OF SEX. 



siphonophore series. Here the larva develops at first into a simple 

 medusa-like individual, but this buds off a manifold series of " persons," 

 which, by dislocation or even migration, become arranged in all the beauty 

 of the siphonophore colonies, which surpass even Hydractinia in their 

 division of labour. It is difficult enough in some cases to distinguish 

 between true "persons" — which Hreckel calls " Medusomes" — and mere 

 organs like protective bracts, which are also budded off. 



Siphonophore Colony, showing the float (n), the 

 swimming-bells {d), and the nutritive, repro- 

 ductive, and other "persons", beneath. — 

 From Lang, after Ha;ckel. 



In another direction, viz., among the true jelly-fishes [Aa-aspeda), 

 where an active habit greatly preponderates, we still find the occurrence of 

 asexual multiplication. Some forms [e.g.^ Pc/agm) are entirely free; at the 

 opposite extreme a few {^Lticernarida) may be described as sedentary ; 

 between these we find the common aurelia, which settles down in its youth, 

 and gives rise by division to what afterwards become the large sexual 

 jelly-fishes (see fig. p. 202). 



There remains two classes of coelenterates, — the Ctenophora, like Beroe, 

 which represent a climax of activity, and never divide; and the Actinozoa 



