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the Medusae would propel the entire colony, consisting of 

 both hydriform and medusiform persons, through the water, 

 and locomotion being thus effected, it would be unnecessary 

 for the medusiform persons to become detached. Then 

 polymorphism produced changes in both the hydriform and 

 the medusiform persons, resulting in the very complicated 

 free-swimming colonies of Siphonophora found at the 

 present day. 



The Ctenophora were probably derived from an ancestral 

 Hydromedusa near to the common ancestor of the Gymno- 

 blastea and the Siphonophora. The primitive Ctenophora 

 must first have lost their hydriform stage in the same 

 manner as it was lost by the ancestral Trachylarida. This 

 would result in their becoming free-swimming Medusae, like 

 the medusiform persons of the Gymnoblastea, but difi'ering 

 from them in developing directly from the egg. They must 

 then have undergone a series of changes which may be seen 

 partly effected in the remarkable transition form described by 

 Haeckel as Ctenaria, which resulted in the narrowing of 

 the margin of the bell so as to produce a nearly spherical 

 form with a small mouth opening (as in Pleurobrachia) , and 

 in the formation of eight bands of modified ciliated ectoderm 

 running meridionally down the outside from pole to pole. 

 The enteric cavities also became modified, and the tentacles 

 were reduced to two, and became retractile into laterally 

 placed sacs. The evolution of the various groups of Cteno- 

 phora from a Pleurobrachia-like common ancestor is easy to 

 trace. 



Keturning to the common ancestor of all the Coelenterata, 

 we find that the second series of forms diverging from this 

 point leads to the primitive Scyphomedusae and Actinozoa, 

 and is characterised by the hydriform stage remaining 

 simple and single in place of producing a colony. It acquired, 

 however, the power of giving off pieces of its body as buds. 



