SUPPLEMENTARY NOTE. 231 



precisely the same way and during tlie same time, the shape of the one being always exactly that of 

 the other, until, finally, the complete separation of the two puts an end to their dependence. 



Dr. Greef regards his Protohijdra as representing the lowest type of a coelcutcrate animal, and, 

 in accordance with the descent hypothesis, he assumes it as the true ancestral form of the whole of the 

 Coelenterata. 



The unsuitableness of the Hydroida for preservation in a fossil state leaves us almost entirely 

 without direct evidence as to the forms which may have been presented by their remote ancestry. 

 Reasons, however, have been already given which would seem to justify our going as low as the 

 Protozoa for the ancestral form of the IIybroida, whose relations to the K/Uzopocla would seem to be 

 traceable through the graptolites. See above, p. 179. 



