100 THE EVOLUTION OF CONTINUITY 



practically identical to that of the free-swimming medusiform 

 gonophore points clearly to a similar process of evolution 

 in the past ; that is, the plan is due to sporosac-gonoblas- 

 tidium compression. But if so, where is the vegetative- 

 zooidal structure which must have borne the gonoblastidium 

 in the original ancestral type ? 



The most reasonable supposition seems to be that the 

 ancestral type came to specialise in gonophore structure 

 at the expense of the vegetative, till this became vestigial, 

 and finally failed to develop at all. We might picture a 

 primitive Continuously Zooidal Individual acquiring the 

 special variation of producing only one gonophore, which 

 developed to such a size at the expense of the colony that 

 this became arrested. We might further suppose that the 

 gonophore would swim away carrying with it its tiny 

 " parent " colony, which would die off and disappear. In 

 course of time it might well be that vegetative struc- 

 ture was omitted altogether in the development of the 

 gonophore, so that this structure came to represent the 

 Individual. 



This is pure speculation, but it is reasonable to conclude 

 that in the evolution of the Individual Medusa there has 

 been some form of reduction leading to complete disap- 

 pearance of ancestral vegetative zooidal structure. 



In all medusoid organisms the dome of the bell represents 

 the proximal end, and the remembrance of this fact is 

 important in the study of megazooidal evolution. We 

 might say that the " resting position " of a medusa is 

 dome-downwards, and in fact medusae sometimes do rest 

 in this position at the bottom of the sea, behaving for the 

 time being like sea-anemones. 



It is true that the Medusa and the Sea-anemone are 

 very different in appearance, but in fundamental structure 

 they are practically the same. In both cases there has 

 been a compression of a developing zooidal system to form 

 a megazooid ; an organism in no way colonial in structure 

 though manifesting distinct repetition in its plan. But 

 the Sea-anemone has been derived through the compression 

 of tubular or vegetative zooids, whereas the Medusa repre- 

 sents the compression of sporosacs, or zooids of a special 

 form adapted for producing sexual elements. If the manu- 



