THE EVOLUTION OF SYMMETRY 223 



external form to the developing new type, but the hollow 

 interior is not so easy of explanation. It may be that the 

 factor of diminishing density mentioned on page 164 was 

 involved. 



The cylindrical tube the zooid represents is the basis 

 of megazooidal symmetry. There are two types of mega- 

 zooid, the anemonoid and the medusoid, the former prac- 

 tically immobile, and the latter free-swimming, and in both 

 there is an external symmetry of form, and an internal 

 symmetry of parts. 



In the anemonoid megazooid the external symmetry is 

 that of a short cylinder, and it reflects the internal symmetry 

 of structure. The theory offered on page 88, and the accom- 

 panying figures, explaining the evolution of the anemone- 

 megazooid, serve to show how the organism came to be 

 radially symmetrical, and to avoid repetition the reader is 

 requested to refer to them. We would only point out once 

 more that the new form of Continuity constituted by the 

 megazooid was the result of compression during develop- 

 ment. The fertilised ovum had the power of becoming 

 many tubular zooids on a colonial plan, and an evenly 

 compressing force, which we believe to have been water- 

 pressure, accepted the hereditary gift of the zooidal tubes, 

 but caused them to develop in close lateral continuity 

 round a central stem, and the megazooid was the result. 

 The radial symmetry of the medusoid-megazooid is to be 

 explained in a similar way (page 94), the hereditary gift 

 being here sporosacs or sexual zooids, and not tubular 

 vegetative zooids. 



