MEGAZOOIDAL INDIVIDUALS 91 



megazooid from which was derived the vertebrate segment. 

 It is in the medusoid plan that the origin of the " coelom " 

 of higher types can be recognised. 



The Medusiform Megazooid. 



To the casual observer there would seem to be few 

 resemblances between a jellyfish and a sea-anemone, but the 

 fact remains that in both cases the structural plan is mega- 

 zooidal and originally of Continuously Zooidal derivation. 

 How then are the divergences in form and habit to be 

 explained ? 



The truth is that the differences depend on the different 

 types of ancestral zooids which have been compressed in 

 lateral Continuity. In the case of the sea-anemone the 

 ancestral zooids have clearly been of typical cylindrical, 

 tubular, or " vegetative " form, while in that of the jellyfish 

 they have been of a structure whose characters reflected 

 special adaptation for the function of reproduction. 



Here lies the whole crux of the matter, and it is of the 

 greatest importance to grasp clearly the distinction just 

 mentioned, as in no other way can we satisfactorily ex- 

 plain the evolution of invertebrate and also vertebrate 

 Segmental organisms. 



But here a few words of explanation are necessary. The 

 present chapter deals with Megazooidal Individuals, and we 

 have now to consider the Mono-megazooidal of medusiform 

 or jellyfish type ; but to do so it is necessary to hark back 

 to the Continuously Zooidal Individual, for it is only 

 through the reproductive structure of the zooidal colony 

 that the derivation of the medusiform megazooid can be 

 made clear. 



As was stated on page 81, many marine zooidal colonies 

 are zooidal throughout, the sexual elements being produced 

 by structures which, though quite unlike the vegetative 

 zooids in appearance, are zooids adapted for reproduction. 

 But there are others in whom the sexual zooid is represented 

 by a sexual megazooid of medusoid or jellyfish form in a 

 state of imperfection or perfection, and in whom the Indi- 

 vidual is partly zooidal and partly megazooidal. In certain 

 colonies these sexual medusoids, or " gonophores," become 



