92 THE EVOLUTION OF CONTINUITY 



separated to lead an independent existence and grow into 

 large jellyfish which fertilise their own ova and originate 

 new sessile zooidal colonies ; but such medusoids are, of 

 course, not megazooidal Individuals. Apart, however, from 

 matters of structural detail the plans of the gonophore and 

 of the Individual Medusa are the same, and this is also 

 true of their derivation. It is to be understood that the 

 immediately following remarks deal with medusiform mega- 

 zooids which are parts of Continuously Zooidal Individuals, 

 and which offer themselves as links between the latter and 

 Individual Medusae. 



The Evolution of Sexual Structure in Marine 

 Zooidal Colonies. 



In these colonies the fundamental vegetative structure 

 is the tubular zooid, and this, specially adapted and altered, 



AV 



A. 



Fig. 27. — A, diagram of Hydrozoal sporosac. a, central 

 hollow protrusion of the colonial " body-cavity " ; ec, ecto- 

 derm ; en, endoderm ; a, sexual elements. B, diagram of 

 a vegetative zooid for comparison. A is for all purposes 

 like a b which has failed to form a mouth, and instead has 

 formed sexual elements in its walls. 



is the basis of the various sexual structures exhibited. In 

 its simplest form the sexual structure of the colony takes 

 the form of a zooid which is not tubular nor possessed of 

 a mouth leading to a body-cavity, but which has a globular 

 form, has no external opening, and forms in the walls 

 enclosing a blind protrusion of its producer's somatic cavity 

 the sexual elements. This special sexual zooid is termed a 

 " sporosac " (Fig. 27). 



From some primitive Hydrozoal type which produced 



