80 THE EVOLUTION OF CONTINUITY 



proximal parts of ancestral zooids. Thus, in Fig. 19, A 

 shows three zooids of Hydra type remaining in continuity, 

 while B shows them as having developed narrow tubular 

 connection with their producers. It is from the narrow con- 

 nections that we presume gemmation habitually occurred 

 in the primitive types, as it is from the coenosarc that new 

 zooids are budded in modern hydrozoal colonies. 



Such zooids as roughly drawn below are what is termed 

 vegetative zooids ; from their walls, as distinct from the 

 connecting coenosarc, they never bud other zooids, nor do 

 they form sexual elements. Cyclically, they remain per- 

 manently arrested, and the growth-cycle advances to its 



Fig. 19. — Suggested significance of the Hydrozoal colony's 

 coenosarc. z, zooids ; ec, ectoderm ; en, endoderm ; t, 

 tentacles ; 6, body-cavity ; c, coenosarc ; x, gemmation-areas. 



termination through coenosarc gemmation, with the result 

 that from some distal portion of the coenosarc a sexual zooid 

 ultimately appears. Naturally from any part of the coenosarc 

 gemmation may take place and finally result in the pro- 

 duction of sexual zooids. 



There are several subclasses of Hydrozoa, and many 

 orders in these, for an account of which the reader is referred 

 to biological textbooks, but for our present purpose the 

 general plan of Continuously Zooidal growth may be taken 

 as is represented above in Fig. 19, B. 



It is to be emphasised that all Hydrozoa are not 

 Continuously Zooidal Individuals. For example, Hydra is 



