THE CONTINUOUSLY ZOOIDAL 79 



whole it may be said that zooidal continuity implies zooidal 

 arrest. 



On the presumption that the growth-cycle advances 

 on through the successive zooids of a series, it is clear that 

 sooner or later sexual zooids, or ones in whom certain cell- 

 cycles terminate, must be produced, and when this has 

 occurred the growing Individual will offer the picture of a 

 branching colony composed of two classes of zooids, one 

 apparently devoted solely to nutritional and supporting 

 functions, and the other to reproduction. It was this picture 

 which gave to such zooidal colonies the special credit of 

 exhibiting " alternation of generations." But the truth is 

 that this phenomenon, in its proper wide sense, is exhibited 

 in all living Individuals. Every intermediate zooid in a 

 colony starts life, so to speak, with the intention of becoming 

 sexual, and the fact that many cells and many zooids are 

 permanently arrested and modified and never attain to sex 

 does not affect the true significance of the " alternation of 

 generations." Fundamentally, it is Arrest, the product 

 of Continuity, which brings it about that the Continuously 

 Zooidal Individual gives a permanent exhibition of two kinds 

 of zooids. 



If we look around for examples of Continuously Zooidal 

 Individual we find them in considerable variety amongst 

 the Hydrozoa — one of the two classes into which the Ccelen- 

 terata are divided. All the Hydrozoa are not Continuously 

 Zooidal Individuals, but those to whom the term is applicable 

 are plant-like organisms inhabiting usually salt and rarely 

 fresh water, and composed of many distinct zooids united 

 by a branching framework, or " ccenosarc," to form a zooidal 

 colony. While we may regard the typical growth habit as 

 essentially equivalent to what Hydra would exhibit were 

 its budded zooids to remain attached to each other, and 

 while we may hazard the guess that the original primitive 

 Continuously Zooidal type presented this appearance, the 

 Hydrozoal colony of present and recent times produces its 

 zooids not directly from other zooids but from the hollow 

 tubular framework just mentioned. The significance of the 

 ccenosarc is, however, very probably as roughly represented 

 in the next diagram (Fig. 19); that is, it may be taken to 

 represent the extension and narrowing of the basal or 



