18 



tuba stage in the development of the Discomedusse) must 

 have given rise to two divergent series of forms — the one 

 the ancestors of the Hydromedusse and the Ctenophora, 

 and the other the ancestors of the Scyphomedusse and the 

 Actinozoa. 



In the first series the hydriform fixed stage became 

 emphasised, the body became longer, and the enteric cavity 

 remained a simple tube (see fig. 12, B) ; and, in place of 

 becoming detached and modified into a Medusa-like form 

 when mature, it acquired the power of budding to a remark- 

 able extent, the buds being in the form of processes from 

 the body, and formed of both cell layers. By means of 

 this property, fixed tree-like colonies were formed, on which, 

 after a time, a special set of buds were produced which, in 

 place of remaining like their predecessors in a hydriform 

 condition, became modified and were detached as free- 

 swimming medusiform persons in which the reproductive 

 elements were developed. This specialisation of the two 

 sets of buds in the common ancestor of the Hydromedusas 

 brought about the state of affairs generally known as 

 "alternation of generations." The ovum of the medusiform 

 person developes into a hydriform person which produces 

 by budding a tree-like colony upon which certain buds 

 become medusiform persons, are detached, and produce 

 ova and spermatozoa. 



In the ancestors of the Trachylarida the life-history 

 became simplified, probably by the hydriform fixed stage 

 being more rapidly hurried over until it came to be of little 

 importance, and was finally suppressed altogether — the result 

 being that in that group (including the Trachomedusse and 

 the Narcomedusae) at the present day we find the ovum 

 developing directly into the Medusa. 



On the other hand, in many of the Calyptoblastea and 

 Gymnoblastea, we find that the medusiform persons become 



