216 THE EVOLUTION OF THE META20A 



strobilation even if this word suits those cases only where 

 we have a probably primary multiple transverse division (e.g. 

 the polydisc strobila). The products of this special form of 

 transverse division develop into medusae simultaneously tak- 

 ing over the sexual function. Since scyphomedusae are deve- 

 loped primarily by way of transverse division, it can be 

 understood that they never reproduce asexually (e.g., by 

 way of budding), as can occcasionally be observed in hydro- 

 medusae. 



It is interesting to notice that in Hydrozoa, which do not 

 include any species that live as primarily solitary animals, 

 transverse division has reappeared in those species that have 

 secondarily adopted a solitary life. This is known for Proto- 

 hydra (besides a longitudinal division), and apparently abnor- 

 mally even in Hydra. The transverse division of the coenosarc 

 in the hydrorhiza which can lead to the formation of podo- 

 cysts can perhaps also be attributed to it as well as the decom- 

 position of the Polypodium chain. Here we can also inc- 

 lude the isolated case of hydromedusa (Gastroblata raffaeli) 

 which reproduce asexually by means of longitudinal fission 

 (Fig. 38). 



A direct transverse division cannot lead to the formation 

 of cormi. We cannot know how the formation of cormi began 

 during the geological past. Yet if we refer to conditions that 

 exist in Ciliata and above all if we take into consideration those 

 that exist in the primarily solitary Anthozoa we can come to 

 the conclusion that the formation of cormi had begun in all 

 probablity as an imperfect division when budding had deve- 

 loped from division due to the great inequality of its products: 

 in this way the larger "half" of the divided polyp grew into 

 the maternal animal, and the smaller "half" into the bud. The 

 longitudinal division, too, had possibly been secondarily de- 

 veloped from the primary transverse division, such as it occurs 

 in Ciliata and in Turbellaria by way of an oblique division. In 

 the higher sessile animals we find budding preserved as the 

 only form of the asexual reproduction. 



