122 THE EVOLUTION OF THE METAZOA 



attributed to the skin layer in their fully developed state only. 

 Recently the theory of an active wandering of cnidocytes has 

 been doubted by some authors (Brien and Reniers-Decoin, 

 1950). To this it can be answered that it is easy to observe 

 this wandering, which is frequently very complicated, in a 

 living object (cf. the most recent work by Lenhoff, 1959). 



In conclusion, we can state that conditions that can be ob- 

 served in the muscular system of Cnidaria show (in comparison 

 with conditions that can be observed in the freely living Tur- 

 bellaria) a generally retrograde development in polyps as the 

 primary form of Cnidaria, a fact due to their sessile way of 

 life. Yet at the same time they have been reorganized and 

 specialized, concordantly with their mode of life. These changes 

 which are partly progressive and partly regressive can be clear- 

 ly identified as early as in Anthozoa. Here the development 

 followed the direction, solitary Hexactiniaria-colonial Hexacti- 

 niaria-colonial Octactiniaria. It is therefore wrong to put the 

 Octactiniaria (as has been done for example by Hyman) as a 

 supposedly more primitive form into the first place of the 

 system of Anthozoa. The Octactiniaria with their sarcosepta 

 (whose number is constantly eight) follow the rule ; first poly- 

 merization with an unfixed number of multiplied "parts," fol- 

 lowed by an oligomerization with a reduced and fixed number 

 of "parts." In the extremely specialized Antipatharia, among 

 Hexactiniaria, whose anthopolyps have a strongly decreased 

 size and which are, moreover, very short, the septal muscles 

 have been mainly lost, while at the same time other strata of 

 the muscular tissue also show a very poor development; the 

 skin, however, has again become covered with cilia. Thus the 

 changes in the muscle system have gone much further in the 

 colonial Hexacorallia than in the typical Octactiniaria. 



The muscular conditions that can be observed in medusa 

 of Scyphoza and Hydrozoa which are even more specialized 

 can be deduced from those of polyps, and not conversely as 

 this has been done by numerous zoologists, Hyman among 

 them. Neither the septal muscles nor sphincters can be found 



