CNIDARIA AS THE ONLY COELENTERATA 



53 



animal by way of the invagination of the surface stratum of the 

 body. This is how we have to understand the origin of nephri- 

 dia which serve as excretory and emunctory organs. Another 

 point to be mentioned here is that Dollo's rule does not seem 

 to be valid in connection with the formation of cilia, i.e. even 

 after they had been lost during the phyletic development of 



Fig. 8. Secondarily ciliated Hydrozoa; A — B, sporosac of Dicoryne 



conferta Alder (after AUman); C, Halammohydra octopodides Remane; 



(after Remane). 



an animal they can be later reintroduced. This can be proved 

 by some clear instances that can be observed among Hydro- 

 zoa. It has been known since Allman's days that in the species 

 Dicoryne conferta Allman, which belongs to Hydroidea (Atheca- 

 ta), completely retrograded medusoids which lived otherwise 

 as completely immobile forms as its cormus, regain their ability 

 to swim again by means of a secondary development of cilia 

 (the so-called sporosacs, see Fig. 8 A-B). Halammohjdra, a Hydro- 

 zoon which has been recently discovered by A. Remane (1937), 

 5 



