PREVIOUS INTERPRETATIONS OF CNIDARIA 209 



metry. Such a larva is known as a cerinula. The inclination to 

 prolong the pelagic stage can even lead so far, that it is already 

 during the plankton phase that sexual maturity is reached 

 (Djctylactis benedeni). In these larvae we find a combination 

 of transient larval characteristics and of the definite properties 

 of the grown-up individuals. A higher level of development 

 can be observed above all in their ciliation. "Zoanthellae" 

 with their oblong form, possess a long cilia-line along 

 the ventral side; "zoanthinae" whose form resembles that of 

 a vat, have a circlet of cilia and they swim in a special way by 

 means of pulsations, somewhat similar to the type of swim- 

 ming that can be observed in medusae. As early as in the 

 Anthozoa we can find cases (Zoantharia) where the planktonic 

 phase of the planula has been given up. This planula itself has 

 a life of short duration only and it soon develops into an 

 oozooid. 



No actinulae can be found among Scyphozoa where medusae 

 are the prevalent generation. In the metagenetic species 

 we find the planula stage to be completed early; in Lucerna- 

 riidae, as has already been mentioned, planulae do not rise 

 into the pelagic zone, they have no cilia, and they creep for 

 a short while only over the sea bottom (Fig. 35). 



Among Hydrozoa, freely sw^imming larvae which resemble 

 an actinula can again be found in the very specialized Athecata 

 (Myriothelidae, Tubulariidae). They become free only as such 

 and they are certainly of an independent origin. They have 

 been developed parallel to the retrogression of the free medu- 

 soid generation. Larvae of Narcomedusae, which have also 

 been called actinulae, have nothing in common either with 

 the larvae of Ceriantharia or with those of the athecate Hydro- 

 idea, except if we allow the homology (homotypy) to be comp- 

 letely watered down— something that has often been done in 

 recent researches. The Scyphozoa which have no actinulae 

 appear between Anthozoa which have actinulae and Hydrozoa. 

 The supposed actinulae of Narcomedusae and of Trachome- 

 dusae are larvae which are medusae and not polyps: in these 



