PREVIOUS INTERPRETATIONS OF CNIDARIA 147 



evolved. The whole system has been taken over from their 

 tubellarian ancestors by two types which have both developed 

 into parasites ; by the Trematoda and by the Cestoda. In all 

 other cases we can observe a very radical reduction of this 

 complex, frequently with a transition to gonochorism. In the 

 Nemertinea which are now quite generally considered as 

 being direct descendants from the Turbellaria and thus their 

 closest relatives, we find all the richly developed "decoration" 

 lost, and of all the oppulence of their auxiliary organs there 

 is hardly the formation of a conduit = canal left. Though 

 hermaphrodites can still be found among the Nemertinea, 

 the latter have nevertheless abandoned the pairing associated 

 with internal fertilization. It is in the pelagic species Phallone- 

 mertes only that simple copulatory organs can be found in 

 the male animials (yet even this case is problematic because 

 we do not know whether these organs are a remnants of a 

 primary apparatus, or whether they are secondarily developed 

 organs, due to the secondary pelagic way of life). In Carcino- 

 nemertes even the direct contact of gonads with the external 

 world has been relinquished, and the gametes are secreted 

 into the digestive organ. The Rotifera, however, have preserv- 

 ed more numerous elements of the complex structure of 

 the turbellarian genital system the female animals have the 

 division of the ovary into a germarium and a vitellarium, 

 and the male animals have developed a special copulation 

 organ. The generally hermaphroditic Gastrostricha do not 

 differ widely in their genital apparatus from that found 

 in Rotatoria. The turbellarian genital apparatus has been 

 somewhat more preserved in the parasitic Acanthocephala. 

 On the other hand, the auxiliary organs show a complete 

 retrogression in Endoprocta which have adopted a sessile 

 way of life; in these the situation resembles that which can 

 be observed in Cnidaria with the difference that in the former 

 the canals of gonads are always developed while in the polyps 

 of Cnidaria the remains of these canals can be found only 

 in Anthopolyps, just as in Ctenophora which have developed 



