164 THE EVOLUTION OF THE METAZOA 



complete gastrovascular system. Since at the first glance similar 

 complications were also known to occur in Spongiae and 

 Ctenophora, the three animal types were united into the 

 group of Coelenterata(Leuckart). In addition to this there deve- 

 loped the idea that the two principal body cavities (the intes- 

 tine and the coelom) which appear in the "more highly" 

 developed animal types, are in these animals mutually combin- 

 ed. This, however, has been proved to be wrong. It is almost 

 unbelievable how obstinately the idea has been preserved 

 according to which there should be a genetic relationship 

 between the intestinal cavity and the coelomic cavities (i.e. 

 the part of the coelomic cavities which surrounds the intestine 

 and which we have called a perigastrocoel); this has led to 

 the formulation of the so-called enterocoele theory w^hich is 

 doubtlessly wrong and w^hich has been the cause of numerous 

 controversies. 



We have to make here a sharp distinction between three 

 facts ; first, between the digestive system of a single individual 

 and that of cormi. It is only in the cormi of Cnidaria that the 

 digestive organs of zooids that form a cormus are directly 

 connected into a uniform canal system. The connecting canals 

 (solenia, as they are frequently called) are impersonal, i.e. they 

 are purely the organs of the coimi. The conditions that exist 

 in the Spongiae are basically different from those that occur 

 in Cnidaria (the Ctenophora, on the other hand, do not form 

 cormi), and it is therefore clear that the Spongiae are completely 

 disassociated from Coelenterata and also from Eumetazoa. 



Secondly, in the Cnidaria w^e must make a sharp distinction 

 between the digestive system of polyps— whether these be 

 primarily or secondarily solitary animals, or whether the 

 digestive system belongs to a zooid— and the "gastrovascular" 

 system of medusae. The peripheral part of the digestive system 

 of medusae with exception of the tentacular canals is a new^ 

 formation and it has no homologue in the polyp. The principal 

 part of the gastrovascular system is not a continuation of 

 the peripheral part of the digestive canal of the anthopolyps. 



