CXIDARIA AS THE ONLY COELENTERATA 41 



were convergently developed pelagic larval forms, i.e. they 

 developed independently from each other in completely diffe- 

 rent animal groups, beginning with Tiirhellaria and continuing 

 up to the very last Oligomeria, or close to the very threshold 

 of Chordoma. 



Unfortunately, there have been numerous zoologists not 

 only in former days (e.g. Hatschek, Cori) but even, now (e.g. 

 Carter, Marcus) who have been induced to attribute great 

 phyletic significance to these larval forms. This led finally to 

 an unnecessary increase in the number of proposed animal 

 systems. One example should suffice to show how this exag- 

 gerated appreciations of similarities between various larvae— in 

 this case between trochophores — led to wrong conclusions. We 

 think here of Mollusca. The veliger larva of Mollusca has 

 been rather generally considered as a kind of trochophore. 

 Because of an external similarity, yet frequently not for this 

 reason only, it has been thought that there is a close rela- 

 tionship between Mollusca and Annelida. According to this 

 interpretation the primitive Mollusca have been believed to 

 have developed from primitive Annelida by way of an omis- 

 sion of their metamerism. In the not very distant past, earnest 

 attempts have been made to find in the ontogeny of Mollusca, 

 in their veliger larva, traces of a former metamerism wdth the 

 cavity of the former perigastrocoel (eucoelom, as it is called by 

 American authors). It has been thought that these have been 

 found, in the Mollusca, in much the same way that they have 

 been described for the Echiuroidea. The Mollusca, however, 

 are genuine and primary Ameria, i.e. they are from their very 

 beginning unsegmented Eumetazoa, and it can be proved that 

 in their phyletic past they have never been segmented animals. 

 They represent the climax in the progressive development with- 

 in the phylum of Ameria. They are therefore not directly 

 related to AnneUda, either in the sense that we see in Annelida 

 a development from the primitive Mollusca, or, vice versa 

 as a result of a partly regressive development due to a secon- 

 dary omission of the segmentation. 



