ANNELIDA 341 



possible to unite satisfactorily either with the Oligochaeta 



Por Hirudinea, the following points, in addition to the al- 

 ►gether aberrant cleavage phenomena, are remarkable: the 

 formation of the mesodermal bands and the very peculiar 

 manner in which the nervous system is formed. A germ 

 band in the sense of the Hiradinea is not present, but, on 

 the other hand, the fundament of the nervous system differs 

 from that of the Chaetopoda. To be sure, the origin of the 

 chain of ganglia from a ventral ectodermal invagination has 

 been repeatedly described for the Annelida, but this conclu- 

 sion has not gained currency. At all events, the origin of 

 the nervous system and mesodermal bands of Branchiobdella 

 merits renewed investigation. 



General Considerations regarding Annelida. 



The embryology of the Annelida affords us some hints 



regarding the phylogenetic derivation of the Annelid stem 



Lud its genetic relationships to other groups of animals, and 



Jso regarding the origin of metameric segmentation. These 



mggestions are significant, even though they do not as yet 



[furnish a foundation of positive knowledge, but serve only 



[to support theories of greater or less probability. 



If we consider the larval forms of the Annelida, we see 



|that their different shapes, however variously they may be 



pexpressed, are referable to the Trochophore. The Trocho- 



ihore is the typical larval jor'm of the Annelid stem. Even in 



the derived and much-modified groups, such as the OUgo- 



Ichoeta and Echiuridas, as Avell as in the aberrant genus Myzo- 



Istoma, the larval Trochophore form can be recognized more 



►r less distinctly. Dinophilas corresponds in its shape 



fand organization to a so-called polytrochal larva, which it 



was possible to derive directly from the Trochophore (comp. 



:p. 278). The embryos of the Hirudinea exhibit the greatest 



[resemblance to those of the Oligochaeta. However, they are 



[much more modified than these, and consequently cannot be 



[traced directly to the Trochophore, though this may be 



[accomplished through the mediation of the Oligochaeta. 



Most likely the Trochophore of the Annelida embodies 



