ANNELIDA 



341 



possible to unite satisfactorily either with the Oligochfeta 

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

 together 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 Chsetopoda. To be sure, the origin of the 

 chain of o-ano-Ha 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 Br anchiob delta 

 merits renewed investigation. 



General Considerations regarding Annelida. 



The embryology of the Annelida affords us some hints 

 regarding the phylogenetic derivation of the Annelid stem 

 and its genetic relationships to other groups of animals, and 

 also regarding the origin of metameric segmentation. These 

 suggestions 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 

 expressed, are referable to the Trochophore. The Trochu- 

 phore is the typical larval form of the Annelid stem. Even in 

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

 chxta and Echiuridee, as well as in the aberrant genus Myzo- 

 stoma, the larval Trochophore form can be recognized more 

 or less distinctly. Dlnophilas corresponds in its shape 

 and 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 Oligochasta. 



Most likely the Trochophore of the Annelida embodies 



