416 GEORGE W. TANNREUTHER 



interpret the trochophore as representing a simplified form, an 

 ancestor which, were it living to-day, would be classified as a 

 ctenophore, and that the three distinct larval types, namely, 

 Miillers larva, the Pilidium, and the trochophore, all represent, 

 with more or less change of form, a group of ancestral Cteno- 

 phora, from which sprang the Polyclada, and through them all 

 the plathelminthes, the Nemertfans, and the annelids. 



Considering the life history of the different groups in question 

 from the standpoint of resemblances and differences, the above 

 phyletic scheme becomes a tenable one. If the rotifers repre- 

 sent a primitive or ancestral type, and are preserved, with modi- 

 fications, in the larvae of annelids and other forms, what position 

 in the phyletic scheme do they occupy, more especially when 

 considered from the standpoint of cleavage and early develop- 

 ment? The larvae of the annelids not only show resemblances 

 to that of the rotifers, but, in addition, they possess in concen- 

 trated form the rudiments of the future adult annelid body. 

 Also the mesodermal structure in the trochophore must un- 

 doubtedly represent the mesoderm of the ancestral type. The 

 point at issue is, however, not so much what the completely 

 formed larvae of annelids or the rotifers as such possess, but is 

 there any parallelism in their development in reaching this point, 

 which is of any phyletic value? 



If cell homology have any significance, according to some 

 writers we must conclude that the cells whose products are 

 homologous must be themselves homologous, even though they 

 may have the same or different origin and position in cleavage. 

 Light on the systematic position of rotifers may be gained by 

 comparison of their development with that of other groups. The 

 studies on cell lineage no doubt are invaluable in determining 

 phyletic relationships, as has been proved by results along this 

 line of investigation. Characters of forms, as manifested during 

 cleavage, in many instances are as constant as are anatomical 

 characters in later stages, and must therefore be as truly in- 

 herited. And since coenogenetic changes may be supposed to 

 affect the later stages of development first, we may expect to 

 find earlier stages retaining longer their primitive characters. It 



