418 GEORGE W. TANNREUTHER 



not invaginate with the large cell 3D, and thus take no part in 

 the formation of the future gut, but, by a rapid proliferation, 

 their products grow posteriorly and form the major portion of the 

 foot, which is absorbed before birth. In the polyclade Planocera, 

 as in the rotifers, 3D has the same fate; likewise, 3B, 3C, and 3A, 

 with their derivatives, take part in the invagination, but do not 

 contribute to the gut, and are later absorbed as food. 



As stated, the sequence of cleavage in the formation of the 

 quartettes in the rotifers is comparable to those of the poly- 

 clades and annelids, but the position taken by the resultant 

 generations of cells is different. This difference, however, is 

 adaptive. The cells, instead of remaining in a more spherical 

 mass, are drawn out in an anteroposterior direction. The 

 cleavage cells take up this early position in accordance with 

 their later formation and the needs of the future animal. The 

 rotifers, in their early development, possess characters which 

 are common to both the polyclades and the annelids. In ad- 

 dition, however, they possess characteristics which are peculiar 

 to the polyclades and others which are distinctly annelidian. 



When rotifers are compared with the adult Dinophilus, some 

 very striking points of resemblance are recognizable. In both 

 the bands of cilia, which are the free-swimming organs of loco- 

 motion, represent the prototroch or remnants of it. The adult 

 Dinophilus remains to a certain extent at the stage of the an- 

 nelid larva or a stationary larva which has become sexually 

 mature. Here the trochophoral characteristics persist in common 

 with the worm like-form of the annelids. 



In the rotifers the trochophore stage persists, and among 

 some of the rotifers at least, it is the end or climax stage in 

 development. Many of the rotifers, however, possess in ad- 

 dition a worm-like body and are capable of an annelid creeping 

 motion, independent of the trochal cilia, which indicates to some 

 degree a specialization in a definitely directed line. As pre- 

 viously stated, there is a distinct parallelism in the development 

 of the two forms. The sequence of cleavage is the same, but 

 there is a dissimilarity in the position taken by the cleavage 

 cells. This difference is, however, adaptive. In the rotifers the 



