516 



Causes and Course of Organic Evolution 



overlooked. The class has generally been divided into the 

 Entoprocta that has been regarded as the more primitive, 

 and the Ectoprocta that is greatly modified in the mature 

 state and somewhat even in the trochophore larva. As al- 

 ready stated (p. 391) we incline to reverse this view. The 

 former includes three existing genera, of which one Urnatella 

 is fresh-water, while Loxosoma and Pedicellina are marine. 



Fig. 22. — a, trochophore stage of Pedicellina; 6-e, stages in development of 

 marine polypide of Polyzoa; e.s., eyes; d.b., dorsal brain; v.b., ventral brain; 

 in, mouth; a, anus; v, ventral area; nep, nephridia; eg, cement glands; coa, circle 

 of tentacles. 



In Fig. 22a is shown in normal position the somewhat ad- 

 vanced trochophore stage of Pedicellina as given by Hatschek, 

 which unfortunately in many text-books is represented as 

 standing or floating on its head. The area in front of the 

 prseoral ciliary ring corresponds to the trochal disc of Roti- 

 fera, and here has been called the "ciliated disc." As in roti- 

 fers "the larva of Pedicellina, when swimming, always carries 

 this organ directed forward." In front of it, and similarly 

 derived as in rotifers according to Harmer, is the_dorsal brain- 



