. The causes and course of organic evolution; . wholly marine. X few from both divi-sions are solitary animals, but the great majority have, bycontinuous budding, gradually evolved an extensive colonialsystem. For the solitary state is naturally the more i)rimi-tive, the colonial is more evolved. The changes which result, on fixation of the previouslyfree-swimming larva, as set forth diagrammatically in , a-e, are in all cases so fundamental, and are further com-plicated in most by extensive budding even in the larval state,that the relation of the Polyzoa to the Rotifera has often been
. The causes and course of organic evolution; . wholly marine. X few from both divi-sions are solitary animals, but the great majority have, bycontinuous budding, gradually evolved an extensive colonialsystem. For the solitary state is naturally the more i)rimi-tive, the colonial is more evolved. The changes which result, on fixation of the previouslyfree-swimming larva, as set forth diagrammatically in , a-e, are in all cases so fundamental, and are further com-plicated in most by extensive budding even in the larval state,that the relation of the Polyzoa to the Rotifera has often been 516 Causes and Course of Organic Evolution overlooked. The class has generally been divided into theEntoprocta that has been regarded as the more primitive,and the Ectoprocta that is greatly modified in the maturestate and somewhat even in the trochophore larva. As al-ready stated (p. 391) we incline to reverse this view. Theformer includes three existing genera, of which one Urnatellais fresh-water, while Loxosoma and Pedicellina are Fig. 22.—a, trochophore stage of Pedicellina; 6-e, stages in development ofmarine polypide of Polyzoa; , eyes; , dorsal brain; , ventral brain;in, mouth; a, anus; v, ventral area; nep, nephridia; eg, cement glands; coa, circleof 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 asstanding or floating on its head. The area in front of theprseoral 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 carriesthis organ directed forward. In front of it, and similarlyderived as in rotifers according to Harmer, is the_dorsal brain- Phylogeny of Animals 517 mass; above this, in Loxosoma as in some rotifers, a pair ofsimple, but in this instance temporary, eyes arise. Belowthe ciliated ring,
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