. The causes and course of organic evolution; . tion. Alike in themature animal Dinophilus, and in the larva of the annelidanOphryotrocha puerilis figured (Fig. 21 e), the so-called telo-trochal larva, typical of Mysostoma and of the adult rotifercompounded above, has become a polytrochal type by reten-tion of the ciliary rows and commencing segmentation. Butneither in Dinophilus nor in the larva are setae as yet from before backward is then gradually estab-lished, as i^ointed out by Korschelt. Thus there arises a polysegmental condition, which in manypoints still largely
. The causes and course of organic evolution; . tion. Alike in themature animal Dinophilus, and in the larva of the annelidanOphryotrocha puerilis figured (Fig. 21 e), the so-called telo-trochal larva, typical of Mysostoma and of the adult rotifercompounded above, has become a polytrochal type by reten-tion of the ciliary rows and commencing segmentation. Butneither in Dinophilus nor in the larva are setae as yet from before backward is then gradually estab-lished, as i^ointed out by Korschelt. Thus there arises a polysegmental condition, which in manypoints still largely retains rotiferan characters. Such existsnow in that simple group of fresh-water annelids, the Aph-aneura, which A. Sedgwick has suggested should be placedamongst the Archiannelida. It along with Dinophilus sug-gests that many genera once existed which formed graded andperfect transitions from primitive rotiferan and trochophoretypes to planarian worms on the one hand, and to highly seg-mented annelidans on the other. Phylogeny of Animals 513. CO
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