ANNELIDA 



The original mode of locomotion of the uniformly ciliated, 

 radial ancestor described, which had arisen from the gas- 

 trula stage by means of some further differentiations, was 

 spiral, such as we may still see in the ciliated planulae of 

 many lower animals. It depended upon a combination of 

 a prog-ressive movement in the direction of the longitudinal 

 axis with a rotation of the entire body about this axis. 

 The ancestral forms of the Platyhelminthes have perhaps 

 been directly developed from such a uniformly ciliated stock- 

 form by the assumption of the creeping mode of life, and 

 the ancestors of the Ctenophora may have been developed 

 by a change in the method of pelagic swarming and by the 

 formation of rows of ciliary plates. Whereas in the latter 

 case the rotation around the longitudinal axis sank into 

 insignificance, and the combined force of the ciliary plates 

 was concentrated on propulsion in the direction of the longi- 

 tudinal axis, in those forms which effected the transition 

 to the Trochophore-like ancestor a change of movement 

 took place. In these cases, though the body as a whole 

 ceased to rotate, the rotatory movement was retained in the 

 trochal organ in the form of a regular circular wave of 

 contraction, Avhereby this organ was in position to under- 

 take a function in relation to the body (now progressing 

 in a constant position) similar to that of the ship's screw 

 in relation to the hull of the ship. Hand in hand with this 

 alteration in the mode of locomotion went a higher differentia- 

 tion of the ciliary locomotor apparatus, by means of which 

 the passage from the original uniform coat of cilia to 

 distinct locomotor organs was brought about. As such are 

 to be mentioned the tuft of cilia functioning as the rudder 

 and the rows of cilia, but especially the preoral ciliated 

 band. 



It is possible that the bilaterally symmetrical distribution 

 of the body-masses was directly developed in connection 

 with the above-mentioned changes in the form of motion 

 by means of which the body was balanced in its forward 

 movement. At any rate, one of the most important factors 

 for^the development of bilateral symmetry is to be sought 

 in the shifting of the mouth-opening, which now moved 



