A. STUDY OF METAMEEISM. 465 



but have gone further, and postulated it as the ancestral adult. 

 They have been led to believe that such a minute few-celled 

 larva has evolved into the complicated segmented Annelid, and 

 into the equally complicated but unsegmented Mollusc. The 

 resemblances between the adult Mollusc and Annelid they 

 have been content to call " parallel developments." 



For all the evidence we have at present we might satisfy the 

 facts just as well by assuming that a large many-celled unseg- 

 mented form stood as the bottom of these two groups, having 

 a trochosphere as its larval form. The Rotifers and related 

 forms would then be interpreted as arrested forms. The 

 one conclusion would be, I think, as justifiable and as 

 easily maintained as the other, and both impossible to 

 demonstrate. 



In the face of so much conflicting embryological indecision, 

 it seems to me we have in reality arrived with certainty no 

 nearer to the solution of metamerism. 



There have been only a few attempts to explain the meta- 

 meric repetition as the result of mechanical action. Hu- 

 brecht's (22) explanation for the Nemertian is scarcely a me- 

 chanical explanation, since it presupposes a repetition of parts 

 and an ability to regenerate in the worm itself. Kennel's (24) 

 ingenious attempt to interpret division of animals as the result 

 of external stimuli is scarcely a mechanical explanation. 



His (19) has attempted to explain the metamerism of the 

 Vertebrate as an embryological phenomenon — as the result 

 of series of breaks occurring in the two lateral mesodermic 

 sheets of the embryo. Meyer's (29) view, referred to above, is 

 distinctly a mechanical hypothesis. The repetition that he 

 assumes to have come into a long Nemerto-Turbellarian an- 

 cestor was the result of the movements of the body in swim- 

 ming. Matiy objections are easily raised against Meyer's 

 romantic speculation. As Hatschek has pointed out, those 

 Annelids that are adapted for swimming are heteronomously 

 segmented, while Meyer's hypothesis seems to demand first a 

 homonomously segmented form as the result of its own activity. 

 In the second place Meyer's sketch starts off on Lamarckiau 



