TYPE ANNELIDA. 217 



embryologist. The evidence at present available seems, however, to point, 

 in the case of the Trochophore, to its being an example of the law, and to 

 this extent the first of the two views stated, above is probably correct. 



But, on the other hand, this may not be the case with the second part 

 of the theory. If the view as to the origin of metameriHui which is advo- 

 cated in this work be correct, then the Annelid cannot be regarded as 

 having arisen directly by a process of reproduction by budding of the 

 Trochophore. It is not a colony of Trochophore individuals, but a single* 

 elongated Trochophore whose organs have undergone repetition, producing 

 a high grade of metamerism. To this extent the second of the views may 

 be correct, but this does not necessarily imply that the Annelids are to be 

 derived from a Nemertean-like form in which the metamerism is not quite 

 so perfect. Metamerism, as here explained, is simply the following out 

 into the higher individualities of the phenomenon of discontinuous growth 

 or reproduction by division which characterizes the cell, and it is quite 

 possible that there may be no more genetic connection between the meta- 

 merism of the Nemertean and that of the Annelid than there is between 

 that of the Cestode and that of the Nemertean. It may have arisen quite 

 independently in the two forms, and in fact when the details of 

 metamerism are examined in the two groups considerable differences are 

 to be seen. 



The view here advocated in regard to the origin of the Polychaeta may 

 be briefly expressed as follows : The Polychaeta — and with them the 

 Annelida in general — have had for their ancestor a non-metameric form 

 of which the Trochophore is the larval representative, and this in the 

 course of its development elongated, the elongation being accompanied hy 

 the repetition hy a budding process of certain organs, a high grade of 

 metamerization being thus produced. 



The relationships of the Trochophore seem to be with the Turbellaria. 

 The nervous system, consisting of the apical thickening and lateral nerve- 

 cords, is very similar to that of Turbellaria, and also it is interesting to 

 notice the similarity of structure of the head-kidney with the Turbellarian 

 nephridium. The exceedingly small development of the parenchyma is 

 probably a secondary condition, and the presence of an anus is an im- 

 portant advance upon the Turbellaria. An undoubted similarity in many 

 respects exists between the Rotifera and the Trochophore, and the former 

 have been regarded as persistent Trochophores or else as forms descended 

 from the Trochophore. This latter view in one of its phases has already 

 been considered (p. 195), and an important difference in the relation of the 

 supracesophageal ganglion to the prototroch mentioned. Reasons have also 

 been given for the belief that the Rotifera are descended from Turbellarian 

 ancestors, and it seems probable that the line of descent of the Rotifers 

 was identical for a time with that followed by the Trochophore, the 

 former group branching off from it shortly before the Trochophore ancestor 

 made its appearance on the scene. In this respect the Rotifers and the 



