THE ANCESTRY OF THE OHORDATA. 71 



&c., one or more of the mesoblastic organs may not be 

 repeated; while in both Oligocheets and Polychsets there is 

 a marked tendency to a division of labour between and 

 specialisation of structure of individual segments or even 

 regions of segments in various parts of the body. It thus 

 appears that even among Annelids alone the fact of segmenta- 

 tion is not a circumscribed idea, but may include several phe- 

 nomena which clearly differ from each other in degree, and 

 possibly are also unlike in kind. For while in the case of Nais, 

 &c., this repetition is complete, and is thus used as an obvious 

 and simple mode of reproduction, yet in other worms it appears 

 only to be concerned in increasing the length of one individual 

 without adding to the number. Now, if these two conditions 

 are merely various expressions of the same phenomenon the 

 question at once arises as to wliich is its more primitive mani- 

 festation. Was segmentation originally a repetition of all 

 the organs for purposes of reproduction, which process 

 has become subsequently commuted into mere increase in 

 bulk, or is this complete repetition to be regarded as the 

 final term in a series of which the first was increase in 

 bulk? Segmentation, as we know it, may clearly be viewed 

 from either of these two standpoints. With regard to the 

 Annelids, many authors have held that the former is the correct 

 one ; the question whether this is so or not cannot be dis- 

 cussed here, but in the case of the Chordata examination will 

 show that their segmentation is of the latter class, and is the 

 result of a summation of repetitions ; and, being so, it is by no 

 means a unique condition, which can unite forms otherwise 

 unlike, as Chordata and Annelids, but is rather a result of the 

 common tendency to repeat parts already present, which ten- 

 dency occurs more or less in almost all animals. But before 

 communicating the features of Choi'date anatomy, which point 

 to this as the mode of origin of the segmentation of the class, 

 it will be best to establish the fact that repetitions of this sort 

 are common, and to examine the comparative evidence as to 

 the manner in which they occur. It will then be seen that 

 segmentation on the plan found in the Vertebrates are really 



