THE ANCESTRY OF THE CHORDATA. 539 
&e., one or more of the mesoblastic organs may not be 
repeated; while in both Oligochzts and Polychzets 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 inkind. 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 which 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 increasein 
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 tendeacy to repeat parts already present, which ten- 
dency occurs more or less in almost all animals. But before 
communicating the features of Chordate 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 
