SECT. XI 



DEVELOPMENT 



167 



liavc seen, this is not the case ; the number of 

 segments varies not only in the different species of 

 the genus, but, as it appears, in different individuals 

 of the same species. These two characteristics of the 

 Apodidae, the great number and the varying number 

 of the segments, ought almost of itself to constitute 

 them the natural transition form between the Annelids 

 and the Crustacea. In the Annelids we have a large 

 and variable number of segments, in the higher 



IMG. 40. Second larval stage of Lepidurus productns (after Brauer). 



Crustacea a comparatively small, and for each group 

 a fixed, number of segments. Between these two the 

 Apodidae form the true link, having a diminishing 

 number of segments, diminishing, that is, by a con- 

 siderable number remaining undeveloped, and so 

 rudimentary as to be useless to the animal, and there- 

 fore liable to vanish. 



In this section on the Nauplius we have appealed 

 to the developmental history of A pus in support of 



