38 THE VOYAGE OF H.M.S. CHALLENGER. 



Crustacea, I am inclined to agree on the whole with the suggestion set forth by Dr. A. 

 Packard in his valuable treatise on the Phyllopod Crustacea of North America, believing 

 that the Nebaliidse may have descended from some Copepod-like ancestors, whereas they 

 do not show any relation whatever to the Podophthalmia, the latter having in all 

 probability developed independently by a separate line from some Nauplius- or Zoea-like 

 form. From the same stem that gave origin to the Nebaliidas, I think that also the 

 other Branchiopoda may be derived, the latter having apparently become rather con- 

 siderably modified in various ways to adapt them to the somewhat exceptional conditions 

 under which they live, whereas the Nebaliidae have still preserved much of the external 

 appearance which may have distinguished the progenitors of the order, while their 

 internal organisation has become much more modified. 



