STATE GEOLOGIST. 13 



CHAPTER 11. 



ORDER CLADOOERA. 



This very extensive ^roup contains a variety of types, but there 

 are sufficiently evident connecting links uniting the extremes of 

 structure. The Gymnomera which, following the usual custom, 

 we include here, stand distinct from the other groups, yet have suf- 

 ficiently evident cladoceran affinities. It is very unfortunate for 

 aetiological speculation that this the only truly marine group 

 should stand thus isolated from its fellows. Ascording to the no- 

 tions at present prevailing, the Phyllopods stand nearest the primi- 

 tive type of Crustacea. There are unmistakable hints at an early 

 origin for that group, and not less evident are certain analogies 

 with both Cladocera and Copepoda. 



There has, however, recently been made an attempt to derive the 

 Phyllopods from an original cladoceran stem with, as we think, 

 somewhat unsatisfactory results. Do we not the rather see in both 

 groups two like phases which may be looked upon as incidental and 

 comparatively trivial. The shelled and the shell-less phasis appears 

 in both. The most closely shelled Phyllopod is unmistakably nearer 

 Branchipus even than any of the Cladocera. It would seem that 

 the brief and imperfect embryonic nauplius condition of the latter 

 sufficiently indicated their later origin. Again no fanciful analogy 

 can unite the Ostracoda with the Lynceidae. We know of no 

 recent discoveries casting discredit on the remark of Balfour: "the 

 independent origin of the Ostracoda from the main crustacean 

 stem seems probable." 

 Prof. Packard says:^ 



"We imagine that when a permanent body of fresh water became 

 established, as, for example, in perhaps early Silurian times, the 

 marine forms carried into it in the egg-condition, possibly by birds 



i"A Monograph of the Phyllopod Crustacea,"'etc., Xllth Annual Rep. U. S. Oeol. 

 cmd Oeog. Surv. Terr, 



