ORDER CLADOCERA. 



This very extensive group 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 suffic- 

 iently evident cladoceran affinities. It is very unfortunate for setio- 

 logical speculation that this the only truly marine group should stand 

 thus isolated from its fellows. According to the notions at present 

 prevailing, the PhyUopods stand nearest the primitive type of Crusta- 

 cea. There are unmistakable hints at an early origin for that group, 

 and not less evident are certain analogies with both (JIadocera and 

 Copejwda. 



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

 PhyUopods fiom an original cladoceran stem with, as we think, some- 

 what unsatisfactory results. Do we not rather see in both groups two 

 like phases which may be looked upon as incidental and comparatively 

 trivial "? The shelled and the shell-less phases appear in both. The 

 most closely shelled Phyllopod is unmistakably nearer Branchipus even 

 than any of the Cladocera. It would seem that the brief and imper- 

 fect embryonic nauplius condition of the latter sufficiently indicated 

 their later origin. Again, no fanciful analogy can unite the Ostracoda 

 with the Lynceidce. We know of no recent discoveries casting dis- 

 credit on the remark of Balfour: "The independent origin of the 

 Ostracoda from the main crustacean stem seems probable." 



Professor 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 

 [sic 7] or by high winds, hatched young, which, under fovorable con- 

 ditions, changed into Slda, Moina and I)aphnia-\\Ve forms. The Clad- 

 ocera are, then, probably the more generalized forms, from which the 

 PhyUopods, at this time, and probably ever since Devonian times, iwr 

 excellence a fresh-water assemblage of forms, took their origin. What- 

 ever affinity there may be between the shelled PhyUopods and the 



*"A Monograph of the Phyllopod Crustacea," etc., A'll/h Anntial Rep. U. S. Geol. and Geog. Surv . 

 Terr. 



