ORDER CLADOCERA. 
This very extensive group contains a variety of types, but there 
are sufficiently evident connectlng 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 ztio- 
logical speculation that this the only truly marine group should stand 
thus isolated from its fellows. According to the notions at present 
prevailing, the Phyllopods 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 Cladocera and 
Copepoda. 
There has, however, recently been made an attempt to derive the 
Phyllopods 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 Lynceide. 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?] or by high winds, hatched young, which, under favorable con- 
ditions, changed into Sida, Moina and Daphnia-like forms. The Clad- 
ocera are, then, probably the more generalized forms, from which the 
Phyllopods, at this time, and probably ever since Devonian times, par 
excellence a fresh-water assemblage of forms, took their origin. What- 
ever affinity there may be between the shelled Phyllopods and the 
2 *“A Monograph of the Phyllopod Crustacea,” ete., X7/ih Annual Rep. U.S. Geol. and Geog. Surv. 
err. 
