PACKABD.] GENEALOGY OF CRUSTACEA. 419 



of these Crustacea. There are no marine allies of the Phyllopods. 

 Moreover all the fossil forms appear to have been fresh-water forms, 

 their remains occuring in fresh- water strata. No fossil Phyllopods 

 have occurred as yet previous to the Devonian Period. 



The difQculty is (and this is a point apparently overlooked by Fritz 

 Miiller, Dohrn, Claus, and Balfour) to account for the origination of the 

 Phyllopods at all from any marine forms. The only explanation we can 

 suggest is that the Phyllopods have arisen through Limnetis directly 

 from some originally marine Cladocerous type like the marine forms now 

 existing, such as Evadne. We imagine that when a permanent body of 

 fresh water became established, as, for example, in j)erhaps early Silu- 

 rian times, the marine forms carried into it in the egg-condition, possi- 

 bly by birds or by high winds, hatched young, which, under favorable 

 conditions, changed into Sida, Moina, and Daphnia-like forms. The 

 Cladocera 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. This 

 view, ifc seems to us, accords with the well known facts in the biology 

 and palaeontology of these forms. 



The view which we believe Dohrn entertains, and to which Mr. Bal- 

 four gives some support (though Clans opposes it), that the Ostracoda 

 may have decended from ancestors with a larger number of appendages 

 than they have at present, appears to us to be negatived by the fact 

 that their valves are so abundant in the lowest palaeozoic rocks. The 

 tyj)e appears to have persisted and to have remained unchanged from 

 the Potsdam Period to the present day, and is more marine than fresh 

 water. So close do the lower Cladocera approximate to Cypris that the 

 transformation of an Ostracode^ into a Cladoceran, and a Cladoceran 

 into a Phyllopod, is much more easily imaginable than a hyijothetical 

 Protophyllopod ancestor for the Phyllopods. 



All this clears the way for the view that the Malacostraca had an in- 

 dependent origin from some Nauplius, through, we will admit, some an- 

 cestral Protophyllopod form which was succeeded by a protozoea, and 

 finally a zoea, the ancestor of the existing Decapods as a whole 5 and it 

 also leaves open a field for the independent evolution of the Phyllocari- 

 dan type, composed of gigantic Nebalia-like Silurian forms, which also 

 have originated at a much earlier date than the Decapods, and have 

 held somewhat the same relations to the Decapods as the Eurypterida 

 did to the Limuli. 



In conclusion, therefore, we consider the Phyllopods as a whole, 

 especially the Apodidce and BrancMpodidce, to be a comparatively recent, 

 highly specialized group, which were developed under exceptional bio- 

 logical conditions in bodies of fresh water, and which, as in Apus, show 

 that this branch of the Crustacean genealogical tree has culminated. 

 The irrelative repetition of the segments and appendages (in Apus) 

 gives evidence that the type, so far from being ancestral, is one com- 

 paratively modern, specialized, and fully worked out. 



1 Balfour remarks that "the independent origin of the Ostracoda from the main 

 Crustacean stem seems probable." Page 424. 



