448 Prof. K. Grobben on the Genealogy and 



double eye by a redupHcature of the skin. Now when, in the 

 case of the Cladocera, in the development of the compound 

 eye we find conditions which can only be understood on the 

 theory of an original roofing-over of the head by the shell, 

 these conditions appear as a character which has become 

 established by inheritance, and belonged to an ancestral form 

 whose shell enclosed the head at tlie sides, and was conse- 

 quently developed in a similar manner to that which we find 

 in the Estheridse. 



From all the reasons which have been adduced I can only 

 decide in favour of the view that young forms of Estheridae, in 

 wliich tlie head was still unobscured by the shell, were the 

 ancestors from whicli the Cladocera were developed ; the pecu- 

 liarities of the Cladocera which were previously mentioned 

 are best understood on this assumption. A possible objection 

 must, however, be considered ; for if the overgrowth and 

 fusion of the compound eyes already makes its appearance in 

 £sthe7na-]a\vsd, in which the head still projects freely from 

 the shell, this fact can only be regarded as a disturbance of 

 the sequence of events in the ontogeny, but not as a proof of 

 the view that the fusion and overgrowth of the eyes have 

 arisen independently of the encasement of the head by the 

 shell. But just as little can it serve as an argument against 

 the theory which I have represented above, that the fusion 

 and overgrowth of the eyes have arisen in the phylogeny only 

 in consequence of the covering of the head by the shell, and 

 therefore after and not before this. 



Finally, I will quote the view expressed by Balfour* as to 

 the origin of the Cladocera, according to which " the Clado- 

 cera have arisen from some Phyllopod form resembling 

 Estheria by a process of regressive metamorphosis." 



Since the Cladocera possess such an extensive structural 

 agreement with the Estheridas, that is, in the first instance, 

 with the young stages of the latter, they are to be regarded as 

 a very young branch of the Crustacea which have only lately 

 split off from Estheridae, such as we see them represented at 

 the present time, and have become adapted to the pelagic 

 mode of life. Lastly, I derive a similar conception on the 

 part of Claus t from the genealogical tree of the Entomo- 

 straca which this investigator has set up, in which no special 

 branch is shown for the Cladocera, which are supposed to be 

 included in the Phyllopod group. 



* F. M. Balfour, ' A Treatise on Comparative Embryology,' German 

 edition, i. Bd., 1880, p. 438. 



t C. Claus, " Neue Beitriige zur Morphologie der Crustaceen," Arbeiten 

 desZool. Institutes zu Wien, Bd. vi., 1885, p. 105. 



