446 Prof. K. Grobben on ike Genealogy and 



elonp:atccl form of the foliaceous foot is rediscovcrablc among 

 the Ch\docera in genera such as Sidn^ which in all charac- 

 teristics prove to be the most primitive. Lastly, mention 

 must be made of the compound eyes, which, as in Estheria^ 

 meet together in the median plane to form a double eye, and, 

 as in the genus referred to, are surrounded by a reduplicature 

 of the skin, with the slight difference that the chamber which 

 is formed above the eye by the overgrowth is in the case of 

 the Cladocera completely closed *. 



Clans has also suggested the two possibilities that the 

 Cladocera are to be derived from larval forms of the Estheridai 

 or from a common ancestor with the latter, Avithout, however, 

 pursuing this question further. The passage referred to in 

 Clans runs as follows : — " For my part there is no question 

 of the fact that they [namely the Cladocera] are to be brought 

 into closer relationship with the larval forms of the Estheridae, 

 and are to be derived, if not from these, at any rate from a 

 common older ancestral form." 



In my opinion this question may be answered with some 

 degree of certainty by the theory that the Cladocera are to be 

 derived from young stages of the Estheridai. 



The reasons to be adduced in favour of this are the follow- 

 ing. In the first place the small number of body-segments 

 in the Cladocera, a character which cannot be regarded as a 

 primitive one, since extensive segmentation of the body must 

 be assumed to have existed in old forms of Annulosa, and in 

 the present case is easily to be proved by tiie fact tliat the 

 forms Branchipus and Apus allied to Estheria, which is so 

 close to the Cladocera, exiiibit the same peculiarity f. Con- 



* Grobben, he. cii. 



t There is probably no need to make especial mention of the fact that 

 the number of the body-segments in the Euphyllopods is usually not so 

 great as to necessitate our thinking of a secondan/ multiplication of the 

 body-segments, of Avhich instances are indeed found in the auitnal 

 kingdom. The large number of appendages in the case of Apus is, in my 

 opinion, to be explained by the theory that the appendages themselves 

 have multiplied within the Umits of the segment. But should the 

 number of the appendages of this form actually correspond -with the 

 number of the body-segments which have coalesced to form a few larger 

 annuli, the large number of body-segments which in this case we should 

 have to recognize in Aims would have to be regarded as having been 

 secondarily augmented. 1 would add merely incidentally that I cannot 

 accept the multiplication of the ventral ganglia in Apus, which keeps 

 pace with the increase in the number of the legs, as a proof that tne 

 body-rings of Apus are to be regarded as complexes of metameres. 



llut also supposing that the body-rings of Apus determine the number 

 of the metameres, with regard to the increase in this number in many 

 species of Apus {e. (j. to about 45 in Aims Lucasanus, Pack.), a secondary 

 multiplication of the body-segments would have to be taken into conside- 

 ration, at least in the case of the more richly segmented species. 



