Classijication of the Crustacea. 451 



The idea that the hxteral shoot formed by the Ostracoda 

 branched off deep down from the Phyllopod stem also finds 

 expression in the genealogical tree of the Entomostraca 

 which Claus lias set up *. The close affinity between the 

 Ostracoda and the slicll-bearing Phyllopods in particular has 

 likewise been frequently alluded to, as, for instance, by 

 Dohrn t and Claus %, as well as by Korschelt and lleider§. 



The question as to whether the ancestral forms of the 

 Ostracoda possessed a body composed of numerous segments 

 is to be answered in the affirmative from the standpoint which 

 has already been advanced, that an extensive segmentation of 

 the body is to be assumed also for the old forms of Phyllo- 

 pods, as being a phylogenetically older condition. In the 

 Ostracoda we have to deal with a group of Crustacea which 

 has proceeded from richly segmented ancient Phyllopods, of 

 the habitus of the Estheridse, by the process of loss of segments 

 of the body. 



The Copepoda. 



There is no other section of Crustacea in which the entire 

 development of the body has undergone such manifold modi- 

 fications in accordance with the different mode of life as in the 

 case of the present group. To enter into all these modifica- 

 tions not only lies outside the task which we have imposed 

 upon ourselves, but also would in no way contribute towards 

 answering the question which has been propounded. Here, 

 as before, only those Copepoda which prove to be phylogene- 

 tically the oldest forms need be considered, and tliese arc the 

 Branchiura [Arguhis), and among the Eucopepoda the Cala- 

 nid» (among which Cetochilus possesses the most primitive 

 characters). 



In respect of morphology the Branchiura are an extremely 

 interesting group. The forms at present existing, which are 

 represented only by two genera and few species, are parasitic 

 as regards their nutrition, but liave nevertheless retained the 

 faculty of free locomotion. To the last-mentioned circum- 

 stance is probably also to be ascribed the retention of old 

 characters. 



That the Branchiura are to be assigned to the Copepoda, 



• Claus, *Neue Beitrage zur Morphologie der Crustaceen,' p. 105. 



t Dohrn, * Geschichte des Krebstammes,' pp. 133 and 149. 



X Claus, ' Untersuchung zur Erforschung der genealogiscben Grund- 

 lage des Crustaceensystems,' p. 97. 



§ E. Korschelt and K. Ileider, * Lehrbuch der vergleichenden Entwick- 

 lungsgeschiehte der wirbellosen Thiere.' Specieller Theil. ii. Heft, Jena, 

 1891, p. 500. 



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