49 



giveD off at different points. It ends in the existing Phyllo- 

 poda, the nearly allied Cladocera being shown as a slightly 

 divergent side branch. 



The Copepoda, the Cirripedia, and the Ostracoda have all 

 probably arisen independently from the ancestral stem, and 

 have become considerably modified in different directions. 

 The Copepoda are probably the oldest group. They retain 

 the median Nauplius eye and have simple biramous append- 

 ages. A number of the Copepoda are certainly degenerate 

 forms, and some of them are very much modified. 



The Cirripedia probably arose further up the main 

 stem, and in the ancestral condition were characterised 

 by the possession of a large carapace, in the form of a 

 bivalve shell. We see the remains of this stage in the 

 " Cypris " larva of existing Cirripedes. The group has 

 undergone very great modifications since, due doubtless to 

 the change from a free-swimming to a sessile existence. A 

 considerable amount of degeneration has also taken place 

 amongst the Cirripedia. 



The Ostracoda probably arose from a point still further 

 up in the series of the Proto-Phyllopods, and have under- 

 gone a good deal of modification. A bivalve shell has been 

 developed to such an extent as to enclose the entire body. 

 In some members of the group degeneration resulting in the 

 loss of the heart and the compound eyes has taken place. 



The higher Crustacea have arisen from the main axis 

 of the group, below the point of separation of the Cladocera 

 from the Phyllopoda, but probably not much further back, 

 and the NebaliadaB are, according to Claus,* the nearest 

 forms we know to the primitive Malacostraca. Possibly after 

 this, as suggested by Balfour, t the thoracic appendage 



* Ueber den Bau und die systematische Stellung von Nehalia. Zeitsch. f. 



Wiss., Zool, Bd. xxii (1872). 



+ See Balfour, Gomp. EmhryoL, vol. i, p. 420. 



E 



