286 TEXTBOOK OP ZOOLOGY 



and resembles very closely the adult Cyclops of modern Copepoda. 

 The Zoea transfonns during further molts and growth to a stage with 

 thirteen segments and a distinct cephalothorax which resembles the 

 adult Mysis and therefore is called the Mysis stage. Gammarus is 

 also in about this category of phylogenetical development. Follow- 

 ing the next molt the mysis stage becomes a juvenile shrimp with 

 nineteen segments. The life history of the barnacles and Sacculina 

 has illustrated quite forcibly the possibility of such a relationship. 

 There are extinct forms also whose adult condition was that of one 

 of these developmental stages. This idea generally has served as 

 a great stimulus to the study of embryology and the theory of 

 evolution as well as serving to establish natural relationship of 

 animal groups. 



Phylogenetic Advances of Arthropoda 



(1) Greater specialization of segments, (2) paired, jointed ap- 

 pendages, (3) chitinous exoskeleton, (4) gill and tracheal respira- 

 tion, (5) dioecious reproduction, (6) development of eyes and 

 other sense organs, (7) green glands and malpighian tubules (in- 

 sects) as excretory organs, (8) organization of social life. 



