414 ESSENTIALS OF ZOOLOGY 



lated to ancestral conditions. That is, the individual in its develop- 

 ment tends to repeat in an abbreviated fashion the history of the 

 development of the race. Briefly stated ontogeny recapitulates 'phy- 

 togeny. There is still some doubt as to the validity of this generali- 

 zation in direct application. 



A classical example Avhich is frequently cited is that of the devel- 

 opment of the shrimp, Penaeus, which hatches out as a nauplius larva, 

 having a single median eye and only three pairs of appendages. 

 Following the molt, this nauplius changes to become the Prozoea 

 stage, possessing six pairs of a])pendages. The next molt brings on 

 segmentation and some change in form. This stage is called the Zoea 

 and resembles very closely the adult Cyclops of modern Copepoda. 

 The Zoea transforms 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 ^lalpighian tubules (in- 

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



References 



Greaser, E. P.: Decapod Crustaeeans of Wisconsin, 1932, Tr. Wisconsin Acad. 



Sc, Arts, and Letters, No. 27. 

 Hay, W. P.: Crustacea of Indiana, ISOH, Proc. Indiana Acad. Sc. 

 Hegner, R. W. : Invertebrate Zoology, New York, 193.'}, The Macmillan Company. 

 Ward, H. B., and Whipple, Geo. C. : Fresh- Water Biology, New York, 1918, John 



Wiley & Sons, Inc. 



