232 



AN INTRODUCTION TO ZOOLOGY 



grows into the Zo<za stage (Fig. 127, A). The cephalothorax 

 and abdomen are distinct at this time ; eight pairs of appendages are 

 present (I-VIII) and six more are developing (ai-fle). TheZoaea 

 grows and molts and becomes a My sis (Fig. 127, B) with eight 

 pairs of appendages (I-VIII) on the cephalothorax. Finally the 

 Mysis passes into the adult shrimp, which possesses the 

 characteristic number of appendages (I-XIX) each modified 

 to perform its particular function. The Nauplius of Pen&us 

 resembles the larvae of many simple crustaceans; the Zoaa is 

 somewhat similar to the condition of an adult Cyclops (Fig. 119); 

 the Mysis is like the adult Mysis (Fig. 125) ; and finally the adult 

 Penceus is more specialized than any of its larval stages, and be- 

 longs among the higher Crustacea. The above facts have con- 

 vinced some zoologists that Pen&us recapitulates in its larval 

 development the progress of the race; that the lobster has lost 

 many of these stages, retaining only the Mysis; and that the cray- 

 fish hatches in practically the adult condition. Tho Nauplius 

 stage of the latter is supposed to be represented by a certain 

 embryonic phase (Fig. 114). 



The law of biogenesis should not be taken too seriously, since 

 it has been criticized severely by many prominent- zoologists, 

 but it has furnished an hypothesis, which has concentrated the 

 attention of scientists upon fundamental embryological processes, 

 and has therefore had a great influence upon zoological progress 

 (175, 177, 187). 



