5 2 



PHYLOGENY OF THE CRUSTACEA. 



four-jointed ambulatory appendages terminating in a hook. The caudal fork 

 becomes more distinct. 



In the sixth stage (fig. 236) the second and hindermost pair of feet be- 

 comes formed (/'") and the maxillae of the second pair lose their ambulatory 

 function, and begin to be converted into definite masticatory appendages by 

 the reduced jointing of their palp, and the increase of their cutting blades. 

 By the seventh stage all the appendages have practically attained their 



Fn 



FIG. 236. SIXTH STAGE IN THE DEVELOPMENT OF CYPRIS. (From Claus.) 

 MX' , first maxilla ; MX" . f . second maxilla ; /". and f". first and second pair ot 

 feet ; Fit. caudal fork ; L. liver ; S.D. shell gland. 



permanent form ; the second pair of maxillae has acquired small branchial 

 plates, and the two following feet have become jointed. In the eighth and 

 ninth stages the generative organs attain their mature form. 



The larva of Cythere at the time of birth has rudiments of all the limbs, 

 but the mandibular palp still functions as a limb, and the three feet (2nd 

 pair of maxillae and two following appendages) are very rudimentary. 



The larvae of Cypridina are hatched in a condition which to all intents 

 and purposes resembles the adult. 



PJiylogcny of the Crustacea. 



The classical work of Fritz Mu'ller (No. 452) on the phylogeny of the 

 Crustacea has given a great impetus to the study of their larval forms, and 

 the interpretations of these forms which he has offered have been the subject 

 of a very large amount of criticism and discussion. A great step forward 

 in this discussion has been recently made by Claus in his memoir (No. 448). 



The most fundamental question concerns the meaning of the Nauplius. 

 Is the Nauplius the ancestral form of the Crustacea, as is believed by Fritz 

 Miiller and Claus, or are its peculiarities and constant occurrence due to 

 some other cause ? The most plausible explanation on the second hypothesis 



