464 



CLADOCERA. 



out in the usual way, and continue to increase in number at successive 

 moults : the shell is rapidly developed. The chief point of interest in 

 the development of this form is the close resemblance of the young larva 

 to a typical adult Cladocera (Claus). This is shewn in the form of the 

 shell, which has not reached its full anterior extension, the rudimentary 

 anterior antennae, the large locomotor second pair of antennae, which differ 

 however from the corresponding organs in the Cladocera in the presence 

 of typical larval hooks. Even the abdomen resembles that of Daphnia. 

 These features perhaps indicate that the Cladocera are to be derived 

 from some Phyllopod form like Estheria by a process of retrogressive 

 metamorphosis. The posterior antennas in the adult Estheria are large 

 biramous appendages, and are used for swimming ; and though they 

 have lost the embryonic hook, they still retain to a larger extent than 

 in other Phyllopod families their Nauplius characteristics. 



The Nauplius form of the Phyllopods is marked by several 

 definite peculiarities. Its body is distinctly divided into a ceph- 

 alic and post-cephalic region. The upper lip is extraordinarily 

 large, relatively very much more so than at the later stages. 

 The first pair of antennae is usually rudimentary and sometimes 

 even absent ; while the second pair is exceptionally large, and 

 would seem to be capable of functioning not only as a swimming 

 organ, but even as a masticating organ. A dorsal shield is 

 nearly or quite absent. 



Cladocera. The probable derivation of the Cladocera from a form 

 similar to Estheria has already been mentioned, and it might have been 

 anticipated that the deve- 

 lopment would be similar 

 to that of the Phyllopods. 

 The development of the ma- 

 jority of the Cladocera takes 

 place however in the egg, 

 and the young when hatched 

 closely resembles their pa- 

 rents,though in the egg they 

 pass through a Nauplius 

 stage (Dohrn). An excep- 

 tion to the general rule is 

 however offered by the case 

 of the winter eggs of Lepto- 

 dora, one of the most primi- 

 tive of the Cladoceran 

 families. The summer eggs 

 develop without metamor- 

 phosis, but Sars (No. 461) 



an 1 



FIG. 209 A. NAUPLIUS LARVA OF LEPTODORA 

 HYALINA FROM WINTER EGG. (Copied from Bronn ; 

 after Sars.) 



an 1 , antenna of first pair; an 2 , antenna of 

 second pair ; md. mandible ; f. caudal fork. 



