384 



CLADOCERA. 



swimming appendages the second pair of antenna? and mandibles. The 

 first pair of antenna? has not been detected, and a dorsal mantle to form 

 the shell is not developed. At the first moult the anterior pair of 

 antenna? arises as small stump-like structures, and a small dorsal shield 

 is also formed. Rudiments of six or seven pairs of appendages sprout 

 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 loconiotor 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 antenna? in the adiilt 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 cephalic and post- 

 cephalic region. The upper lip is extraordinarily large, relatively 

 very much more so than at the later stages. The first pair of 

 antennas 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 masti- 

 cating 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 de- 

 velopment 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 resemble their parents, 

 though in the egg they pass 

 through a Nauplius stage 

 (Dohrn). An. exception to 

 the general rule is however 

 offered by the case of the 

 winter eggs of Leptodora, one 

 of the most primitive of the 

 Clacloceran families. The 

 summer eggs develop without 

 metamorphosis, but Sars (No. 

 461) has discovered that the 

 larva leaves the winter eggs 



an ' 



FIG. 209 A. NAUPLIUS LARVA OF LEPTODORA 

 HYALINA FROM WINTER EGG. (Copied from Bronn ; 

 after Sars.) 



aw 1 , antenna of first pair; an-, antenna of 

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



