LEPTODORA HYALINA. 17 



higher point in the Ostracoda, such as Cypris, in which also the bivalve 

 shell reaches its highest development. 



The absence of brauchial lamelte ou the legs in Leptodora is also to 

 be correlated with the rudimentary condition of the carapace ; these 

 lamellaa, which attain their greatest development in Lynceidao and 

 Daphniada3, in which also the carapace is most fully developed, probably 

 serve to maintain a current of water through the shell, rather than to 

 act themselves as direct respiratory organs, and they are therefore need- 

 less when the shell is absent or rudimentary. 



From these arguments Weismann concludes that Leptodora is the 

 most primitive form of Cladocera wtli which we are acquainted — a 

 conclusion supported in a remarkable manner by what httle is known of 

 its development. 



Leptodora, Uke Cladocera in general, appears to foi-m two kinds of 

 ova, which may be distinguished as %vinter ova and summer ova. The 

 two kinds of ova differ markedly in appearance and in their subsequent 

 mode of development, though both kinds may occur at one time in the 

 ovary of a single female. The difference between the two appears to 

 depend mainly on the winter ova being fertihsed while the summer ova 

 develope parthenogenetically. This, however, is not definitely proved ; all 

 that is known is that males occur very rarely or not at all until late in the 

 autumn, and that their occurrence is apparently coincident vdth. the 

 development by the females of winter ova. From the summer ova are 

 developed young which undergo no marked ruetamorphosis, and resemble 

 their parents at the time of leaving the egg. The winter ova, on the 

 other hand, produce embxyos which when hatched have the foi*m known 

 as " NaupUus," and only attain the form of their parents after a long and 

 gradual series of metamorphoses, accompanying the successive moultings 

 of their skin. 



The occurrence of this NaupUus larva is of very great interest. A 

 Nauphus is characterised by possessing only three pairs of appendages 

 (con-esponding to the two pairs of antennas and the pair of mandibles of 

 the adult), a rudimentary unsegmented abdomen, and a single median 

 eye instead of the double eye of the adult. Nauphus larvae are vei-y 

 general among the Entomostraca, but till the Ufe-history of Leptodora 

 was worked out by Sars in 1873 the Cladocera were supposed to stand 

 alone among Entomostraca in never passing through the NaupUus stage. 

 Leptodora serves in this respect to connect the Cladocera with the other 

 Entomostraca, and in this way fully proves its title to rank as one of the 

 most primitive forms of Cladocera known. 



The NaupUus larvae of different Crustacea generaUy have some 

 resemblances to the adult fonns, and this is particularly weU shown by 

 the NaupUus of Leptodora, in which, while the anterior antennse are 

 very smaU, the posterior antennae are very large indeed, thus fore- 

 shadowing their enormous size in the adult. As these early 

 stages are of very great interest and importance, figm-es of some of 



