LEPTODORA HYALINA. 51 



From tliese arguments Weismann concludes that Leptoclora is the 

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

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

 its development. 



Leptodora, like Cladocera in general, appears to form two kinds of 

 ova, which may be distinguished as winter 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 fertilised while the summer ova 

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

 that is known is that males occur very rai'ely or not at all until late in the 

 autumn, and that then: occurrence is apparently coincident with the 

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

 developed young which undergo no marked metamorphosis, and resemble 

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

 other hand, produce embryos which when hatched have the form known 

 as Nauplius, 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 Nauplius larva is of very great interest. A 

 Nauplius is characterised by possessing only three pairs of appendages 

 (corresponding to the two pairs of antennae 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. Nauplius larva? are very 

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

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

 alone among Entomostraca in never passing through the Nauplius 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 Nauplius larvae of different Crustacea generally have some 

 resemblances to the adult forms, and this is particularly well shown by 

 the Nauplius of Leptodora, in which, while the anterior antennae are 

 very small, the posterior antennas are very large indeed, thus fore- 

 shadowing their enormous size in the adult. 



As these early stages are of very great interest and importance, 

 figures of some of the most important larval stages of Leptodora 

 accompany this paper, in the hope that they may facilitate the 

 recognition of the forms when actually met with. 



The winter ova, it appears, give rise in March or April to larvas 

 which hatch in the form of Nauplii ; these moult several times, and 

 gradually acquire the form cf the adult. This spring brood maybe always 

 recognised by the fact that they retain even when adult the median eye 

 of the Nauplius, in addition, to the large double compound eye of the 

 adult; the larval eye persisting as a small black spot on the under 

 surface of the brain* The spring brood gives rise to females only ; these 



