[From the American Journal of Science and Arts, Vol. IX. March, 1875.] 



19. Dimorphic Development and Alternation of Generation in 

 the Cladocera. — Dr. G. O. Saes has discovered a remarkable di- 

 morphism and alternation of generation in Leptodora hyalina. 

 (Om en dimorph Udvikling samt Generationsvexel hos Leptodora, 

 Forhandlinger Vidensk.-Selsk., Christiania, for 1873, p. 15, and 

 plate.) The development from the ordinary snmmer-eggs, as al-' 

 ready described by E. P. Miiller, is without metamorphosis and 

 like that of ordinary Cladocera, the young when excluded from 

 the egg agreeing essentially with the adult ; while, according to 

 Sars' observations, the young are excluded from the winter-eggs 

 in a very imperfect condition, quite unlike the known young of 

 any other Cladocera, and pass through a marked post-embryonal 

 metamorphosis. In the earliest observed stage of the young of 

 this form, the body is obovate, wholly without segmentation, the 

 compound eye wanting, while there is a simple eye between the 

 bases of the antennuke, the swimming arms (antennas) well devel- 

 oped, and the six pairs of legs represented only by minute pro- 

 cesses projecting scarcely beyond the sides of the body; but the 

 most remarkable feature is the presence of a pair of appendages 

 tipped with cilia and nearly as long as the body, which are evi- 

 dently homologous with the mandibular palpi of other Crusta- 

 ceans, although these appendages have always been supposed to 

 be wanting in the species of Cladocera. Two subsequent stages, 

 gradually approaching the adult form, are described. The adults 

 from the winter-eggs have no vestige of the mandibular palpi left, 

 yet the simple eye — which is wholly absent in ordinary individuals 

 developed from summer-eggs — is persistent, and thus marks a dis- 

 tinct generation. Three stages of the ycung from winter-eggs 

 are beautifully figured upon the plate accompanying the riiemoir. 



This remarkable species has, still more recently, been made the 

 subject of a very elaborate memoir by Prof. Weismann of Frei- 

 burg (liber Ban unci Lebenserscheinungen von Leptodora hyalina, 

 Zeitschrift fiir wissensch. Zool., xxiv, Sept., 1874, pp. 349-418, 

 plates 33-38), who, however, had not observed the peculiar devel- 

 opment of the winter-eggs. The occurrence of this genus in Lake 

 Superior is noticed in this Journal, vol. vii, p. 161, 1874. 



s. i. s. 



20. Development of the European Lobster. — Dr. Saes has also 

 recently published, in the Proceedings of the same Society for 

 1874, a paper of 27 pages, illustrated by two autographic plates, 



