374 CRUSTACEA ENTOMOSTRACA. 



interesting researches, undertaken with the purpose of finding the answer 

 to the question, " On what does the onset of the sexual period in the life- 

 cycle of the Daphnidae depend ? " and has arrived at results differing 

 considerably from those of Weismann. He finds that when the partheno- 

 genetically produced offspring of one parent (Simocephalus vetulus O.F.M.) 

 are divided into two batches, and, while kept at a warm temperature, are 

 submitted, the one to conditions of abundant nourishment, the other to a 

 starvation diet, those in the first batch continue to reproduce females by 

 parthenogenesis, while those in the second produce males, or winter eggs. 

 The second result is also produced by cold, even when food is abundant, 

 but Issakowitsch is inclined to attribute it to the falling off in nutrition 

 resulting from the lower rate of metabolism. He concludes that "nourish- 

 ment and temperature (the latter by its influence on nourishment) are the 

 determining factors for the occurrence or disappearance of the sexual in- 

 dividuals ; and that there is no cyclical succession of generations in the 

 Daphnidae." 



The conditions under which males are produced, in the above 

 named Phyllopod genera, are more obscure. The presence of 

 males in a colony of the genus Apus is of rare occurrence. Thus 

 an examination of the successive generations of A. cancriformis 

 inhabiting pools in certain clay pits, carried on for six consecutive 

 years failed to reveal to von Siebold a single male among 8,521 

 specimens which he examined. Colonies are however occasionally 

 found in which males are present, though always in smaller 

 numbers than the females. In some species of Artemia males 

 are also of rare occurrence, while in others they are plentiful. 

 The males of Limnadia Hermanni Brongn. are unknown. 



The females usually carry the eggs about with them on ap- 

 pendages specially modified for this purpose (Apus}, or in a 

 brood pouch beneath the shell on the dorsal surface (Daphnidae), 

 or in ventral marsupia (Branchipus). 



In the Phyllopoda the young leaves the egg as a somewhat 

 modified nauplius larva, with three pairs of appendages (the 

 mandibular palp, which is absent in the adult, being at this 

 stage well developed) ; and the mature condition is reached by 

 a, complicated metamorphosis. In the Cladocera, on the other 

 hand, whose large eggs contain abundant yolk and are protected 

 in the brood pouch of the mother, the young are hatched in the 

 'orm of the adult, though passing through a nauplius stage 

 within the egg. That remarkable and aberrant form Leptodora 

 offers however an exception to this rule in that while the summer 

 eggs develop in this manner, the winter eggs (which are 

 fertilized) hatch out as nauplius larvae (Sars.) 



