1898] REPRODUCTION OF THE ROTIFERA 49 



and similar organisms which swarm in such situations. It is thus 

 very liable to be killed by drought, since it does not possess the 

 power of withstanding drying exhibited to a marked degree by 

 certain rotifers. But the rise of temperature preceding and causing 

 the drying up of the water may not only act directly on the 

 Hydatina as Maupas believes, determining the appearance of males 

 and of resting-eggs, but it also brings into play the factor emphasised 

 by Nussbaum by diminishing the supply of food, for the Euglenae 

 encyst themselves and become aggregated into masses, in which 

 state they cannot be ingested by the rotifers. Lauterborn considers 

 that in Hydatina, and probably also in other species of similar 

 habits, the power of ready response to changes in these two factors 

 of its environment is a special adaptation ensuring the production 

 of resting-eggs when the colony is threatened with extermination by 

 the drying up of its habitat. It has already been pointed out that 

 in the Philodinidae, some of which live in a minimal quantity of 

 water among damp moss or earth, the same end has been reached in 

 another way, the adult animals becoming encysted in an envelope of 

 hardened mucus, and in this state exhibiting an almost incredible 

 power of resistance to prolonged drying. 



Lauterborn finds that the pelagic Botifera may be divided into 

 three categories : — 



(1) Perennial forms, which occur in greater or less abun- 



dance all the year round. 



(2) Summer forms, which are found only in the summer 



months. 



(3) Winter forms, including a few species whose occurrence 



is limited to the colder months of the year. 

 The species belonging to the last two classes he finds to be 

 strictly ' monocyclic,' in the sense in which Weismann has applied 

 that term among the Cladocera ; that is to say, the sexual period 

 occurs only once a year, in autumn in the case of summer forms, in 

 spring in the winter forms, when resting-eggs are produced to tide 

 over the unfavourable seasons of winter and summer respectively. 

 The 'perennial' species, on the other hand, are di- or poly- cyclic, the 

 sexual period occurring at least twice in the year, in spring and in 

 autumn, but without any interruption of the process of partheno- 

 genetic reproduction, which goes on all the year round, so that the 

 species never disappears from the gatherings. It is pointed out 

 that those species which, in the pelagic fauna of lakes, show this 

 perennial character, are for the most part forms known to occur also 

 in smaller ponds and pools, while the monocyclic summer and winter 

 forms are largely truly pelagic (limnetic) species inhabiting the open 

 waters of larger ponds and lakes. These results show considerable 

 agreement with those obtained by Weismann among the Cladocera. 



n 



