220 Merkel Henry Jacobs 



The method of reproduction of Philodina is of interest. There 

 are two ovaries, one on each side of the ahmentary canal, and each 

 is combined with a large vitellarium. The eggs undergo a partial 

 development within the body of the mother and at the time of 

 laying are relatively large and are protected by fairly thick shells. 

 The point of special interest in connection with the reproduction 

 of Philodina is that there are no male individuals; this is also true 

 of the other members of the group Bdelloida. Although these 

 animals are among the commonest of our rotifers and have been 

 under observation for over two hundred years, males have never 

 been observed, and it seems reasonably certain that they do not 

 exist. Parthenogenesis appears to be the only method of repro- 

 duction. 



The eggs hatch two or three days after being laid and the young 

 from the first behave exactly like the adults which they closely 

 resemble in most respects. They seem to be somewhat less resist- 

 ant to desiccation, but there is much individual variation and the 

 difference is not very great in any event. They feed freely and 

 grow rapidly; egg laying may begin seven or eight days after 

 hatching at which time they have not yet reached their full size. 



As to the length of hfe of Philodina under favorable conditions, 

 unfortunately nothing definite can be said except that it is con- 

 siderably longer than has been supposed in the case of other roti- 

 fers. Hydatina is said to Hve about two weeks; in the course of 

 these experiments rotifers isolated at the time of hatching have 

 been kept alive for more than six weeks, and under more favorable 

 conditions they would probably have lived still longer. 



IV THE BEHAVIOR OF PHILODINA 



/ Behavior under Ordinary Conditions 



The behavior of Philodina is more complicated than that of 

 most rotifers owing to the two different modes of locomotion. 

 Ordinarily the animals are found in one of the following four states, 

 (i) swimming, (2) creeping, (3) attached by the foot and feeding 

 or (4) contracted. The causes which determine the succession 



