232 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



designates as a seasonal dimorphism of one species, R. hebetata, with a 

 winter form, f. hiemale, which changes to a summer one, f. semispina, and 

 vice versa. 



With our present knowledge another interpretation is, I believe, cer- 

 tainly possible, namely, that M. hebetata and R. semispina are two dis- 

 tinct species with separate seasonal (predominant) distribution, and that 

 under the stimulus of certain environmental factors they mutate with a 

 coefficient at present unknown, the one to the other. The fact that the 

 actual change (mutation) from Gran's winter form ( hebetata) to the 

 summer one {semispina) was observed by him in plankton taken in 

 February in the middle of the Arctic winter does not particularly favor 

 the seasonal dimorphism interpretation. 



Mutations in Desmids. 



Desmids eaten by Daphnia and Cypris continue to divide in the diges- 

 tive tract. This has been observed by Cnshman (1904) in the three 

 genera Cosmarium, Euastrum, and Micrasterias. The newly forming 

 semi-cells are, however, unlike the parent cell, being of bizarre and con- 

 torted form, possibly as a result of the stimulus of the secretions of the 

 digestive tract. 



The occurrence in nature in the plankton of Loch Morar of desmids 

 one semi-cell of which has the structure of Xanthidium subhastiferum 

 and the other of its var. murrayi has been noted by W. and G. S. West 

 (1903). This phenomenon is similar to that in diatoms and dinoflagel- 

 lates and presumably originates' in like fashion. 



Mutations in Ciliates. 



In pure cultures of Paramecium caudatum Calkins (1906) found 

 among the daughter cells arising by the early divisions of an excon- 

 jugant a strain of P. aurelia which continued to exhibit the characters 

 of this species, to wit, two micronuclei, and smaller size, for about three 

 months through forty divisions, when the culture reverted to P. caudatum. 

 Calkins is inclined to doubt the validity of a number of the characters 

 previously used to separate the two species and is inclined to combine 

 them as one. In my opinion the phenomenon with which he was deal- 

 ing is similar in all important phases to that in Ceratium, and may be 

 regarded as a process of mutation, occurring in the course of asexual 

 reproduction and producing a weak species, P. aurelia. 



