502 



SCIENCE 



[N. S. Vol. XXV. No. 639 



admitted by Biitschli in his Thierreich 

 monograpli. The species which Dr. Volk lists 

 from the Elbe near Hamburg exhibit a high 

 degree of coincident distribution. His data 

 indicate that about 85 per cent, of the re- 

 corded species in the genera named are found 

 in the same habitats and that 80 per cent, of 

 these were taken at some time in the period 

 named on the same day. Within the genua 

 Vorticella, represented by eighteen species, the 

 average percentage of coincident occurrence 

 on the same day in all the collections was 50 

 per cent.; in Stentor, 75 per cent. Among 

 these instances of coincident distribution of 

 the species of a genus are numerous cases of 

 the coexistence of the most closely related 

 species in the same habitat. 



The fresh-water Eotifera are notoriously 

 cosmopolitan in distribution. About 92 per 

 cent, of the 246 species reported by Professor 

 Jennings from the United States are found 

 on other continents. The species of the group 

 exhibit in common with the fauna and flora 

 with which they are associated, certain well- 

 defined restrictions in distribution, due to en- 

 vironmental factors combined in various types 

 of habitat, such as open water, and shore vege- 

 tation. The genus Brachionus is found pre- 

 dominantly in open water of warm shallow 

 ponds and streams where it occurs in enormous 

 numbers during a large part of the year. 

 After eliminating synonyms arising in large 

 part from failure to examine literature or to 

 ascertain the seasonal form-cycle, there remain 

 at least about twelve common forms of specific 

 value in this genus along with a considerable 

 number of so-called variants. One of these 

 species, B. dorcas, is predominantly a winter 

 species, and another B. militaris, predomi- 

 nantly a shore and vegetation-loving species. 

 The remaining forms are all typical members 

 of the limnetic fauna of warm shallow waters 

 and usually together throughout a large part 

 of the summer, in many parts of the world. 

 In common with practically all other organ- 

 isms of the limnetic faima and flora these 

 species of Brachionus run a rhythmical course 

 of fluctuating recurrent cycles of increase and 

 decrease in numbers of three to six weeks' 

 duration. A single instance drawn from the 



records of the Illinois State Laboratory of 

 Natural History of my unpublished statistical 

 work upon the plankton of the Illinois River 

 will suffice to illustrate the character of these 

 rhythmical fluctuations and the relations 

 which they bear to the breeding periods of 

 Brachionus, During the rise of the ' pulse,"^ 

 parthenogenesis prevails, and at its maximum 

 and during its decline sexual reproduction ap- 

 pears as indicated by the presence of male 

 eggs, males and winter eggs. The imper- 

 fections of the plankton method, the difficulty 

 of specific identification of male rotifers and 

 detached eggs introduce a considerable degree 

 of incompleteness in these statistical records. 

 The trend of the statistics remains, however,, 

 significant. 



BKACHIONUS ANGULARIS. — NUMBEK PER CUBIC METEE 

 ILLINOIS EIVEK, 1896, 



Within the breeding period of B. angularis 

 covered by this table, Brachionus pala, B. 

 amphiceros, B. hidens and B. hakeri were also 

 found with male or winter eggs or both, and 

 B. cluniorhicularisj B. melhelmi, B. militaris,. 

 B. mollis, B. punctatus, B. rhenanus, B. rubens 

 and B. tuberculus ran a like cycle of smaller 

 amplitude, but no males or winter eggs were- 

 found attached to the individuals counted. 

 No effort was made during the collection of 

 these statistics to determine whether these 

 remaining species were breeding sexually or 

 not It is quite possible that they were. The 

 list includes a number of instances of most 

 closely related couplets. Opinions differ as to 

 the specific, subspecific or varietal standing 

 of some of the forms in the list. 



We have then in Brachionus coincident dis- 

 tribution of nearly all the species of the genua- 



