152 



tute less than a seventh of the total representatives of the species. 

 Their distribution throughout the year (Table I.) accords with the 

 results obtained by Lauterborn ('98), who found that the average 

 length of the posterior spine from January to May and from October 

 to December was from 78 to 48/i, while from June to September it 

 was from 28.5 to 21 //. In Table I. it will be seen that the longer- 

 spined forms which I have referred to A. cochlearis var. macracantha 

 and var. typica occur in the plankton from January to May 3 1 , and 

 then disappear, returning again, in small numbers, October. 25. The 

 short-spined variety referred by me to A. cochlearis var. stipitata 

 and the spineless var. tecta are, on the other hand, continued during 

 the summer. The natural result would be that the average length 

 of the spines in the species as a whole would fall during the summer 

 months. It is apparent that this tendency on the part of A. 

 cochlearis to become shorter and smaller during the summer months 

 does not bear out the contention of Wesenberg-Lund ('98) that 

 winter individuals are smaller and summer ones larger among 

 perennial rotifers. He reports var. tecta as "die Hauptform des 

 Winters " in several Danish lakes, and the variety with a long 

 horn as a summer form, found in July-August. 



Of these varieties, macracantha, typica, and stipitata intergrade 

 in our waters with numerous connecting links, while var. tecta is not 

 connected with the other forms by many individuals with inter- 

 mediate characters. Lauterborn ('98) also notes the greater inde- 

 pendence of this variety in the waters of the Rhine. 



In Table I. the seasonal distribution of these three varieties, the 

 .long-spined (typica and macracantha}, the short-spined (stipitata), 

 and the spineless (tecta} are given separately. It will be noted that 

 the long-spined form has the distribution above mentioned, that 

 var. tecta runs throughout the whole year, and that var. stipitata is 

 absent in midwinter and is a common summer form. The relative 

 numbers of the varieties fluctuate in different years. For example, 

 var. tecta was relatively but one fourth as abundant in 1897 as in 

 1898. As shown in Table I., whenever coincidently present in the 

 plankton all the varieties respond to the causes which produce the 

 rhythm of occurrence, the rise, culmination, and decline of the pulses 

 being much alike in all of the varieties. 



About three eighths of the females noted in 1898 were ovigerous, 

 carrying as a rule but a single egg. Instances of two eggs were 



