32 TWELFTH ANNUAL REPORT. 



seen to enter the arterial opening and emerge from the ventricles. 

 This process would be impossible if the anterior valve were as de- 

 scribed by Claus and Weismann; while being really more like the 

 venous valves, it is easily and frequently permitted. The current 

 of the blood in this case stagnates except near the heart. 



The rapidity of the pulsations of the heart varies with age and 

 condition of rest or motion. 



In D. schfefFeri this variation may range from about 150 per 

 minute to perhaps 250, 200 being probably a fair average. In a 

 young Simocephalus I have observed a heart beat 300 times in a 

 minute. Again, in a specimen of D. Schseflferi at rest the heart 

 was beating 170, but during the spasmodic motion of feet and an- 

 tennge the pulse rose to over 200. 



I. — GrENUS MoiNA. Baird. 



The systematic position of this genus has been the theme of 

 some discussion, it being claimed, with good reason, that there are 

 many resemblances to the Lyncodaphnidse (P. E. Mueller considers 

 it a transition to the Bosminidse and lyncodaphnids); on the other 

 hand, Leydig and Kurz regard it more closely allied to the Sididte, 

 with equally good reason. The long antennae, long narrow anten- 

 nules and many peculiarities in form, etc., suggest the macrothroid 

 crustaceans; the extended abdomen and especially the location of 

 the male seminal opening are like Daphnella, which Moina resem- 

 bles in motion and habit very strikingly. The absence of the pig- 

 ment fleck is no more a characteristic of the Sididae than of other 

 groups. After all has been said, the immediate affinities of the 

 genus are acknowledged to be with the Daphnidse. 



The true place of the genus, as it appears to the writer, was 

 hinted at by Birge (Notes on Cladocera). Moina seems to be the 

 pivotal point of the Cladocera, at least of the families above men- 

 tioned. Without going into phylogenetic speculation, it is sug- 

 gestive that this genus can and does by preference live in very im- 

 pure water and may therefore have had an early origin. From 

 Moina diverges the stem of the Daphnidae by way of Ceriodaphnia, 

 Simocephalus and Daphnia. These two latter genera are intimately 

 connected by Simocephalus daphnoides, Herrick. Scapholeberis 

 is connected with Ceriodaphnia through S. angulata, Herrick. 

 The Sididae seem to diverge by the way of Daphnella, through 

 which by means of Pseudo-sida the genus Sida is reached, and final- 

 ly Limnosida, Latona and Holopediura. The relationships of the 

 curious Polyphemidae are less evident. 



