160 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 
scribed, Instead of nearly spherical or oval chambers they are really 
tubes which connect on one hand with the opening above, and below 
with the general cavity of the limb, whence the blood returns to the 
abdomen. The current is very rapid through these tubes. The blood 
having been returned to the abdomen, courses in the well-known man- 
ner through the post abdomen and flows over the intestine, thence 
over the back flowing stream to the posterior lower opening of the 
pericardial chamber. 
The study of the actions of the heart is rendered more difficult by 
the fact that in order to secure the greatest possible transparency, the 
living animal must be covered and a little pressure applied, which is 
frequently attended with abnormal variations of the circulation. In 
particular, if the usual exit of the blood be stopped by the coeca of the 
intestine, as is frequently the case, the operation of the heart may be 
reversed, when a vigorous stream may be seen to enter the arterial 
opening and emerge from the ventricles. This process would be im- 
possible if the anterior valve were as described by Claus and Weis- 
mann; 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 con- 
dition of rest or motion. 
In D. schefferi 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. schefferi at rest the heart was beating 170, 
but during the spasmodic motion of feet and antenne the pulse rose 
to over 200. 
GENUS 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 Lyncodaphnide (P. E. Mueller considers it a tran- 
sition to the Bosminide and Lyncodaphnide); on the other hand, Leydig 
and Kurz regard it more closely allied to the Sididw, with equally good 
reason. The long antennze, long narrow antennules and many peculi- 
arities in form, ete., suggest the macrothroid crustaceans; the ex- 
tended abdomen and especially the location of the male seminal 
opening are like Daphnella, which Moina resembles in motion and habit 
very strikingly. The absence of the pigment fleck is no more a char- 
acteristic of the Sididw than of other groups. After all has been said, 
the immediate affinities of the genus are acknowledged to be with the 
Daphnide. 
