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 difiicult 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. schcefferi this variation may range from about 150 per minute 

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

 Simocephahis I have observed a heart beat 300 times in a minute. 

 Again, in a specimen of i>. schcefferi at rest the heaj-t was beating 170, 

 but during the spasmodic motion of feet and antennae 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 Lyncodaphnidce (P. E. Mueller considers it a tran- 

 sition to the Bosminidce and Lyncodaphnidce); on the other hand, Leydig 

 and Kui z regard it more closely allied to the Sididce, with equally good 

 reason. The long antennte, long narrow antennules and many peculi- 

 arities in form, etc., suggest the macrothroid crustaceans; the ex- 

 tended abdomen and especially the location of the male seminal 

 opening are like Dciphnella, which Moina resembles in motion and habit 

 very strikingly. The absence of the pigment fleck is no more a char- 

 acteristic of the Sididce than of other groups. After all has been said, 

 the immediate affinities of the genus are acknowledged to be with the 

 Daphnidce. 



