No. 6.] AMERICAN MORPHOLOGICAL SOCIETY. 



J 



viciously and the next morning they were found dead. Argn- 

 lus deposits its eggs instead of carrying them around like other 

 crustaceans, arranging them in rows on sticks, stones, etc., 

 with their long diameters parallel. 



When laid they are covered with a jelly envelope consist- 

 ing of beads of jelly arranged in rows parallel with the long 

 diameter of the egg. These harden into a brittle shell. The 

 eggs are fertilized outside the body of the female and there 

 is no copulation. The egg hatches into a typical nauplius, 

 which after one or two moults changes into a metanauplius 

 having highly developed clasping organs in the shape of barbed 

 claws terminating the second maxillae. On putting two small 

 dace into the aquarium with about two hundred of these larvae, 

 the latter made no attempt to use their claspers, but the fish, 

 on recovering from their fright, ate up every one of the larvae. 

 On inquiry it was found that the pond in question had been 

 seined for three years, and that the dace and roach had been 

 sold for pickerel bait. First conclusion, the subject of para- 

 sitism is not so one-sided as would appear at first sight. 

 Second conclusion, the protection of small fish like dace and 

 roach in our fish ponds may be one of the best preventives 

 against such parasites as these. 



V. THE ANATOMY AND DEVELOPMENT OF THE 

 POSTERIOR VENA CAVA IN DIDELPHYS 

 VIRGINIANA (KERR, LINN.). 



C. F. W. McCLURE. 



AN examination, thus far, of forty-eight opossums has brought 

 to light many interesting variations concerning the mode of 

 formation of their posterior vena cava. 



These variations are so pronounced and so closely accord 

 with certain embryonic conditions described by Hochstetter 

 for Echidna aculeata, it seems to the writer as not improbable 

 that the development of the posterior vena cava may take place 

 in Didelphys and Echidna in substantially the same manner. 



