Crustacea of Belgium. 371 



Daphnia resists its action scarcely for a quarter of an horn'. 

 M. Paul Bert ascribes the deatli of sea-fish in fresh ^yater to 

 the difference of density, of osmotic power, and of the power 

 of holding oxygen in solution possessed by the two liquids. 

 Now these small Crustacea do not as yet verify this supposi- 

 tion ; for they continued living for eight days and more in a 

 solution of sugar of the same density as sea-water. From my 

 investigations it would appear that we must attribute the 

 death of the Cyclopida? and Daphnice in sea-water to some of 

 the salts which that water holds in solution. By employing 

 them alone and separately, in the proportions in which they 

 exist in the water of the ocean, we find that the chlorides of 

 sodium and magnesium act like true poisons, and that sulphate 

 of magnesia has no action. 



It was supposed until very lately that Cyclops quadricornis 

 had no heart. Nevertheless a heart exists in it, and is of a 

 pyriform shape, slightly constricted in the middle, with its 

 broadest end in front. The only apertm-e I have been able to 

 distinguish in it is a venous fissure at the antero-superior 

 part. Whilst the heart of Cyclopsina castor is situated under 

 the first thoracic ring, that of Cyclops quadricornis^ on the 

 contrary, is near the extremity of the sixth cephalic somite. 

 It beats very slowly. 



I have entirely passed over the internal reproductive organs, 

 and only attended to the genital apertures, which are less 

 known. 



In Cyclops quadricornis the female genital orifice opens on 

 the ventral median line, in the fm-row which separates the last 

 thoracic from the first abdominal somite. The last thoracic 

 somite forms its upper lip, and is moved by special muscles. 

 Its lower lip belongs to the following segment. 



The investigation of the mode of formation of the oviferous 

 sacs has enabled me to ascertain that the elongated secretory 

 organ lodged in the first and second abdominal segments, and 

 opening at the vulva, is not devoted to the secretion of the 

 sacs, but is a seminal receptacle. The time secretory organ of 

 the oviferous sacs consists of two cuiwed glandular cseca si- 

 tuated beneath the skin of the first abdominal somite. Al- 

 though at first very indistinctly visible, these glands by degrees 

 acquire more distinct outlines. When the female is fecundated, 

 the seminal receptacle, which is enormously swelled, ascends 

 entirely into the fii'st segment of the abdomen, which it fills 

 up, and at the same time pushes upward the glands just men- 

 tioned. These glands, the volume of which has increased at 

 least a hundredfold, extend themselves laterally to the epimera. 

 On each side we find an aperture, which has long been known, 



