History of British Entomostraca. 409 



comes overgrown with moss or parasitic infusoria^ and thus the in- 

 sect's motions become much impeded, and at last entirely destroyed ; 

 the moulting, therefore, seems to be necessary in the full grown in- 

 sect for the preservation of its life, for weak or sickly individuals 

 may be seen frequently so overgrown with moss, &c., that motion 

 and life are both soon arrested, the insect apparently not having 

 strength enough to throw off its exuviae. At the 4th moulting, * 

 the young insect has eggs, which are deposited in the open space 

 on the back, almost immediately after the old shell has been cast 

 oif. The time that the eggs remain there varies according to 

 the season of the year. In summer, according to Jurine, three days ; 

 according to Straus, from four to six. In winter I have found the 

 interval between the eggs, being deposited in the open space, 

 and the young ones being born, to be eight days, as may be seen in 

 the experiments detailed above. Moulting takes place every time 

 after the young are born ; and generally within a very short period 

 of time after the change has taken place, eggs are again depo- 

 sited. Occasionally, however, this does not take place, and then 

 the insect remains without eggs for a space of time equal to 

 that of carrying the eggs, when she moults again, and then has 

 eggs. Straus says, that the young of the same laying are generally 

 speaking all of one sex, the two sexes being seldom to be found to- 

 gether in the same birth. He also says, that the Daphnias cease to 

 produce at the approach of winter or to change their skin, and that 

 they die before the commencement of frost. This does not accord 

 with my experience, having found them in considerable numbers 

 producing young and moulting as late as the month of December, 

 after both frost and snow had taken place. Indeed I have found 

 them as late in the season as February, though not in great num- 

 bers, but about that time they seem to disappear, and perhaps in a 

 severe winter earlier, young ones only being to be met with in the 

 spring. At particular seasons the Daphniae may be found with a 

 dark opaque substance on the back of the shell. This is what Mul- 

 ler has called the Ephippium, from the resemblance the substance 

 has to a saddle. He was the first to take notice of this curious ap- 

 pearance ; but though he describes it very well, and has given an ac- 

 curate representation of it, he does not give any opinion upon the 

 cause or use of the formation. Jurine next notices it, describes it 

 well, traces its gradual formation from matter contained in the 



* Straus says after the third ; Jurine says generally between third and fourth, 

 I have always found it to be after the fourth. 



