268 ORDER CLADOCERA; DAPHNIA. 



The other organs then begin to show themselves ; but it is not 

 until near the end of the fourth day, that any motion is per- 

 ceptible. They come forth into the world, at the end of the fifth 

 day, with the tail curved up within the shell ; and very shortly 

 after birth, this tail maybe seen to spring forth with a sudden jerk, 

 and to assume its natural position. In a short time afterwards, 

 the animal acquires the perfect form, and it speedily increases 

 in size, throwing off' its envelope, and acquiring a new one, at 

 short intervals. In summer these moultings are frequently 

 seen to be performed every two days ; but in colder weather, 

 several days elapse between them. They do not cease with the 

 full growth of the animal, but continue during its whole life. 

 The purpose of this may be, to prevent the animal being injured 

 by the tendency of its shell to become overgrown with parasitic 

 Animalcules and Confervas ; for weak and sickly individuals 

 may be frequently seen so covered with these growths, that their 

 motion and life are soon arrested, the animals apparently not 

 having strength enough to throw off their envelope. 



805. After the third or fourth moulting, the young Daphnia 

 begins to deposit its eggs in the cavity of its back ; these may 

 be frequently seen there, as early as the tenth day of its separate 

 existence. Soon after the young are born, another moulting 

 takes place ; and the egg-coverings, which have been left in the 

 cavity, are thrown off with the shell. In a very short time after- 

 wards, another brood of eggs is seen in the cavity ; these are 

 soon hatched, and another moult takes place ; and these pro- 

 cesses continue to take place through the whole season, until the 

 weather becomes severe. All the Daphnicz then seem to be 

 destroyed ; none having their existence prolonged through the 

 winter. There is, however, a very curious provision for perpetu- 

 ating the race. At particular seasons, the Daphnia may be 

 found with a dark opaque substance on the back of the shell. 

 This has been termed the ephippium, from its resemblance to a 

 saddle. When examined by the microscope, it is seen to be of 

 a dense texture, and to be composed of a mass of hexagonal cells ; 

 and to contain two oval bodies, which are capsules opening like 

 a bivalve shell, each inclosing an ovum covered with a horny 



