38 LECTUKE III. 



fcjetus to move in the egg. At the twelfth hour the body was com- 

 pletely formed, and bent somewhat spirally, the bifurcated anal ap- 

 pendage being doubled backwards towards the head. The re- 

 volutions of the young Rotifer are now so powerful as to threaten 

 every instant to burst the egg-shell, but they often continue two 

 hours. 



The average period of development of a young Hydatina under 

 favourable circumstances is twenty-four hours ; twelve within and 

 twelve without the parent's body. When it proceeds more slowly, 

 Ehrenberg recommends the liberal supply of the green monads 

 (^Chlaynydomonas ptilvisculus, and JEugle?ia viridis). 



Ova deposited in the cold early days of winter remain undeveloped 

 until spring, and are protected by their dense double shell. 



Ehrenberg watched during eighteen days successively an individual 

 Hydatina senta, which was full-grown when singled out, and did not 

 die of old age, which proves this species to live more than twenty 

 days. Such an individual is capable of a four-fold propagation every 

 twenty-four or thirty hours, bringing forth in this time four ova, 

 which grow from the embryo to maturity, and exclude their fertile 

 ova in the same period. The same individual, producing in ten days 

 forty eggs, developed with the rapidity above cited, this rate, raised 

 to the tenth power, gives one million of individuals from one parent? 

 on the eleventh day four millions, and on the twelfth day sixteen 

 millions, and so on. 



Although this rate of production from fertile ova is the greatest 

 hitherto observed, far exceeding that in the class of insects, it is much 

 inferior to the propagative power in the Polygastria. We saw that 

 in the Paramceciwn aurelia, which lives several days, a transverse 

 fissure took place, the individual becoming two every twenty-four 

 hours. It also propagates by ova, which are excluded not singly, 

 but in masses ; which ova rapidly develop and repeat the acts of pro- 

 pagation ; so that the possible increase in forty-eight hours is quite 

 incalculable. Who can wonder that infusions should, with the brood 

 of two or three days only, swarm with these animalcules I 



All the ordinary Infusoria live through the winter beneath the ice. 

 After having been once completely frozen, Ehrenberg found them 

 dead when thawed. They, however, manifest considerable powers of 

 resistance to this effect of extreme cold. Ehrenberg endeavoured to 

 freeze some Infusoria in a watch-glass, and examined the clear ice in a 

 cold room : he observed that those which appeared to be frozen and im- 

 bedded in the mass were actually inclosed in very minute vesicles in 

 the ice. He conceives that they may remain torpid in this state 



