ROTIFERA. 53 



then appeared in the middle of the ovum, which, six hours after 

 exclusion, could be distinguished as the head, with the rudimental 

 dental apparatus of the embryo. At the eleventh hour the wheel- 

 like ciliated organs began to play, and the foetus to move in the egg. 

 At the twelfth hour the body was completely formed, and bent some- 

 what spirally, the bifurcated anal appendage being doubled backwards 

 towards the head. The revolutions 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, 

 i^Chlamydomonas pulvisculus, and Ettglena viridis). 



Ova deposited in the cold early days of winter remain undeveloped 

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

 berg 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 Paramcecium aurelia, which lives several days, a trans- 

 verse fission took place, the individual becoming two, every twenty- 

 four hours. It is affirmed also to propagate by ova, which are 

 excluded, not singly, but in masses ; which ova rapidly develope and 

 repeat the acts of propagation ; 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. 



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, manifested considerable powers 

 of resistance to this eflfect of extreme cold. Ehrenberg endeavoured 



E 3 



