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FROM SIMPLE PERCEPTIBLE MATTER. 559 
organs of the larger animals is also found to exist. There are very often 
observed in animal dissections a small number of full-grown worms, 
filled with an innumerable quantity of eggs without any young in their 
proximity ; and I was often astonished to find in the considerable num- 
ber of my dissections of animal bodies (I have brought from Africa 
alone intestinal worms of 196 species of animals, all of which I have 
myself dissected, and of some from 40 to 50 individuals, ) only a few 
alive, although these were completely filled with eggs. Thus from la- 
borious observations this opinion has become more and more firmly 
fixed in my mind, that it is much more astonishing how the great fe- 
cundity of the Entozoa should be so limited by the living organs, than 
that it should be possible that living worms should inhabit them, and, 
considering their diffusion, escape observations which are generally su- 
perficial. The Epizoa present to the observer quite a different propor- 
tion, although these can for the most part be limited voluntarily by the 
animals. The circumstances favourable to their cyclical development in 
most cases overcome those which limit it; and a careful observer might 
follow with ease the formation and development of their innumerable eggs. 
It is not the small size of the Entozoa which forms the difficulty, but 
solely their inaccessible station in the interior of living animal bodies. 
3. Infusoria.—Of an entirely different nature is the difficulty of ob- 
servation in the Infusoria, the third strong hold of the generatio equi- 
voca: it lies in their minute size. Observations of Infusoria, which 
I pursued with great zeal and repeated on every occasion, showed me 
the necessity of a more definite determination of their forms, which I 
endeavoured to acquire by drawings and measurements of them. These 
severe and often-repeated investigations of individuals enabled me fre- 
quently to recognise the most decided traces of a higher internal organi- 
zation than had previously been ascribed to them. In the year 1819 
Thad already observed that the motion of the zoological monads (Monas 
pulvisculus) was by no means a mere rolling effected by a change of 
the centre of gravity, as it was thought to be; but I perceived, from 
the throwing off of very minute particles of the dirty water, and from 
an apparent whirling at the anterior part of the animalcule, the presence 
of oarlike cilia, which at times even became visible. Some of these ob- 
servations I made known in 1820, in an Appendix to a Memoir by my 
friend Friederich Nees von Esenbeck in the Regensburg Botanical Jour- 
nal, part ii. p. 535. My friend and subsequently fellow-traveller Dr. 
Hemprich often witnessed my observations and experiments, and has also 
given, in his Grundrisse der Naturgeschichte, 1820, p. 289 to 291, a sum- 
mary account of what I had ascertained at that time (see preface, p.viii.). 
I was not then myself desirous of making publicly known any of those 
observations, because I saw on the one hand that they were capable of 
being carried to much greater perfection, and on the other hand, I pos- 
sessed at that time only a very incomplete thirty-shilling wooden com- 
