INFUSORIAL ANIMALCULES. 17 



amining those works in which they are accurately delineated. In the great work 

 of Dr. Ehrenberg, who has devoted his life to the study of animalcules, they are 

 beheld in all their beautiful and singular proportions. This splendid volume, of 

 folio size, contains sixty-four plates, filled with several hundred infusorial shapes, 

 drawn and colored from nature. Some resemble globes, trumpets, stars, boats, 

 and coins ; others assume the forms of eels and serpents ; and many appear in 

 the shape of fruits, necklaces, pitchers, wheels, flasks, cups, funnels, and fans. 



But the minuteness of these beings is no less surprising than the diversity of 

 their forms. The Monad, the smallest of all known living creatures, swarms by 

 myriads in a drop of water ; for it has been computed that within this small 

 space, no less than Jive hundred millions could be comprised ; and this calculation 

 is not to be regarded as unworthy of confidence, inasmuch as the Monad is never 

 found to attain a length greater than the twelve thousandth part of an inch. In 

 a cubic inch of a certain kind of mould, consisting entirely of animalcules, more 

 than forty-one millions of distinct beings were estimated by Ehrenberg to exist ; 

 a fact which, when taken in connexion with others, of the same nature, render it 

 highly probable, that the living beings of the microscopic world surpass in num- 

 ber those which are visible to the naked eye. 



STRUCTURE. The outer covering of infusorial animalcules is of two kinds ; 

 the first soft and yielding, resembling the skin of the leech and slug, and so far 

 capable of expansion and contraction as to adapt itself to the state of the ani- 

 malcule whether distended or not ; the second presenting the appearance of a 

 firm, transparent shell, yet possessing a flexibility like horn. Those animalcules 

 that are protected by the latter integument are termed loricated, from the Latin 

 word lorica, a shell ; while the name illoricated or shelless, is assigned to those 

 which are invested with the softer and more perishable covering. The materials 

 that compose the shell vary in different species ; in many instances it consists 

 entirely of flint, and in others of lime united with oxide of iron ; in some cases it 

 is combustible and in others not. In several kinds, the lorica, in the form of a 

 jar or cylinder, entirely surrounds the animalcule, while in others it is shaped like 

 a shield, and protects the living atom to which it belongs, as the shell of the 

 turtle defends its sluggish inhabitant from external danger. 



When the loricated infusoria die, their shells yet remain, uninjured for ages, 

 and in several parts of the world have been discovered accumulated in such vast 

 quantities as to form extensive deposits of marl, lime, and flint, of which we 

 shall speak more particularly hereafter. 



It was formerly believed, that the smaller species of animalcules were entirely 

 destitute of external organs ; but such improvements have now been made in 

 the construction of microscopes, and the organization of the living objects has 

 been rendered so much more distinct, from the practice of feeding them on color- 

 ed substances before examination, that this supposition has been shown to be 

 entirely unfounded, even in the case of monads. 



These external organs vary in kind in different animalcules, but the one which 



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