INFUSORIAL ANIMALCULES. 17 



this calculation is not to be regarded as unworthy of confidence, inasmuch as the 

 Epistylis is known to attain a length no greater than the twenty-four hundredth 

 part of an inch. In the case of the Ophrydin already described, a cubic inch of 

 the gelatinous matter in which they are embedded has been estimated to contain 

 no less than eight million distinct beings ; a fact which, when taken in connection 

 with others of the same nature, render it highly probable that the living beings of 

 the microscopic world surpass in number 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 animalcule 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 iUoricated or shettess 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. 



Many animalcules which may be classed as illoricated have yet a firmer covering 

 than others belonging to this division. It consists of a gelatinous membrane hav- 

 ing a bell-shaped cylindrical or conical figure, closed at the lower end, but open at 

 the other, through which opening the animalcule may protrude itself. 



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 colored sub- 

 stances before examination, that this supposition has been shown to be entirely un- 

 founded. 



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

 is the most remarkable, and is common to all Infusoria, is a slender filament like a 

 hair, situated near the mouth, and from its striking resemblance to an eyelash is 

 known by the name of cilium, the Latin word for eyelash. 



The cilium is employed by the animalcule for the purpose of motion, and also 

 for that of procuring food. Using this member as an oar, the creature moves 

 swiftly through the water, and so curious is the action of this propeller, that 

 the very stroke which effects a progressive motion, causes at the same time a 

 current to set towards the mouth of the animalcule, bearing its prey and food 

 within its reach. 



In addition to the offices of the cilia* just described, they are supposed by some na- 

 turalists to be the principal instruments for respiration to the Infusorial world ; inas- 

 much as similar appendages are found encircling the gills or beard of the oyster and 

 muscle, and other animals of the like nature. It is by means of the gills that these 

 creatures inhale the air contained in the water, and the cilia, by causing currents to 



* CiUa, the plural of (Mum. 



