INFUSORIAL ANIMALCULES. 13 



SECTION II. General External Forms, Coverings, Organs, 

 and Members of Infusoria. 



Before entering on the classification of infusorial ani- 

 malcules, as determined by their internal structure, it will 

 be well to make a few remarks upon their general appear- 

 ance and external characters, as exhibited by the micro- 

 scope. The forms and members of large animals may be 

 said, in one respect, to differ but little from each other ; 

 the comparative anatomist being enabled to trace, by easy 

 gradations, one common type throughout the whole, the 

 varieties being occasioned by a greater development of 

 certain parts, and the suppression of others. Such, how- 

 ever, is not the case with Infusoria. The general forms of 

 Infusoria will be best conceived by a reference to the 

 drawings, inasmuch as words would be found insufficient 

 to convey an idea of the vast varieties which they assume. 

 Some are egg-shaped ; others resemble spheres ; others, 

 again, different kinds of fruit, eels, serpents, and many 

 classes of the invertebrated animals, funnels, tops, cylin- 

 ders, pitchers, wheels, flasks, &c. &c. 



The covering, or outer tunic, of Infusoria, is of two kinds; 

 the one soft and apparently membraneous, yielding to the 

 slightest pressure, and accommodating itself to the state of 

 repletion or otherwise of the animalcule, and thus resem- 

 bling the tunic of the naked molusca and annelida, as 

 slugs, leeches, &c. ; the other, stiff, rigid, and hard, having 

 the appearance of a shell, though, from its flexibility and 

 transparent nature, it is more like horn. The creature 

 identified with the former of these is termed the naked, 



